Cloudy weather on an alien world

WEATHER FORECASTERS on exoplanet GJ 1214b would have an easy job. Today’s forecast: cloudy. Tomorrow: overcast. Extended outlook: more clouds.

The planet, which is known as GJ 1214b, is classified as a super-Earth because its mass is intermediate between that of Earth and Neptune. Recent searches for planets orbiting other stars (‘exoplanets‘) have shown that super-Earths like GJ 1214b are among the most common type of planets in the Milky Way galaxy. Because no such planets exist in our Solar System, the physical nature of super-Earths is largely unknown.

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

Artist's view of exoplanet GJ 1214b

An artist’s view of exoplanet GJ 1214b. The weather forecast is for cloudy skies.

But now a team of astronomers led by Laura Kreidberg and Jacob Bean of the Department of Astronomy and Astrophysics at the University of Chicago, have detected clear evidence of clouds in the atmosphere of GJ 1214b from data collected with the Hubble Space Telescope. The Hubble observations used 96 hours of telescope time spread over 11 months. This was the largest Hubble programme ever devoted to studying a single exoplanet.

The researchers describe their work as an important milestone on the road to identifying potentially habitable, Earth-like planets beyond our Solar System. The results appear in the January 2 issue of the journal Nature.

Pushing the limits

“We really pushed the limits of what is possible with Hubble to make this measurement,” said Kreidberg, a third-year graduate student and first author of the new paper. “This advance lays the foundation for characterising other Earths with similar techniques.”

“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,” explained Bean, an assistant professor and the project’s principal investigator.

GJ 1214b is located just 40 light-years from Earth, in the direction of the constellation Ophiuchus. Because of its proximity to our Solar System and the small size of its host star, GJ 1214b is the most easily observed of the known super-Earths. It transits, or passes in front of its parent star, every 38 hours, giving scientists an opportunity to study its atmosphere as starlight filters through it.

Kreidberg, Bean and their colleagues used Hubble to precisely measure the spectrum of GJ 1214b in near-infrared light, finding what they consider definitive evidence of high clouds blanketing the planet. These clouds hide any information about the composition and behaviour of the lower atmosphere and surface.

Four planets in a row

An artist’s rendering comparing the size of GJ 1214b, another, larger exoplanet, and Earth and Neptune.

Unearthly weather

The planet was discovered in 2009 by the MEarth Project, which monitors 2,000 red dwarf stars for transiting planets. The planet was next targeted for follow-up observations to characterise its atmosphere. The first spectra, which were obtained by Bean in 2010 using a ground-based telescope, suggested that the planet’s atmosphere either was predominantly water vapour or hydrogen-dominated with high-altitude clouds.

More precise Hubble observations made in 2012 and 2013 enabled the team to distinguish between these two scenarios. The news is about what they didn’t find. The Hubble spectra revealed no chemical fingerprints whatsoever in the planet’s atmosphere. This allowed the astronomers to rule out cloud-free atmospheres made of water vapour, methane, nitrogen, carbon monoxide, or carbon dioxide.

The best explanation for the new data is that there are high-altitude clouds in the atmosphere of the planet, though their composition is unknown. Models of super-Earth atmospheres predict clouds could be made out of potassium chloride or zinc sulphide at the scorching temperatures of 230 degrees Celsius found on GJ 1214b. “You would expect very different kinds of clouds to form than you would expect, say, on Earth,” Kreidberg said.

The launch of NASA’s next major space telescope, the James Webb Space Telescope (JWST), later this decade should reveal more about such worlds, Kreidberg said. “Looking forward, JWST will be transformative,” she said. “The new capabilities of this telescope will allow us to peer through the clouds on planets like GJ 1214b. But more than that, it may open the door to studies of Earth-like planets around nearby stars.”

Adapted from information issued by the University of Chicago. Images courtesy NASA, ESA and G. Bacon.

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