Archive for exoplanets

Protoplanetary disk AS 209 imaged by ALMA

Posted in astronomy with tags , , on March 5, 2018 by Tim Kendall

ESO press release: Nestled in the young Ophiuchus star-forming region, 410 light-years from the Sun, a fascinating protoplanetary disc named AS 209 is slowly being carved into shape:

“This wonderful image was captured using the high-resolution ALMA telescope, revealing a curious pattern of rings and gaps in the dust surrounding a young star. Protoplanetary discs are dense, rotating planes of gas and dust that surround newly formed stars; providing the matter that one day becomes orbiting planets, moons and other minor bodies. At less than one million years old, this system is very young, but already two clear gaps are being sculpted from the disc. The outer gap is deep, wide, and largely a dust-free zone, leading astronomers to believe that a giant planet almost the mass of Saturn is orbiting here — around 800 light-minutes from the central star, and more than three times the distance between Neptune and the Sun! As the planet carves out its path, dust piles up at the outer edge of its orbit, creating ever more defined rings in the disc. The thinner, inner dust gap could have been formed by a smaller planet, but astronomers have raised the intriguing possibility that the large and distant circling planet in fact created both paths. This inferred Saturn-like planet so far from its central star raises fascinating questions about planet formation at the edges of protoplanetary discs on particularly short timescales.”

The link to the associated research paper by D. Fedele et al. can be found here. Given the importance of molecules in star formation, and the apparent prevalence of protoplanetary discs, every new image of this sort continues to hint that on astronomical scales, the building blocks of life are everywhere around us. Scientific American magazine have recently given a new overview of the TESS mission, due for launch in April and designed to find transiting exoplanets around stars within ~100 pc of the Sun. Because nearby, these will be the most ready targets for study of their atmospheres, where they are shown to exist, by JWST. For a very different, extragalactic note, I  recommend Astronomy Picture of the Day’s delightful treatment today of the Hubble Ultra-Deep Field.

More: “Planet Nine” has not yet been observed. Scientific American have an excellent new round-up today as to why this is so, with insight from the research teams involved on the practical and scientific difficulties they face in attempting to pin down the hypothetical ten Earth mass body, at least twenty times more distant from the Sun than Neptune, which has been invoked to explain the unusual properties of the orbits of recently discovered small trans-Neptunian objects.


Implications of an upper mass bound on exoplanets

Posted in astronomy with tags , , on January 22, 2018 by Tim Kendall

Image: Illustration of a massive exoplanet orbiting a solar-type star. A new paper, accepted by the Astrophysical Journal, is by K. Schlaufman (Johns Hopkins Univ.), in arXiv today and clarifies a question which I attempted to address in a recent post, here; the findings of an earlier paper (to which I drew attention in that post) are reinforced. Observational evidence is combined with insights from theory to address whether a boundary exists between planets which form like gas-giant planets through core accretion, or like stars, through gravitational instability. From the abstract of today’s paper (abridged):

Celestial bodies with a mass of M ~ 10 M_Jup have been found orbiting nearby stars. It is unknown whether these objects formed like gas-giant planets through core accretion or like stars through gravitational instability. I show that objects with M <~ 4 M_Jup orbit metal-rich solar-type dwarf stars, a property associated with core accretion. Objects with M >~ 10 M_Jup do not share this property. This transition is coincident with a minimum in the occurrence rate of such objects, suggesting that the maximum mass of a celestial body formed through core accretion like a planet is less than 10 M_Jup. Consequently, objects with M >~ 10 M_Jup orbiting solar-type dwarf stars likely formed through gravitational instability and should not be thought of as planets.

These findings apply to solar-type stars, and to how planets around other such stars, much like our Sun, form in general. The findings of both these papers, taken together, strongly point to the 4 – 10 Jupiter mass range as marking a real, distinctive boundary between bona-fide planets and more massive (sub)stellar objects.

Seven temperate rocky planets orbiting the red dwarf TRAPPIST-1

Posted in astronomy with tags , , on February 23, 2017 by Tim Kendall

pia21421The TRAPPIST-1 star, an ultracool red dwarf located 12 parsecs from the Sun, has seven Earth-size planets orbiting it. This artist’s concept appeared on the cover of the journal Nature on Feb. 23, 2017.  It shows the expected physical state of water at the probable temperature of each planet: (courtesy: NASA)

This exoplanet system is called TRAPPIST-1, named for The Transiting Planets and Planetesimals Small Telescope (TRAPPIST) in Chile. In May 2016, researchers using TRAPPIST announced they had discovered three planets in the system. Assisted by several ground-based telescopes, including the European Southern Observatory’s Very Large Telescope, Spitzer confirmed the existence of two of these planets and discovered five additional ones, increasing the number of known planets in the system to seven.The new results were published Wednesday in the journal Nature, and announced at a news briefing at NASA Headquarters in Washington. Using Spitzer data, the team precisely measured the sizes of the seven planets and developed first estimates of the masses of six of them, allowing their density to be estimated. Based on their densities, all of the TRAPPIST-1 planets are likely to be rocky. Further observations will not only help determine whether they are rich in water, but also possibly reveal whether any could have liquid water on their surfaces. The mass of the seventh and farthest exoplanet has not yet been estimated – scientists believe it could be an icy, “snowball-like” world, but further observations are needed. “The seven wonders of TRAPPIST-1 are the first Earth-size planets that have been found orbiting this kind of star,” said Michael Gillon, lead author of the paper and the principal investigator of the TRAPPIST exoplanet survey at the University of Liege, Belgium. “It is also the best target yet for studying the atmospheres of potentially habitable, Earth-size worlds.”

The paper in Nature is available to subscribers: Gillon, M. et al. Nature (2017) and a summary at Nature News here. The planets have radii between about three-quarters that of Earth up to 1.13 times that of Earth, and their derived masses yield densities ranging between 0.6 and 1.2 times that of Earth (image courtesy NASA):

This chart shows, on the top row, artist conceptions of the seven planets of TRAPPIST-1 with their orbital periods, distances from their star, radii and masses as compared to those of Earth. The bottom row shows data about Mercury, Venus, Earth and Mars.

Update: more accurate and precise eccentricities are found, < 0.02 for the six innermost planets, together with a more constrained mass for the seventh, h, in a new preprint by S. Wang et al., 2017 Apr 17. Overall, derived masses for the outer planets d, e, f and g decrease compared to the estimations from the discovery paper.

Radial velocity discovery of Proxima Centauri b

Posted in astronomy with tags , on August 26, 2016 by Tim Kendall

The sky around Alpha Centauri and Proxima Centauri

“It is true. We are convinced that there is a planet orbiting Proxima now. The evidence goes as follows: a signal was spotted back in 2013 on previous surveys (UVES and HARPS). The preliminary detection was first done by Mikko Tuomi, our in-house applied mathematician and his Bayesian codes. However, the signal was not convincing as the data were really sparse and the period was ambiguous (other possible solutions at 20 and 40 days, plus a long period signal of unknown origin). We followed up Proxima in the next years but our two observing runs were 12 days, barely sufficient to secure a signal which ended up being 11.2 days. So the Pale Red Dot was designed with the sole purpose of confirming or refuting its strict periodicity, plus carefully monitor the star for activity induced variability. We got very lucky with the weather so we obtained 54 out of 60 observations. The photometric monitoring telescopes (ASH2 and several units of Las Cumbres Observatory Global Telescope network), worked flawlessly so we could see the effect of spots, flares and rotation of the star, which also had a footprint on the spectra. However, nothing indicated that spurious variability would be happening at 11.2 days. So that’s basically it: the Pale Red Dot campaign also detects the same period, and confirms that the signal has been in phase for the 16 years of accumulated observations. This is a requirement for a proper Keplerian orbit. Features like starspots are more short lived plus affect the velocities in the time-scales of the rotation of the star, which is now confirmed at ~83 days.”

