Bigger planets have bigger atmospheres

Exoplanets enveloped in clouds

Scientists used the Hubble Space Telescope to characterize the atmospheres of two exoplanets in the Milky Way. The planets GJ 436b and GJ 1214b, 36 and 40 light-years away from Earth, whose atmospheres have not yet been precisely analyzed, are apparently covered by clouds, as the researchers report in two publications in the journal Nature.

Both planets are in the middle mass range - between smaller rock planets like Earth on the one hand and larger gas giants like Jupiter on the other - and thus belong to the most common type of planets within the Milky Way. GJ 436b is categorized as "warm Neptune" because it is much closer to its star than cold Neptune is to the sun. GJ 1214b, on the other hand, is referred to as "super earth" due to its size. Both planets pass in front of their stars in such a way that the starlight filtered through their atmospheres allows detailed investigations of these atmospheres.

Size comparison

The spectral images recorded with the Hubble telescope show no chemical fingerprints or other abnormalities in the atmosphere of GJ 436b. "Either this planet has a high layer of clouds that block the view, or it has a cloudless atmosphere that is free of hydrogen, which would make it very unlike Neptune," said Heather Knutson of the California Institute of Technology in Pasadena. "Instead of hydrogen, it could be rich in heavier molecules - like water vapor, carbon monoxide and carbon dioxide - that would compress the atmosphere and make it difficult for us to detect chemical signatures."

Similar observations of the planet GJ 1214b had earlier revealed an equally unreliable spectrum. However, there was evidence that its atmosphere is dominated by water vapor or hydrogen and by dense clouds at high altitudes. With the help of Hubble, the scientists led by Laura Kreidberg from the University of Chicago were able to take a closer look at GJ 1214b and found further evidence of clouds that envelop the planet and thus make information about the composition and behavior of the lower atmosphere and the surface inaccessible. The spectrum that has now been recorded shows no chemical fingerprints either, but with such an accuracy that the astronomers can rule out cloud-free compositions of water vapor, methane, nitrogen, carbon monoxide or carbon dioxide for the first time.

In any case, the clouds on the two exoplanets are unlikely to resemble those that we can see in our solar system. Possible candidates include dust clouds from potassium chloride or zinc sulfide. “Both planets tell us about the variety of types of planets that exist outside of our own solar system. In this case, we see that we may not know them as well as we thought, ”says Knutson.