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

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.


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