Image [section, Proxima Centauri is the orange-red star in center of this image]: ESO press release. The alpha Centauri AB pair are off to the upper left of this image and of course exceedingly bright. Here is link to the Nature paper. Text: Huge and heartfelt congratulations to the Pale Red Dot Team. Also, the website contains an interview by Guillem Anglada-Escudé, who led this work, with Didier Queloz, co-discoverer of 51 Peg b back in 1995, and it is well worth reading to compare the stories of the two discoveries, as well as everything else on, for a sense of the field.

At the observational frontier of local solar space: RECONS and the ten parsec census

Posted in astronomy with tags , , on April 28, 2016 by Tim Kendall

RECONS-grabImage credit: A. Riedel and the RECONS group, P.I. Todd J. Henry taken from a new visualization by the RECONS consortium ( which can be seen on youtube. This is my final post (for now) on the subject of the completeness of our knowledge of stellar and planetary systems near the Sun. Stars here are plotted coloured broadly by spectral type and sized approximately by luminosity class (dwarfs or giants). The horizontal blue circle is the galactic plane crossed by the equatorial plane (grey circle) plotted at 2, 5, 10 and 25 parsecs distance. RECONS is preparing a 10 parsec census for publication and leads the field in both observation and visualization. The latest data were presented last year by Henry to the 227th meeting of the American Astronomical Society, and I reproduce the abstract in part below. In the meantime, Bihain & Scholz (2016) have investigated the projected distribution of brown dwarfs around the Sun and listed 26 brown dwarfs within 6.5 parsecs distance (as compared to 136 stars) in their Table 1 (also below):

The sample of stars, brown dwarfs, and exoplanets known within 10 parsecs of our Solar System as of January 1, 2015 is presented. All systems have trigonometric parallaxes of 100 mas or more with errors of 10 mas or less. Included in the sample are 12 systems in the southern sky added to the sample via new parallaxes from the RECONS (REsearch Consortium On Nearby Stars, effort at the CTIO/SMARTS 0.9m.The census consists of 366 stars (including the Sun and white dwarfs), 37 brown dwarfs, and 34 planets (eight in our Solar System and 26 exoplanets). Red dwarfs clearly dominate the sample, accounting for 75% of all stars known within 10 pc, while brown dwarfs are currently outnumbered 10 to 1 by stars. The completeness of the sample is assessed, indicating that additional discoveries of red, brown, and white dwarfs within 10 pc, both as primaries and secondaries, are likely, although we estimate that roughly 90% of the stellar systems have been identified. The evolution of the 10 pc sample over the past 70 years is outlined to illustrate the growth of the sample. The luminosity and mass functions are described. In contrast to many studies, once all known close multiples are resolved into individual components, the true stellar mass function rises to the end of the main sequence. With far fewer brown dwarfs than stars, different formation scenarios for objects that fuse hydrogen and those that do not are likely. Of 270 stellar primaries, 28% have companion stars, only 2% have brown dwarf companions, and 6% have detected planets. The planetary rate so far is low but climbing, while searches for brown dwarf companions to stars within 10 pc have been quite rigorous, so the brown dwarf companion rate is unlikely to rise noticeably. Overall, the solar neighborhood is dominated by small stars that are potentially orbited by many small, as yet unseen, planets.

bihain-grabBrown dwarfs near the Sun. Red dwarfs like Barnard’s star are missing. The coldest known brown dwarf WISE J0855-0714 (~250K) is third on the list following Luhman 16AB, given its WISE designation in the table. WISE J0720-0846B is the mid-T companion to Scholz’s star, a 6 pc distant M9 dwarf.

High precision radial velocity planet searches in the near-infrared

Posted in astronomy with tags , , on April 23, 2016 by Tim Kendall

Spinvelocity-vs-mass-BetaPicb-and-solarsystemSystem along with the recently measured spin rate of the planet Beta Pictoris b. Credit: ESO/I. Snellen (Leiden University). Spin rates are a different matter, but even radial velocities (RV) are notoriously hard to measure for the majority of nearby stars, which are M dwarfs, and this extends to brown dwarfs as well. Very new instrumental developments are beginning to allow RV to be measured in the near-infrared, where the spectra of these stars are less crowded with lines, allowing line widths to be measured properly, with RV and spin rates also much easier to determine. A natural extension into the planet-seeking arena follows. A new paper by Jonathan Gagné, Peter Plavchan (who is a pioneer in this field) and many co-authors has been accepted to the Astrophysical Journal and appears on arXiv:

We present the results of a precise near-infrared (NIR) radial velocity (RV) survey of 32 low-mass stars with spectral types K2-M4 using CSHELL at the NASA IRTF in the K-band with an isotopologue methane gas cell to achieve wavelength calibration and a novel iterative RV extraction method. We surveyed 14 members of young ( 25-150 Myr) moving groups, the young field star ε Eridani as well as 18 nearby (< 25 pc) low-mass stars and achieved typical single-measurement precisions of 8-15 m/sec with a long-term stability of 15-50 m/sec. We obtain the best NIR RV constraints to date on 27 targets in our sample, 19 of which were never followed by high-precision RV surveys. Our results indicate that very active stars can display long-term RV variations as low as 25-50 m/sec at 2.3125 μm, thus constraining the effect of jitter at these wavelengths. We provide the first multi-wavelength confirmation of GJ 876 bc and independently retrieve orbital parameters consistent with previous studies. We recovered RV variability for HD 160934 AB and GJ 725 AB that are consistent with their known binary orbits, and nine other targets are candidate RV variables with a statistical significance of 3-5σ. Our method combined with the new iSHELL spectrograph will yield long-term RV precisions of 5 m/sec in the NIR, which will allow the detection of Super-Earths near the habitable zone of mid-M dwarfs.

ESO/ALMA imaging of planet formation in an Earth-like orbit

Posted in astronomy with tags , , , on April 2, 2016 by Tim Kendall
( ALMA‘s best image of a protoplanetary disc to date. This picture of the nearby young star TW Hydrae reveals the classic rings and gaps that signify planets are in formation in this system. Credit: S. Andrews (Harvard-Smithsonian CfA); B. Saxton (NRAO/AUI/NSF); ALMA (ESO/NAOJ/NRAO). The star TW Hydrae is a popular target of study for astronomers because of its proximity to Earth and its status as an infant (or T Tauri) star about 10 million years old. Its distance has been recently re-calculated to be as close as 38 pc. The star itself is slightly less massive than the Sun, spectral type K8IVe (as given in an excellent recent review of young stars in nearby stellar associations here). It also has a face-on orientation as seen from Earth, giving astronomers a rare view of the complete protoplanetary disc around the star.

