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Hubble detects high clouds surrounding alien worlds

Paul Sutherland, Feature writer
Jan 2, 2014, 8:00 UTC

Sen—Astronomers’ abilities to learn more about the ever-increasing numbers of alien planets around other stars continues to impress. The latest result, achieved using the Hubble Space Telescope, has been to observe clouds in the atmospheres of exoplanets dubbed “super-Earths”.

Even the powerful orbiting eye cannot actually image these clouds in the way that artistic representations suggest. But the information is teased out of the starlight collected by Hubble to provide the clear evidence of high clouds blanketing the planets.

The worlds where this weather has been detected are both relatively close on the cosmic scale of things. Unromantically labelled, they are GJ 436b, which lies 36 light-years from Earth in the constellation of Leo, and GJ 1214b, 40 light-years away in the constellation Ophiuchus.

Previous efforts to analyse these planets’ atmospheres have been unsuccessful. But by using a spectroscope on Hubble, the light from their parent stars was broken up into its separate chemical fingerprints. Because both planets pass in front of their stars as viewed from Earth, this technique allowed their atmospheres to be examined too as the starlight passed through them on it way towards Hubble.

It is an important breakthrough because it points to the possibility in future of pinpointing which rocky, Earth-like worlds in habitable zones of their star systems might have suitable atmospheric conditions for life as we know it to exist.

The two planets examined by Hubble are midway in mass and size between the Earth and gas giants like Jupiter. GJ 436b is described as a “warm Neptune” because it is much closer to its star than our own icy Neptune is to the Sun. GJ 1214b is considered to be a “super-Earth” because of its size.

Separate papers about Hubble’s observations are published this week in the journal Nature.

The atmospheric study of GJ 436b was led by Heather Knutson of the California Institute of Technology in Pasadena, California. In this case, the data gathered by Hubble’s spectrometer was unable to identify the actual chemistry of the exoplanet’s atmosphere.


An illustration compares the sizes of exoplanets GJ 436b and GJ 1214b with Earth and Neptune. Credit: NASA, ESA, and A. Feild and G. Bacon (STScI)

Knutson said: “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 vapour, carbon monoxide, and carbon dioxide, which would compress the atmosphere and make it hard for us to detect any chemical signatures.”

Similar spectra previously obtained for the other planet, GJ 1214b, was also devoid of features but indicated that its atmosphere was dominated by water vapour or hydrogen, with high-altitude clouds.

A team led by Laura Kreidberg and Jacob Bean of the University of Chicago took a closer look at GJ 1214b with Hubble and found what they believe to be firm evidence of high clouds blanketing the planet and hiding information about the make-up and behaviour of the lower atmosphere and surface.

University of Chicago graduate student Laura Kreidberg describes her team's findings about GJ 1214b

Though the new Hubble spectra also revealed no chemical fingerprints in GJ 1214b’s atmosphere, the data were so precise that the astronomers were able, for the first time, to rule out cloud-free compositions of water vapour, methane, nitrogen, carbon monoxide, or carbon dioxide.

Knutson commented: “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.”