Scientists using NASA’s Hubble Space Telescope have characterized the atmospheres of two nearby exoplanets: the super-Earth GJ 1214b and the Neptune-sized exoplanet GJ 436b.

GJ 436b, or Gliese 436b, orbits a red-dwarf star located in the constellation Leo, about 36 light-years away. Discovered in August 2004, the planet is categorized as a hot Neptune because it is much closer to its star than frigid Neptune is to the Sun.

GJ 1214b is located in the constellation Ophiuchus, some 40 light-years from us. Discovered in 2009, the planet is about 2.7 times Earth’s diameter and is almost 7 times as massive. This super-Earth, also known as Gliese 1214b, orbits a red-dwarf star every 38 hours at a distance of 1.3 million miles, giving it an estimated temperature of 450 degrees Fahrenheit.

Despite numerous studies, the nature of the atmospheres surrounding GJ 436b and GJ 1214b had eluded definitive characterization until now.

The new atmospheric study of GJ 436b, published in the journal Nature, is based on transit observations with Hubble over 2013. The spectra were featureless and revealed no chemical fingerprints whatsoever in the planet’s atmosphere.

“Either this planet has a high cloud layer obscuring the view, or it has a cloud-free atmosphere that is deficient in hydrogen, which would make it very unlike Neptune. Instead of hydrogen, it could have relatively large amounts of heavier molecules such as water vapor, carbon monoxide, and carbon dioxide, which would compress the atmosphere and make it hard for us to detect any chemical signatures,” said study lead author Dr Heather Knutson of the California Institute of Technology in Pasadena.

Previous studies of GJ 1214b yielded two possible interpretations of the planet’s atmosphere. It could consist entirely of water vapor or some other type of heavy molecule, or it could contain high-altitude clouds that prevent the observation of what lies underneath.

But now the astronomers have detected clear evidence of clouds in the atmosphere of GJ 1214b. The Hubble observations also revealed no chemical fingerprints in the planet’s atmosphere, but the data were so precise they could rule out cloud-free compositions of water vapor, methane, nitrogen, carbon monoxide, or carbon dioxide.

“We really pushed the limits of what is possible with Hubble to make this measurement. This advance lays the foundation for characterizing other Earths with similar techniques,” said Laura Kreidberg of the University of Chicago, who is the lead author of a paper on GJ 1214b also appearing in the journal Nature.

“I think it’s very exciting that we can use a telescope like Hubble that was never designed with this in mind, do these kinds of observations with such exquisite precision, and really nail down some property of a small planet orbiting a distant star,” added study co-author Dr Jacob Bean, also from the University of Chicago.

Models of GJ 436b and GJ 1214b predict clouds that could be made out of potassium chloride or zinc sulfide at the scorching temperatures of several hundred degrees Fahrenheit predicted to be found in these atmospheres.

Dr Knutson said: “both planets are telling us something about the diversity of planet types that occur outside of our own Solar System. In this case we are discovering we may not know them as well as we thought.”

“We’d really like to determine the size at which these planets transition from looking like mini-gas giants to something more like a water world or a rocky, scaled-up version of the Earth. Both of these observations are fundamentally trying to answer that question.”

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Knutson HA et al. 2014. A featureless transmission spectrum for the Neptune-mass exoplanet GJ 436b. Nature 505, 66–68; doi: 10.1038/nature12887

Kreidberg L et al. 2014. Clouds in the atmosphere of the super-Earth exoplanet GJ 1214b. Nature 505, 69–72; doi: 10.1038/nature12888