ALMA TW Hya central regions

This is the inner region of the TW Hydrae protoplanetary disk as imaged by ALMA. The image has a resolution of 1 AU (Astronomical Unit, the distance from the Earth to the Sun in our own Solar System). This new ALMA image reveals a gap in the disk at 1 AU, suggesting that a planet with the same orbit as Earth is forming there. Credit: S. Andrews (Harvard-Smithsonian CfA); B. Saxton (NRAO/AUI/NSF); ALMA (ESO/NAOJ/NRAO). The paper “Ringed Substructure and a Gap at 1 AU in the Nearest Protoplanetary Disk”, by S.M. Andrews et al., appearing in the Astrophysical Journal Letters (pdf copy via ESO). These recent observations represent a huge breakthrough in direct imaging at the resolutions required and are very suggestive evidence for the existence of Earth-like planets in nearby interstellar space. From the abstract:
We present long-baseline Atacama Large Millimeter/submillimeter Array (ALMA) observations of the 870 μm continuum emission from the nearest gas-rich protoplanetary disk, around TW Hya, that trace millimeter-sized particles down to spatial scales as small as 1 AU (20 milliarcseconds). These data reveal a series of concentric ring-shaped substructures in the form of bright zones and narrow dark annuli (1-6 AU) with modest contrasts (5-30%). We associate these features with concentrations of solids that have had their inward radial drift slowed or stopped, presumably at local gas pressure maxima. No significant non-axisymmetric structures are detected. Some of the observed features occur near temperatures that may be associated with the condensation fronts of major volatile species, but the relatively small brightness contrasts may also be a consequence of magnetized disk evolution (the so-called zonal flows). Other features, particularly a narrow dark annulus located only 1 AU from the star, could indicate interactions between the disk and young planets. These data signal that ordered substructures on ~AU scales can be common, fundamental factors in disk evolution, and that high resolution microwave imaging can help characterize them during the epoch of planet formation.

Update on the ongoing search for the proposed “Planet Nine”, from Scientific American: The article highlights the research of Fienga et al., (2016) using the Cassini spacecraft data to pinpoint the planet. The planet is likely sub-Jovian, ten Earth masses, eccentric, e ~ 0.6, distant but not that distant, ~ 700 AU, and possibly located in the region of the sky in the direction of the southern constellation of Cetus, with true anomaly 117.8°±11°. I predict that it will be found soon, and there is as good a chance of finding it by its own internal heat, in millimeter data, as by reflected light in the visible part of the spectrum.

Combined VLA/ALMA view of the forming planetary system HL Tauri

Posted in astronomy with tags , on March 26, 2016 by Tim Kendall


Image: Combined ALMA (red) and VLA image of HL Tau. Credit: Carrasco-Gonzalez, et al.; Bill Saxton, NRAO/AUI/NSF. The paper is Carrasco-Gonzalez et al., “The VLA view of the HL Tau Disk – Disk Mass, Grain Evolution, and Early Planet Formation,” accepted by Astrophysical Journal Letters (preprint). The image above is certainly ground-breaking, and has been noted extensively elsewhere. The age of the system is thought to be less than 100,000 (105) years and HL Tau itself is quite Sun-like, spectral type K5. It was already known to host a protoplanet (the bright clump in the yellow VLA data above) about 14 times as massive as Jupiter and about twice as far from HL Tau as Neptune is from our Sun. From the abstract:

The first long-baseline ALMA campaign resolved the disk around the young star HL Tau into a number of axisymmetric bright and dark rings. Despite the very young age of HL Tau these structures have been interpreted as signatures for the presence of (proto)planets. The ALMA images triggered numerous theoretical studies based on disk-planet interactions, magnetically driven disk structures, and grain evolution. Of special interest are the inner parts of disks, where terrestrial planets are expected to form. However, the emission from these regions in HL Tau turned out to be optically thick at all ALMA wavelengths, preventing the derivation of surface density profiles and grain size distributions. Here, we present the most sensitive images of HL Tau obtained to date with the Karl G. Jansky Very Large Array at 7.0 mm wavelength with a spatial resolution comparable to the ALMA images. At this long wavelength the dust emission from HL Tau is optically thin, allowing a comprehensive study of the inner disk. We obtain a total disk dust mass of 0.001 – 0.003 Msun, depending on the assumed opacity and disk temperature. Our optically thin data also indicate fast grain growth, fragmentation, and formation of dense clumps in the inner densest parts of the disk. Our results suggest that the HL Tau disk may be actually in a very early stage of planetary formation, with planets not already formed in the gaps but in the process of future formation in the bright rings.

Strong evidence suggests a super-Earth lies beyond Pluto

Posted in astronomy with tags , , , on January 24, 2016 by Tim Kendall


Burdick-Discovering-Planet-Nine-chart-1200Caltech researchers have found evidence of a giant planet tracing a bizarre, highly elongated orbit in the outer solar system. The object, which the researchers have nicknamed Planet Nine, has a mass about 10 times that of Earth and orbits about 20 times farther from the sun on average than does Neptune (which orbits the sun at an average distance of 2.8 billion miles). In fact, it would take this new planet between 10,000 and 20,000 years to make just one full orbit around the sun. A consequence of Planet Nine is that six distant Kuiper belt objects (magenta) all follow elliptical orbits that point in the same direction in physical space; they have the same argument of perihelion. That is particularly surprising because the outermost points of their orbits move around the solar system, and they travel at different rates. A second predicted consequence of Planet Nine is that a second set of confined objects should also exist. These objects are forced into positions at right angles to Planet Nine and into orbits that are perpendicular to the plane of the solar system. Five known objects (cyan, upper figure) fit this prediction precisely. The Sun is at centre in both plots. Credit: Caltech/R. Hurt (IPAC) [Diagram was created using WorldWide Telescope.]

The researchers, Konstantin Batygin and Mike Brown, discovered the planet’s existence through mathematical modeling and computer simulations but have not yet observed the object directly. “This would be a real ninth planet,” says Brown, the Richard and Barbara Rosenberg Professor of Planetary Astronomy. “There have only been two true planets discovered since ancient times, and this would be a third. It’s a pretty substantial chunk of our solar system that’s still out there to be found, which is pretty exciting.” Brown notes that the putative ninth planet—at 5,000 times the mass of Pluto—is sufficiently large that there should be no debate about whether it is a true planet. Unlike the class of smaller objects now known as dwarf planets, Planet Nine gravitationally dominates its neighborhood of the solar system. In fact, it dominates a region larger than any of the other known planets—a fact that Brown says makes it “the most planet-y of the planets in the whole solar system.” Batygin and Brown describe their work in the current issue of the Astronomical Journal and show how Planet Nine helps explain a number of mysterious features of the field of icy objects and debris beyond Neptune known as the Kuiper Belt. “Although we were initially quite skeptical that this planet could exist, as we continued to investigate its orbit and what it would mean for the outer solar system, we become increasingly convinced that it is out there,” says Batygin, an assistant professor of planetary science. “For the first time in over 150 years, there is solid evidence that the solar system’s planetary census is incomplete.” [more]

Further reading: could you live on Planet Nine? (at and also see this article at the New Yorker, from which is taken the second graphic (above).

