An outline of the paper: Mass, Radii, and Cloud Properties of the HR 8799 Planets. by Marley, Saumon, Cushing, et al.
The planets of HR 8799 strikingly depart from the brown dwarf sequence in colours.
The blueward turn of J-K colours in brown dwarfs marking the transition between the L and T spectroscopic types, is thought to be due to the gradual disappearance of dust clouds. As the clouds cover vanish, the strong effect of water vapour bands reappear and push that infrared colour towards the blue.
Because the planets keep getting redder as they get fainter, Currie et al. talk about “radically enhanced” clouds, whereas Madhusudhan et al. invoke a “new class of atmospheres” for these planets.
What Marley et al. argue in this paper though, is that the atmospheres of the three planets are not that surprising. On the contrary, they make rather good sense in the context of what is known about brown dwarfs.
The catch is in the main difference between planet and brown dwarf atmosphere: gravity. The gravity in giant planet atmosphere is ten to a hundred times smaller than on a brown dwarf. At the same temperature, a lower gravity will strongly affect the cloud regime. The authors consider the different effects in detail.
The first effect of gravity is “textbook” atmospheric physics. At the same pressure, a lower gravity implies a lot more matter in the atmosphere. For instance, the pressure on Titan is 1.5 bars (=1 Earth atmosphere), but the amount of matter overhead is ten times higher – because more mass is needed to create a given pressure in lower gravity. Since the opacity of things like dust is proportional to the amount of matter rather than to pressure, other things being equal, a lower gravity will imply correspondingly thicker clouds.
For this reason, the clearing of clouds can be expected at lower temperature for lower-gravity object, just like the planets around HR 8799 b seem to suggest.
Marley et al. also consider other effects of the different gravity on clouds. The size and position of grain in the clouds of planets and brown dwarfs is set – at least in the models – by a competition between the formation of grains, as warm parts of the atmosphere are dredged up by mixing motions in the atmosphere, and their destruction, as they rain down under their own weight. A lower gravity will directly affect the speed at which grains fall down. It will also modify the mixing of the atmosphere.
According to Marley et al., a reduced gravity will make grain settling slower and mixing easier, thus favouring larger grains higher up in the atmosphere. Both effects therefore go in the same direction: that of favouring the survival of clouds in the upper atmosphere at lower temperature than brown dwarfs.
This will be easy to test in the future: planets slightly cooler than HR 8799 b (around 1000 K) should start showing signs of lifting cloud deck.
Feature Image: Clouds sketch by Roberts Loan (robertsloan2.hubpages.com)
Marois et al. (2008)
Currie et al. (2011)
Madhusudhan et al. (2012)