This Planet's Clouds Are Made of Rocks and Never Move

A couple of weeks ago, we hit the 1,000 exoplanet mark. It took a little over two decades, but astronomers have found and confirmed more than 1,000 planets orbiting distant stars. We're so spoiled, it’s getting to the point where finding exoplanets isn’t the exciting news it once was.

Science is now taking center stage; determining the characteristics of confirmed exoplanets is the next in the search for any habitable worlds. And happily, scientists have at least some of the tools they need for the job. Though we can’t optically see the surface details on distant bodies, astronomers have managed to map the clouds that partially cover Kepler-7b.

Kepler-7b was one of the first five exoplanets NASA’s Kepler Space Telescope found once it began science operations in 2009.  Kepler-7b is a hot Jupiter. Though it has only half the mass of Jupiter, it’s about one and a half times as big. It also orbits extremely close to its star, just 5.6 million miles away—that’s about 0.06 astronomical units; or six percent as far as the Earth is from the Sun. Since finding Kepler-7b more than three years ago, the Kepler telescope has been watching it pretty closely.

Kepler (the telescope) sees in visible light. What astronomers saw with these optical observations was a planet with Moon-like phases. From there, astronomers created a rough map of the planet’s surface, characterized by a bright spot on its western hemisphere. But optical data is limited. This wasn’t enough to tell astronomers whether the bright spot was the result of heat emanating from the planet or reflection of its star’s light off the surface or cloud tops.

This is where NASA’s Spitzer Space Telescope came in. Like Kepler, Spitzer can focus on a single distant star and the planets the orbit it, but unlike Kepler it doesn’t see in visible light. Spitzer gathers infrared light, light on the low end of the electromagnetic spectrum associated with heat. Anything that gives off heat gives off infrared radiation.

By measuring Kepler-7b in infrared light, Spitzer was able to ballpark the planet’s temperature as being between 1,500 and 1,800 degrees Fahrenheit. Given how close the planet orbits its star, this actually is pretty cool. And for astronomers, this was a major clue: If the planet is this cool but close to its star, the light seen in Kepler’s optical observations couldn’t be related to temperature. The bright spots had to be a reflection.

Astronomers concluded that Kepler-7b’s bright spot is light reflected off a layer of clouds located on the planet’s west side. The eastern side of the planet, meanwhile, is relatively clear. The cloud map they created from this data reveals a world with an incredibly stable climate. While clouds on Earth are incredibly varied, the clouds on Kepler-7b don’t seem to move much at all. And that’s not the only way these clouds might differ from those we have on Earth. Given the planet’s scorching temperature, astronomers think the clouds could be made of rock and iron rather than water vapor.

Having two telescope measuring light differently gave astronomers the multi-wavelength tool they needed. And while Kepler’s faulty reaction wheels means the telescope’s days of optically measuring a planet are over, there’s a lot we can hope to learn about distant worlds from old Kepler data and different wavelength measurements to come.