Narrowband optical and infrared views of the Large Magellanic Cloud

Posted in astronomy with tags , , on January 16, 2016 by Tim Kendall

Cosmic dust clouds ripple across this infrared portrait of our Milky Way’s satellite galaxy, the Large Magellanic Cloud. In fact, the remarkable composite image from the Herschel Space Observatory and the Spitzer Space Telescope show that dust clouds fill this neighboring dwarf galaxy, much like dust along the plane of the Milky Way itself. The dust temperatures tend to trace star forming activity. Spitzer data in blue hues indicate warm dust heated by young stars. Herschel’s instruments contributed the image data shown in red and green, revealing dust emission from cooler and intermediate regions where star formation is just beginning or has stopped. Dominated by dust emission, the Large Magellanic Cloud’s infrared appearance is different from views in optical images. But this galaxy’s well-known Tarantula Nebula still stands out, easily seen here as the brightest region to the left of center. A mere 160,000 light-years distant, the Large Cloud of Magellan is about 30,000 light-years across. (Credits: APOD)

The LMC is seen in amazing detail in this very deep 4 frame mosaic of telescopic images, a view that reveals the Milky Way’s satellite to have the appearance of a fledgling barred spiral galaxy. The mosaic includes image data taken through a narrow filter that transmits only the red light of hydrogen atoms. Ionized by energetic starlight, a hydrogen atom emits the characteristic red H-alpha light as its single electron is recaptured and transitions to lower energy states. As a result, this mosaic seems spattered with pinkish clouds of hydrogen gas surrounding massive, young stars. Sculpted by the strong stellar winds and ultraviolet radiation, the glowing hydrogen clouds are known as H II (ionized hydrogen) regions. Composed of many overlapping clouds, the sprawling Tarantula Nebula left of center, is by far the LMC’s largest star forming region. (Credits: APOD). In other interesting news, Bradley Schaefer of Louisiana State University, an expert in the analysis of archival photographic plates, has uncovered a long-term fading of ~20% in the visual brightness of KIC 8462852 between 1890 and 1989. New Scientist reports:

Known as KIC 8462852, or Tabby’s star, it has been baffling astronomers for the past few months after a team of researchers noticed its light seemed to be dipping in brightness in bizarre ways. Proposed explanations ranged from a cloud of comets to orbiting “alien megastructures”. Now an analysis of historical observations reveals the star has been gradually dimming for over a century, leaving everyone scratching their heads as to the cause. The first signs of this space oddity came from NASA’s planet-hunting Kepler space telescope, which continually watched the star’s region of the sky between 2009 and 2013. Most planet-hosting stars show small, regular dips in light when their planets pass in front of them. But Tabby’s star dipped erratically throughout the four years, sometimes losing as much as 20 per cent of its brightness. In September, a team led by Tabetha Boyajian of Yale University, who lends the star its informal name, tried to make sense of this unusual signal. Ultimately they determined that dust from a large cloud of comets was the best explanation. A month later, the star made headlines across the globe thanks to a paper by Jason Wright of Pennsylvania State University and his colleagues, who suggested that “alien megastructures”, such as satellites designed to collect light from the star, could be responsible for the signal. Now Bradley Schaefer of Louisiana State University has discovered that the mystery goes even further. When Boyajian’s team studied the star, they looked at data from a Harvard University archive of digitally scanned photographic plates of the sky from the past century or so to see if the star had behaved unusually in the past, but found nothing. Schaefer decided this unusual star deserved a second look. He averaged the data in five-year bins to look for slow, long-term trends, and found that the star faded by about 20 per cent between 1890 and 1989. “The basic effect is small and not obvious,” he says. To confirm the fade was real, Schaefer went to Harvard to look at the original photographic plates and inspected them by eye for changes, a skill few astronomers possess these days. “Since no one uses photographic plates any more, it’s basically a lost art,” says Wright. “Schaefer is an expert at this stuff.” Schaefer saw the same century-long dimming in his manual readings, and calculated that it would require 648,000 comets, each 200 kilometres wide, to have passed by the star – completely implausible, he says. “The comet-family idea was reasonably put forth as the best of the proposals, even while acknowledging that they all were a poor lot,” he says. “But now we have a refutation of the idea, and indeed, of all published ideas. This presents some trouble for the comet hypothesis,” says Boyajian. “We need more data through continuous monitoring to figure out what is going on.” What about those alien megastructures? Schafer is unconvinced. “The alien-megastructure idea runs wrong with my new observations,” he says, as he thinks even advanced aliens wouldn’t be able to build something capable of covering a fifth of a star in just a century. What’s more, such an object should radiate light absorbed from the star as heat, but the infrared signal from Tabby’s star appears normal, he says. “I don’t know how the dimming affects the megastructure hypothesis, except that it would seem to exclude a lot of natural explanations, including comets,” says Wright. “It could be that there were just more dimming events in the past, or that astronomers were less lucky in the past and caught more dimming events in the 1980s than in the 1900s. But that seems unlikely.” There’s no doubt KIC 8462852 is behaving strangely, so something must be responsible, says Schaefer. “Either one of our refutations has some hidden loophole, or some theorist needs to come up with some other proposal.”

Iron droplet clouds and hot silicates in the atmosphere of lone planetary mass object PSO J318.5-22

Posted in astronomy with tags , , , on November 2, 2015 by Tim Kendall

ps1-lonely_planet-3x3in300dpiRGBimageOnly Deep multi-colour image from the Pan-STARRS1 telescope of the free-floating planet PSO J318.5-22, in the constellation of Capricornus. The exoplanet, or low mass brown dwarf, is extremely cold and faint, about 100 billion times fainter in optical light than the planet Venus. Most of its energy is emitted at infrared wavelengths, hence the very red colour. The image is 125 arcseconds on a side. An update on this object from New Scientist: the paper by Beth A. Biller et al., (2015) “Variability in a Young, L/T Transition Planetary-Mass Object” is accepted to the Astrophysical Journal Letters [preprint]. From the abstract:

As part of our ongoing NTT SoFI survey for variability in young free-floating planets and low mass brown dwarfs, we detect significant variability in the young, free-floating planetary mass object PSO J318.5-22, likely due to rotational modulation of inhomogeneous cloud cover. A member of the 23±3 Myr β Pic moving group, PSO J318.5-22 has Teff = 1160+3040 K and a mass estimate of 8.3±0.5 MJup for a 23±3 Myr age. PSO J318.5-22 is intermediate in mass between 51 Eri b and β Pic b, the two known exoplanet companions in the β Pic moving group. With variability amplitudes from 7-10% in JS at two separate epochs over 3-5 hour observations, we constrain the rotational period of this object to >5 hours. In KS, we marginally detect a variability trend of up to 3% over a 3 hour observation. This is the first detection of weather on an extrasolar planetary mass object. Among L dwarfs surveyed at high-photometric precision (<3%) this is the highest amplitude variability detection. Given the low surface gravity of this object, the high amplitude preliminarily suggests that such objects may be more variable than their high mass counterparts, although observations of a larger sample is necessary to confirm this. Measuring similar variability for directly imaged planetary companions is possible with instruments such as SPHERE and GPI and will provide important constraints on formation.

