New Earth-resembling exoplanets from Kepler

Image courtesy: NASA

Links to the papers detailing the new finds in the Kepler-62 and Kepler-69 systems are here: Borucki et al., “Kepler-62: A Five-Planet System with Planets of 1.4 and 1.6 Earth Radii in the Habitable Zone” Science DOI: 10.1126/science.1234702. Barclay et al., “A super-Earth-sized planet orbiting in or near the habitable zone around Sun-like star”, accepted to ApJ, arXiv preprint. Also see Kaltenegger, Sasselov & Rugheimer “Water Planets in the Habitable Zone: Atmospheric Chemistry, Observable Features, and the case of Kepler-62e and -62f” which is in review for ApJ Letters, arXiv preprint.

It is important to remember that although there are complex statistical arguments in favour of the transit detections being real, and not the result of false positives, an independent experiment to confirm the planet(s) is not possible. From the Barclay et al. paper concerning Kepler-69:

Since planets of this size and period cannot currently have their masses measured (by, for instance, radial velocity measurements) and their planetary nature thus confirmed, we rely on statistical analysis of the likelihood that KOI-172.01 and KOI-172.02 are planets for validation of their planetary status.

False positives ruled out with 99% confidence include a grazing eclipse by a stellar companion, a companion eclipsing binary actually orbiting Kepler-69, and background eclipsing binaries.


In contrast, the prototype of this sought-after class of planet, Kepler-22b (left), also discovered by Borucki, has been confirmed by Keck radial velocity measurements. The planet has 2.4 Earth radii and a radiative equilibrium temperature of 262K, and is the first known planet with measured radius found to orbit in the HZ of a sun-like star. However, we do not know for certain if it is rocky, since the RV measurements do not well constrain the mass.


In other respects, the new discoveries are also better bets for habitability. They have smaller radii, and more likely to have an Earth-like surface gravity. Their probable composition be seen from the plot at right, which uses mass-radius relations calculated for the given compositions to compare various Kepler and other planets, not all of which lie in the HZ. Low density planets such as Kepler-11f lie well to the top of the plot, and probably have hydrogen-helium envelopes. Planets with very high density such as the enigma 55 Cnc e lie towards the bottom of the plot; 55 Cnc e itself has a density over twice that of the bulk Earth and must be composed primarily of metals. Kepler-68b has a density intermediate between the ice giants and Earth, rather like GJ 1214b, which of course orbits an M dwarf. Compositions very rich in water are possible for these planets, as is well-known. Pioneering work on the mass-radius relation for solid exoplanets was carried out by Sara Seager and collaborators in 2007.
Kepler 62, 69 Super-Earths - NASA image

Observationally, progress is being made towards combining information gained from transits (the planetary radius) with mass, at least the minimum mass msini (from radial velocity) so as to populate the plot above with data. A new example of this has been provided today as two other Kepler objects of interest (KOI) have been confirmed by Hebrard et al. (arXiv preprint). One planet is very massive, the other constrained to 0.68 and 1.32 Jupiter mass and radius respectively, with errors only ~ 10%. Multi-epoch observations can then be used to obtain a full orbital solution (i.e. exact mass, with knowledge of the inclination). Limiting this discussion to solar-type stars, this has so far only been achieved for the case of one brown dwarf, HR 7672B. In this way, it can be seen that while we are some way off from the goal of a true Earth analogue, the nature of the observations required to discover such a body are well understood.

Image courtesy: Planetary Habitability Laboratory

Update: The Borucki et al. arXiv preprint is now available. Outside of the habitable zone, two sub-Neptune mass planets have also been found in a 7:5 orbital resonance around the mildly metal-poor G2V star HD 41248.

Update: A remarkable Earth radius planet in an 8.5 hour orbit. A direct derivation of the mass may be possible with further Kepler data and spectroscopy.


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