Planet shrouded in green clouds? NASA cloud mappers seek new Earths

Scientists have discovered that a huge, gassy exoplanet called Kepler-7b is covered with patchy clouds of silicates that might even rain liquid rock -- even though it’s within scalding distance of its parent star.

The findings, set to be published in Astrophysical Journal Letters, are the first to map cloud structures on a world beyond our solar system and could one day be used to study clouds on smaller, more Earth-like planets.

The planet Kepler-7b, whose star sits in the constellation Lyra, was one of the earliest discoveries using NASA’s now-hobbled Kepler space telescope.

“We consider it a hot Jupiter because it’s very close to its star,” Demory said. “We think it’s tidally locked.” This means it can’t rotate freely and always shows the same face to its parent star – just as the moon does toward Earth.

Kepler-7b looks strange. It is 50% larger than Jupiter, but 50% less massive. It orbits so close to its home star — just six-hundredths of Earth’s distance to the sun — that it qualifies as a “hot Jupiter,” said the study‘s lead author, Brice-Olivier Demory, an observational astrophysicist at the Massachusetts Institute of Technology.

But Kepler-7b caught Demory’s eye for another big reason: For a hot Jupiter, it is surprisingly bright.

Because hot Jupiters are so close to their stars, they often absorb a lot of the star’s visible light and re-emit it as thermal radiation. So in the visible spectrum, these hot Jupiters are usually quite dim – they have a low albedo, or surface reflectivity.

But Kepler-7b, Demory said, “is more than three times the highest albedo of most hot Jupiters.”

To make sure Kepler’s readings were coming from visible light and not thermal radiation, they tracked the planet using NASA’s infrared-seeking Spitzer space telescope. Spitzer showed that the planet’s temperature must be about 1,500 to 1,800 degrees Fahrenheit – pretty balmy, but still much cooler than the roughly 2,200 degrees on typical hot Jupiters.

Clearly, the relatively cool planet wasn’t putting out any extra thermal radiation. The light picked up by Kepler must have been getting reflected by cloud cover, the scientists concluded.

“Kepler-7b is very peculiar in terms of density and temperature, which would allow the clouds to form high up in the atmosphere, and sufficiently high to suppress the heat from below,” Demory said.

Building a map, the scientists found that high clouds protect the west, while skies stay clear in the east. The planet could have very different temperatures depending on which side was being examined -- the west would be cooler, the east hotter.

These are not water clouds, Demory pointed out: The planet is far too hot for water to stick around. Kepler-7b’s dense clouds are probably made up of silicon-rich compounds, he said – perhaps perovskite and forsterite, which are found in rocks on Earth. The perovskite could potentially give the clouds a greenish hue, Demory said.

It’s even possible these clouds are raining drops of silicates or perhaps falling gently as mineral snowflakes, Demory said, though he stressed that he is not a mineralogist.

Kepler-7b is too big and gassy and close to its star to be habitable, but finding other overcast gas giants could teach scientists more about the specific conditions that cause these strange clouds to form.

The endgame would be to apply these cloud-mapping techniques to smaller, rockier, more Earth-like planets, Demory said – though researchers don’t have the technology to do so just yet.

“Our goal is to find habitable planets where life could emerge,” Demory said.

Either way, Demory added that many of the exoplanets once deemed uninhabitable may now get a second look.

“Clouds may change climate and habitability of planets,” Demory said. “If we take clouds into account, it may be that a lot more of these planets are more habitable.”