New Scientist gives a glimpse as to the hellish nature of the atmosphere of this bizarre object:

The starless planet, PSO J318.5-22, was discovered in the Pan-STARRS survey in 2013. At about eight times the mass of Jupiter, it’s much more like the giant planets we see orbiting other stars than the small, failed stars called brown dwarfs. That means it probably formed around a star and was somehow shot out of its orbit into lonely deep space. That also makes this planet much easier to study than those that are almost lost in the dazzle from the stars they circle. “You have to work really hard to even see them, whereas this object is just by itself,” says Beth Biller at the University of Edinburgh, UK. Biller’s team measured the planet’s brightness and found that it could vary by up to 10 per cent in just a few hours. The explanation, they say, could lie in its weather systems. “If you think about the Great Red Spot on Jupiter, it would be stormy spots like that,” Biller says. Both worlds have similar rotation periods: 10 hours for Jupiter, and between 5 and 10 hours for the lone planet. But unlike Jupiter, which has cooled from a hot start over the long life of our solar system, this planet retains a scorching surface temperature of about 1100 kelvin – maintained by internal heat since it has no star. Those conditions mean that any clouds it has should be molten, containing liquid metals where on Earth we would have water. “These are likely hot silicates and iron droplet clouds,” Biller says. “This makes Venus look like a nice place.” Caroline Morley, who models exoplanet atmospheres at the University of California, Santa Cruz, thinks the finding may mean that similar planets – whether orbiting stars or not – might show the same behaviour. “It strongly suggests that these objects should be variable [in brightness],” Morley says. “We really want to be able to look at this variability and then connect it to storm systems.” Biller’s team is already trying to tease out a similar analysis from observations of a star called HR 8799, which has planets closely resembling this lone world.

VLT/SPHERE imaging of the circumbinary disk and massive exoplanet around HD 106906

Posted in astronomy with tags , , , , on October 28, 2015 by Tim Kendall

Planet-hunting SPHERE Images First Circumbinary Planet System wi

Image and text credit: ESO, A. M. Lagrange (Université Grenoble Alpes) Observations by ESO’s planet-finding instrument, SPHERE, a high-contrast adaptive optics system installed on the third Unit Telescope of ESO’s Very Large Telescope, have revealed the edge-on disc of gas and dust present around the binary star system HD 106906AB.

HD 106906AB is a double star located in the constellation of Crux (The Southern Cross). Astronomers had long suspected that this 13 million-year-old stellar duo was encircled by a debris disc, due to the system’s youth and characteristic radiation. However, this disc had remained unseen — until now. The system’s spectacular debris disc can be seen towards the lower left area of this image. It is surrounding both stars, hence its name of circumbinary disc. The stars themselves are hidden behind a mask which prevent their glare from blinding the instrument.These stars and the disc are also accompanied by an exoplanet, visible in the upper right, named HD 106906 b, which orbits around the binary star and its disc at a distance greater than any other exoplanet discovered to date — 650 times the average Earth–Sun distance, or nearly 97 billion kilometres. HD 106906 b has a mammoth mass of up to 11 times that of Jupiter, and a scorching surface temperature of 1500 degrees Celsius. Thanks to SPHERE, HD 106906AB has become the first binary star system to have both an exoplanet and a debris disc successfully imaged, providing astronomers with a unique opportunity to study the complex process of circumbinary planet formation.

HD_106906_b_imageImage: This is a discovery image of planet HD 106906 b in thermal infrared light from MagAO/Clio2, processed to remove the bright light from its host star, HD 106906 AB. The planet is more than 20 times farther away from its star than Neptune is from our Sun. This is one of the most extreme separations known and it may be more appropriate to consider HD 106906 b a low mass brown dwarf companion. (Image: Vanessa Bailey).

Direct imaging discovery of a companion to 51 Eridani

Posted in astronomy with tags , , on August 14, 2015 by Tim Kendall


Astronomers have found the smallest planet outside this solar system yet to be directly photographed by a telescope on Earth, a methane-shrouded gas giant much like a young Jupiter. The exoplanet 51 Eridani b orbits the ~20 Myr old F0 star 29.4±0.3 parsecs from Earth in a planetary system that may be much like Earth’s own solar system. The discovery could shed light on how our solar system formed, scientists added. Over the last 20 years, astronomers have confirmed the existence of more than 1,800 exoplanets, or alien planets around other stars. Many of these worlds are quite unlike any planets in Earth’s solar system. For example, so-called “hot Jupiters” are gas giants that orbit their host stars more closely than Mercury does the sun.  More than 1,000 of the exoplanets confirmed to date were discovered by NASA’s Kepler Space Telescope. Kepler indirectly finds planets by detecting a loss of starlight as a world passes in front of its star. However, this new planet was discovered using the Gemini Planet Imager, an instrument on the Gemini South telescope in Chile, which directly detects exoplanets by looking for light from the worlds themselves. “To detect planets, Kepler sees their shadow,” study lead author Bruce Macintosh, lead investigator on the Gemini Planet Imager and an astrophysicist at Stanford University in California, said in a statement. “The Gemini Planet Imager instead sees their glow, which we refer to as direct imaging.” The scientists detailed their findings online Aug. 13 in the journal Science. [via] [Update: A step toward rewriting planet formation models]

The exoplanet 51 Pegasi b detected in reflected visible light

Posted in astronomy with tags , , on April 23, 2015 by Tim Kendall

Wide-field view of the sky around the star 51 Pegasi
Image and further reading: ESO Text: A&A

Twenty years ago, the discovery of 51 Peg b heralded the birth of a new field, exoplanetology. However, the Jupiter-like planet that orbits at only 0.05 AU from its parent star does not transit in front of its star. The emphasis therefore shifted toward the study of other hot Jupiters transiting bright stars, for which sizes and spectra can be obtained. Martins et al. take a new look at the planet with the spectrograph HARPS and provide evidence of reflected light from the planet. In order to do so, they cross-correlate the spectrum of the star Doppler-shifted by the motion of the planet on its orbit. This provides two pieces of information: the maximum Doppler shift of the planetary spectral lines is linked to the inclination of the orbit; and the amplitude of the lines compared to those of the star measures the ratio of the planetary to the stellar flux. They find that the orbit is instead seen edge-on with an inclination of 61° to 90°, meaning that the true mass of 51 Peg b is 0.45 to 0.52 MJup. They also find a surprisingly high flux ratio of about 60 ppm (parts per million), which would imply that 51 Peg b has a large visible albedo and a large radius (albedo of 0.5 and radius of 1.9 RJup). While this is within the realm of possibilities, it begs for an independent confirmation of this difficult measurement. But the study does shows that today’s spectrographs have reached a sensitivity and precision that enables the observation of even non-transiting planets, paving the way for characterizing the population of close-in planets in our neighborhood.

The team that made this new detection was led by Jorge Martins from the Instituto de Astrofísica e Ciências do Espaço (IA) and the Universidade do Porto, Portugal, who is currently a PhD student at ESO in Chile. Needless to say, this is the first detection of an exoplanet in reflected light and it is perhaps fitting that the target is the first exoplanet to be discovered by the radial velocity method, as long ago as 1995. The paper is Martins et al., “Evidence for a spectroscopic direct detection of reflected light from 51 Pegasi b”, A&A 576, A134 (2015).

Update: The Automated Planet Finder (APF) plays an important role in the discovery of two new “super-Earth” exoplanets in orbit around the K0.5V star HD 7924 (arXiv pdf)

An exoplanet with an infernal atmosphere

Posted in astronomy with tags , on April 14, 2015 by Tim Kendall

Exoplanet HD189733b courtesy Ron Miller

( – As part of the PlanetS National Centre of Competence in Research (NCCR), astronomers from the Universities of Geneva (UNIGE) and Bern, Switzerland, have come to measure the temperature of the atmosphere of an exoplanet with unequalled precision, by crossing two approaches.

The first approach is based on the HARPS spectrometer and the second consists of a new way of interpreting sodium lines. From these two additional analyses, researchers have been able to conclude that the HD189733b exoplanet is showing infernal atmospheric conditions: wind speeds of more than 1000 kilometres per hour, and the temperature being 3000 degrees. These results open up perspectives to approach the study of exoplanet atmospheres. They were published in two journals, Astronomy & Astrophysics and Astrophysical Journal Letters.

With a temperature of 3000 degrees and such winds blowing at several thousand kilometres per hour, the HD189733b exoplanet’s atmosphere is truly turbulent. Measurements taken by a team of astronomers from the Universities of Geneva and Bern are given in the framework of the PlanetS NCCR; the figures come from observations made over sodium spectral lines. This element is contained in the exoplanet’s atmosphere, and has been measured by the HARPS spectrometer, an instrument designed at the UNIGE Observatory, and installed on a telescope of the European Star Observatory (ESO) in Chile. Image: Ron Miller/NASA.

The HR 8799 system observed by the Gemini Planet Imager

Posted in astronomy with tags , , , on January 7, 2015 by Tim Kendall


GPI imaging of the planetary system HR 8799 in the near-infrared K band, showing 3 of the 4 planets. (Planet b is outside the field of view shown here, off to the left.) These data were obtained on November 17, 2013 during the first week of operation of GPI and in relatively challenging weather conditions, but with GPI’s advanced adaptive optics system and coronagraph the planets can still be clearly seen and their spectra measured (below). Credit: Christian Marois (NRC Canada), Patrick Ingraham (Stanford University) and the GPI Team.

Stunning exoplanet images and spectra from the first year of science operations with the Gemini Planet Imager (GPI) were featured today in a press conference at the 225th meeting of the American Astronomical Society (AAS) in Seattle, Washington. The Gemini Planet Imager GPI is an advanced instrument designed to observe the environments close to bright stars to detect and study Jupiter-like exoplanets (planets around other stars) and see protostellar material (disk, rings) that might be lurking next to the star.

Marshall Perrin (Space Telescope Science Institute), one of the instrument’s team leaders, presented a pair of recent and promising results at the press conference. He revealed some of the most detailed images and spectra ever of the multiple planet system HR 8799. His presentation also included never-seen details in the dusty ring of the young star HR 4796A. “GPI’s advanced imaging capabilities have delivered exquisite images and data,” said Perrin. “These improved views are helping us piece together what’s going on around these stars, yet also posing many new questions.”

The GPI spectra obtained for two of the planetary members of the HR 8799 system presents a challenge for astronomers. GPI team member Patrick Ingraham (Stanford University), lead the paper on HR 8799. Ingraham reports that the shape of the spectra for the two planets differ more profoundly than expected based on their similar colors, indicating significant differences between the companions. “Current atmospheric models of exoplanets cannot fully explain the subtle differences in color that GPI has revealed. We infer that it may be differences in the coverage of the clouds or their composition.” Ingraham adds, “The fact that GPI was able to extract new knowledge from these planets on the first commissioning run in such a short amount of time, and in conditions that it was not even designed to work, is a real testament to how revolutionary GPI will be to the field of exoplanets.”


GPI spectroscopy of planets c and d in the HR 8799 system. While earlier work showed that the planets have similar overall brightness and colors, these newly-measured spectra show surprisingly large differences. The spectrum of planet d increases smoothly from 1.9-2.2 microns while planet c’s spectrum shows a sharper kink upwards just beyond 2 microns. These new GPI results indicate that these similar-mass and equal-age planets nonetheless have significant differences in atmospheric properties, for instance more open spaces between patchy cloud cover on planet c versus uniform cloud cover on planet d, or perhaps differences in atmospheric chemistry. These data are helping refine and improve a new generation of atmospheric models to explain these effects. Credit: Patrick Ingraham (Stanford University), Mark Marley (NASA Ames), Didier Saumon (Los Alamos National Laboratory) and the GPI Team. Materials courtesy

The mass of Kepler-93b and the composition of terrestrial-type exoplanets

Posted in astronomy with tags , , , , on January 2, 2015 by Tim Kendall

kepler-planet-candidatesMore than three-quarters of the planet candidates discovered by NASA’s Kepler spacecraft have sizes ranging from that of Earth to that of Neptune, which is nearly four times as big as Earth. Such planets dominate the galactic census but are not represented in our own solar system. Image credit: NASA Ames

In a new preprint today Dressing et al. have measured a high density for this planet using archival radial velocity measurements to derive a precise mass. The paper is entitled “The Mass of Kepler-93b and The Composition of Terrestrial Planets” has been accepted to the Astrophysical Journal. Overall the Kepler data seem to suggest a rough cutoff around six Earth masses, above which much lower densities are derived, suggesting the presence of extensive hydrogen and helium envelopes. Above this approximate mass high density iron/silicate terrestrial-type planets appear rare. From the abstract:

Kepler-93b is a 1.478 +/- 0.019 Earth radius planet with a 4.7 day period around a bright (V=10.2), astroseismically-characterized host star with a mass of 0.911+/-0.033 solar masses and a radius of 0.919+/-0.011 solar radii. Based on 86 radial velocity observations obtained with the HARPS-N spectrograph on the Telescopio Nazionale Galileo and 32 archival Keck/HIRES observations, we present a precise mass estimate of 4.02+/-0.68 Earth masses. The corresponding high density of 6.88+/-1.18 g/cc is consistent with a rocky composition of primarily iron and magnesium silicate. We compare Kepler-93b to other dense planets with well-constrained parameters and find that between 1-6 Earth masses, all dense planets including the Earth and Venus are well-described by the same fixed ratio of iron to magnesium silicate. There are as of yet no examples of such planets with masses > 6 Earth masses: All known planets in this mass regime have lower densities requiring significant fractions of volatiles or H/He gas. We also constrain the mass and period of the outer companion in the Kepler-93 system from the long-term radial velocity trend and archival adaptive optics images. As the sample of dense planets with well-constrained masses and radii continues to grow, we will be able to test whether the fixed compositional model found for the seven dense planets considered in this paper extends to the full population of 1-6 Earth mass planets.

The possibility of binary terrestrial-type exoplanets

Posted in astronomy with tags , on December 3, 2014 by Tim Kendall

The possible existence of Earth-like binary planets is being described today at the American Astronomical Society’s Division for Planetary Sciences meeting in Tucson, AZ. Two bodies, each of mass similar to Earth, can form a closely orbiting pair under certain conditions present during the formation of planetary systems. (

There is a good reason to believe terrestrial binary planetary systems may be possible. In a grazing collision if the angular momentum is too high to be contained within a single rotating body it would fission and if the bodies barely touch then they could retain their identity. However, [the intermediate scenario in which a binary planet might form] requires an encounter where the bodies are initially approaching each other at low enough velocity.

To test for this possibility, a simulation technique called Smoothed Particle Hydrodynamics (SPH) was utilized. Smoothed Particle Hydrodynamics represents a body as a collection of tens of thousands of particles, and it has been used to study protoplanetary collisions as well as the giant impact hypothesis of the Moon’s formation.

Using SPH, collisions between two rocky Earth-sized bodies were simulated, with impact velocity and impact parameter (a measure of how head-on a collision is) being varied and the output observed. In the cases where the bodies underwent substantial collision, the scientists replicated previous results in which a did not arise but a moon might form. However, by including interactions where the bodies are close enough to undergo a large tidal distortion, initial conditions were found that led to a terrestrial binary planetary system.

Ancient planets around Kapteyn’s star

Posted in astronomy with tags , , on June 16, 2014 by Tim Kendall

Image source: Planetary Habitability Laboratory; modified July 8, 2014

Typical planetary systems detected by NASA’s Kepler mission are hundreds of light-years away. In contrast, Kapteyn’s star is the 25th nearest star to the sun and it is only 13 light years away from Earth. What makes this discovery different however, is the peculiar story of the star. Kapteyn’s star was born in a dwarf galaxy absorbed and disrupted by the early Milky Way. Such galactic disruption event put the star in its fast halo orbit. The likely remnant core of the original dwarf galaxy is omega Centauri, an enigmatic globular cluster 16, 000 light years from earth which contains hundreds of thousands of similarly old suns. This sets the most likely age of the planets at 11.5 billion years; which is 2.5 times older than Earth and ‘only’ 2 billion years younger than the universe itself (around 13.7 billion years). Dr Anglada-Escude adds: “It does make you wonder what kind of life could have evolved on those planets over such a long time.”

Professor Richard Nelson, leader of the Astronomy Unit at QMUL, who didn’t participate in the research, commented: “This discovery is very exciting. It suggests that many potentially habitable worlds will be found in the next years around nearby stars by ground-based and space-based observatories such as ESA’s PLATO mission. Until we have detected a larger number of them, the properties and possible habitability of the near-most planetary systems will remain mysterious.”

Text taken from the QMUL press release which also contains a short fiction by Alistair Reynolds. The star itself has long been studied and its age and kinematic history would suggest planets were unlikely to exist there. The discovery of planets at Kapteyn’s star gives us the vital insight that planet formation was occurring very early in the history of our Galaxy.

Lick Observatory’s Automated Planet Finder: first robotic telescope for exoplanets

Posted in astronomy with tags , , , on May 15, 2014 by Tim Kendall


Unlike Kepler, however, which focused on distant stars in one small patch of sky, the APF focuses on nearby stars and covers the entire sky. “The planetary systems we’re finding are our nearest neighbors. Those are the ones that will matter to future generations,” said Steve Vogt, professor of astronomy and astrophysics at UC Santa Cruz, who led the $12 million APF project and designed the Levy spectrometer at the heart of the system.

This new facility will lead the way to a census of exoplanets around nearby stars. Links to the first papers are given in this University of California/Lick Observatory press release.

“Necklace” planetary nebula PN G054.2-03.4 in Sagitta

Posted in astronomy with tags , , on March 1, 2014 by Tim Kendall

Credit: NASA, ESA, and the Hubble Heritage Team (STScI/AURA)

The “Necklace Nebula,” also called PN G054.2-03.4, is the exploded aftermath of a giant star that came too close to its Sun-like binary companion. The two stars that produced the Necklace Nebula live in a relatively small orbit about each other. They have a period of 1.2 days and a separation on the order of 5 times the radius of the Sun. Evidence for the existence of the two-body system arises from the nebula’s appearance of a half-light-year-wide equatorial ring of dense material near the inner portion of the nebula. The expanding elliptical ring is composed of bright, dense knots of glowing hydrogen and oxygen gas. Each knot also dons a small tail pointing away from the central star. The clumpy appearance of the ring may have been caused by density fluctuations in the shared material of the binary stars prior to the explosion, or possibly by magnetic field lines present in the giant star as it began to expand and shed off its outer layers.

A fast, collimated outflow of nitrogen gas from the binary system has formed faint lobes and polar caps extending in the direction perpendicular to the ring. Edge to edge, the nebula is nearly 9 light-years long, over twice the distance between our Sun and our nearest stellar companion, Proxima Centauri. Astronomers studying PN G054.2-03.4 predict that the outer lobes of gas were ejected about 10,000 years ago, before the two stars began sharing material. The inner ring of material was created only about 5,000 years ago (and relatively recent on astronomical timescales), and shares the same plane as the orbit of the two stars. The Necklace Nebula is located about 15,000 light-years away in the northern constellation Sagitta. It was recently discovered in 2005 from the Isaac Newton Telescope Photometric H-alpha Survey (IPHAS), a ground-based H-alpha planetary nebula study of the North Galactic Plane.

Update: The first ground-based imaging detection of an exoplanet in the optical (wavelength less than one micron) is β Pic b reported today using the Magellan and Gemini telescopes. Another new study has investigated low mass stellar members of the 40 Myr Tucana-Horologium moving group.

The Omega nebula M17 from the VLT survey telescope

Posted in astronomy with tags , on August 7, 2013 by Tim Kendall

Image credits: ESO/Phil Plait/Discover magazine
Rounding up some of the latest exoplanet and brown dwarf news from the literature this week: firstly I noted the direct imaging discovery of the Jovian analogue GJ 504b which has a projected (on the plane of the sky) separation from its parent (G0V) star of 43.5 AU, causing some problems for the core accretion scenario of planet formation, as explained by Paul Glister over at Centauri Dreams, here.

Direct imaging and cancellation of starlight works well to discover planets in rather distant orbits: microlensing, on the other hand, is a useful method to find closer in planets, although the method is limited by the requirement of close on the sky alignment between the foreground system with planet and a background target star. Recently a 2 Jupiter mass planet has been found only 0.87 AU from an old, field brown dwarf. Neptune mass planets have been found by the method. Microlensing resources can be found here, here and here. From the first linked paper of these:

Gravitational microlensing occurs when a foreground star happens to pass very close to our line of sight to a more distant background star. The foreground star acts as a lens, splitting the light from the background source star into two images, which are typically unresolved. However, these images of the source are also magnified, by an amount that depends on the angular separation between the lens and source. The relative motion between the lens and source therefore results in a time-variable magnification of the source: a microlensing event. If the foreground star happens to host a planet with projected separation near the paths of these images, the planet will also act as a lens, further perturbing the images and resulting in a characteristic, short-lived signature of the planet.

Elsewhere, in a paper accepted to MNRAS, Pinfield et al. have identified two late T dwarfs from WISE which have kinematics (space motions) consistent with them being old objects belonging to the galactic thick disk or even halo population. Schneider et al. inform us of a study of HD 166191, a young F8 star, which shows how WISE data can be used to study infrared excesses above the stellar photosphere, indicating circumstellar and likely planet-forming material. This warm dust disk (~ 150K) is a rarity, hinting perhaps at a phase of ongoing collisional planet-building. Lastly, and on a cosmological note, a gamma ray burst has been found at redshift z ~ 5.9. The burst, one of the most distant yet seen, marks the death of a massive star from a time when the Universe was only enriched in metals – elements heavier than hydrogen and helium forged in stellar interiors – to one tenth of the present day value.

Update: astrometry is the direct positional measurement of the period of a secondary, or companion, from which masses and semi-major axes can be deduced using various Keplerian methodologies. Like microlensing, astrometry is also sensitive to planets at small to intermediate separations, especially for low-mass primaries, as shown by this recent discovery around another field brown dwarf, in a 246 day orbit.

Rarity of gas giant planets orbiting far from their host stars

Posted in astronomy with tags on July 7, 2013 by Tim Kendall


The first of a series of papers detailing the achievements of the Gemini/NICI direct imaging planet-finding campaign has revealed a paucity of giant planets in distant (greater than about 10 astronomical units) orbits around their host stars. The team surveyed a large number of A and B stars, finding that analogues to the HR 8799 system, whose planets lie very far from the host star, are rarities. The campaign would have been able to image many planets on orbits larger than that of Neptune around the Sun if they existed but these expected planets were not observed. While it seems giant planets do exist preferentially close to parent stars, rocky planets may also follow the same trend: the inset shows an impression of the magma exoplanet UCF 1.01, which is perhaps itself similar to alpha Centauri Bb. Tiny transiting systems around M dwarfs and solar-type stars have been found by Kepler, while these new results show that truly huge planetary systems are unusual among a sample of local nearby stars more massive than the Sun. On the gas giant findings:

Eric Nielsen of the University of Hawaii, who leads a new paper about the (NICI) campaign’s search for planets around stars more massive than the Sun, adds that the findings have implications beyond the specific stars imaged by the team. “The two largest planets in our Solar System, Jupiter and Saturn, are huddled close to our Sun, within 10 times the distance between the Earth and Sun,” he points out. “We found that this lack of gas-giant planets in more distant orbits is typical for nearby stars over a wide range of masses.”

Two additional papers from the campaign will be published soon and reveal similar tendencies around other classes of stars. However, not all gas-giant exoplanets snuggle so close to home. In 2008, astronomers using the Gemini North telescope and W.M. Keck Observatory on Hawaii’s Mauna Kea took the first-ever direct images of a family of planets around the star HR 8799, finding gas-giant planets at large orbital separations (about 25-70 times the Earth-Sun distance). This discovery came after examining only a few stars, suggesting such large-separation gas giants could be common. The latest Gemini results, from a much more extensive imaging search, show that gas-giant planets at such distances are in fact uncommon.


Liu sums up the situation this way: “We’ve known for nearly 20 years that gas-giant planets exist around other stars, at least orbiting close-in. Thanks to leaps in direct imaging methods, we can now learn how far away planets can typically reside. The answer is that they usually avoid significant areas of real estate around their host stars. The early findings, like HR 8799, probably skewed our perceptions.”

The paper is Eric Nielsen et al., “The Gemini NICI Planet-Finding Campaign: The Frequency of Giant Planets around Young B and A Stars”, in press at ApJ (arXiv preprint).

Images: top, (image) below, Zeit News (

Mass and radius of the exoplanet HD 97658b

Posted in astronomy with tags on July 2, 2013 by Tim Kendall

A planet more massive than Earth orbits a star less massive than the Sun. Image: NASA Art

HD 97658 is a nearby (21 pc) K1V dwarf whose planet was detected in 2011 by radial velocity measurements. The minimum mass msini is eight Earth masses. Early transit confirmation measurements suggested a low density of 1.4 grams per cubic centimeter, making the planet likely similar in composition to Neptune, with a hydrogen and helium envelope. The new observations give a higher density and suggest a rocky core with a gaseous envelope of lighter elements:

“Measuring an exoplanet’s size and mass leads to a determination of its density, which in turn allows astronomers to say something about its composition,” Dragomir said. “Measuring the properties of super-Earths in particular tells us whether they are mainly rocky, water-rich, mini gas giants, or something entirely different.”

The average density of HD 97658b is about four grams per cubic centimeter, a third of the density of lead but denser than most rocks. Astronomers see great significance in that value – about 70 percent of the average density of Earth – since the surface gravity of HD 97658b could hold onto a thick atmosphere. But there’s unlikely to be alien life breathing those gases. The planet orbits its sun every 9.5 days, at a distance a dozen times closer than we are from our Sun.

The new data are from the Canadian MOST micro-satellite telescope, launched exactly 10 years ago. The paper is Diana Dragomir, et al., “MOST Detects Transits of HD 97658b, a Warm, Likely Volatile-rich Super-Earth,” 2013, ApJ, 772, L2; doi:10.1088/2041-8205/772/1/L2.

Direct imaging discovery of a planet around a young A8 star

Posted in astronomy with tags , , , , , , on June 3, 2013 by Tim Kendall

Image credit: ESO/Digitized Sky Survey 2. Acknowledgement: Davide De Martin
eso1324aA new, 4 – 5 Jupiter mass probable planet discovery has been reported today around the young (10 – 17 Myr) dusty A8 star HD 95086. The companion is detected in the L’ band at 3.8 microns (left, image courtesy ESO/J. Rameau) using the NACO adaptive optics instrument at the ESO VLT. The best detection has a signal-to-noise ratio of 9. The projected separation is 56 AU and the planet is still warm, with surface temperature is around 1000K. The model-dependent mass obtained means this companion is the lightest planet yet directly imaged. The star lies close to the galactic plane as can easily be seen in the main image of the field around HD 95086, but contamination of the detection by a background object is unlikely, and the planetary status of the companion is reinforced by a non-detection in the Ks band (2.18 microns). The astrometry confirms it is co-moving with the star. The paper is “Discovery of a probable 4-5 Jupiter-mass exoplanet to HD 95086 by direct-imaging”, J. Rameau et al., arXiv preprint [pdf], and is accepted to ApJ letters.

In another new development, NASA have shown that the closest red dwarf, Proxima Centauri, will pass close to two background stars in alignments in 2014 and again in 2016 (below) which will allow possible planets around Proxima to be detected by microlensing, as well as yielding an accurate mass measurement for the star itself:

Microlensing occurs when a foreground star passes close to our line of sight to a more distant background star. These images of the background star may be distorted, brightened and multiplied depending on the alignment between the foreground lens and the background source. These microlensing events, ranging from a few hours to a few days in duration, will enable astronomers to measure precisely the mass of this isolated red dwarf. Getting a precise determination of mass is critical to understanding a star’s temperature, diameter, intrinsic brightness, and longevity. Astronomers will measure the mass by examining images of each of the background stars to see how far the stars are offset from their real positions in the sky. The offsets are the result of Proxima Centauri’s gravitational field warping space. The degree of offset can be used to measure Proxima Centauri’s mass. The greater the offset, the greater the mass of Proxima Centauri. If the red dwarf has any planets, their gravitational fields will produce a second small position shift.

Image credit: NASA, ESA, K. Sahu and J. Anderson (STScI), H. Bond (STScI and Pennsylvania State University), M. Dominik (University of St. Andrews), and Digitized Sky Survey (STScI/AURA/UKSTU/AAO). The scalloped appearance of the path of the star is due to the Earth’s orbital motion.

There is more to read on highly accurate astrometry and microlensing, as well as direct imaging, in a new article from Space Review on the specifications and exoplanet detection capabilities of WFIRST-2.4, while for those who prefer more conceptual reading, there is another article about loop quantum gravity and the resolution of black hole singularities. The paper under discussion is Gambini & Pullin, “Loop quantisation of the Schwarzschild black hole”, in Phys. Rev. Lett., (arXiv), the essential idea being that LQG can provide a mathematical description applicable to both black holes and the early Universe which is able to remove (resolve) the problematic singularity introduced by general relativity in both cases.