Weather on Alien Worlds

From high-altitude clouds discovered on a super-Earth to massive, hurricane-force storms on a nearby brown dwarf, a bevy of results show that the age of “astrometeorology” is upon us.

Imagine turning on the radio and hearing a weather forecast. But rather than predicting some mild winter rain in your city, the forecast describes hurricane-force winds brewing on another world, one where a storm of behemoth proportions covers a quarter of the sky.

Hubble observations show that the super-Earth GJ 1214b must have high-altitude clouds. And these clouds are like nothing we've seen in the solar system, perhaps made of potassium chloride or zinc sulfide droplets.
Space Telescope Science Institute
That’s not a What If from the future. The radio broadcast may be imagined, but the forecast itself is real. At the winter meeting of the American Astronomical Society, one theme became clear as team after team announced results: astronomers have moved beyond merely counting alien worlds to studying their compositions and atmospheres.

Not that counting isn’t important — one of the Kepler mission’s defining goals was to find out how common Earth-like planets are (as it turns out, pretty darn common). But even before Kepler, astronomers were detecting transiting planets by the blip in their parent star’s light. Two of these planets, GJ 1214b (a super-Earth) and GJ 436b (a Neptune-size planet), closely orbit red dwarf stars only 42 and 33 light-years away, respectively.

“I call [GJ 1214b] everybody’s favorite super-Earth, in part because it’s so close,” says Laura Kreidberg (University of Chicago). Her team used the Hubble Space Telescope to capture the spectrum of starlight passing through the planet’s atmosphere as it transited across the face of its parent star not just once, but 15 times — 4 days of observation in all. “I feel like a proud parent showing this off,” Kreidberg said as she announced the results at the AAS. They also appear in the January 2nd issue of Nature.

Hubble observed the system in a wavelength band (between 1 and 2 microns) that ought to have shown hints of water vapor, gaseous carbon dioxide or carbon monoxide, or methane molecules. So the team was surprised when, instead, the spectrum showed no features at all. You could say the planet flatlined.

Flatlining isn’t usually a good thing, but in this case, the null result carries a treasure trove of information. For a long time, astronomers have held two views of this particular planet’s atmosphere: either it’s cloud-free and made of heavy molecules, such as water or carbon dioxide, or high-altitude clouds or haze block our view of the atmosphere altogether. Kreidberg’s observations break the tie — they definitively show that GJ 1214b must have high-altitude clouds.

And these are no ordinary clouds. Instead of being made of water vapor, methane, or any of the ordinary cloud-forming material we see in the solar system, the clouds might be of more exotic stuff, like droplets of potassium chloride (a form of potash which, interestingly enough, is pink in liquid form, Kreidberg says) or zinc sulfide. Confirming these cloud models will be difficult with current instruments, but the infrared James Webb Space Telescope, due to launch in October 2018, could unlock the clouds’ secrets.

Watch Kreidberg explain her research here (the good stuff starts at 1:00 after a brief introduction):

GJ 1214b isn’t the first exoplanet to be studied in this way — it’s part of a tidal wave of observations from many different teams. Heather Knutson (California Institute of Technology) found similar, though less conclusive, “flatline” results for the Neptune-size GJ 436b, also published in the January 2nd issue of Nature. And previous Hubble observations have found plenty of high-altitude haze as well as water vapor in the atmospheres of hot Jupiters.

Cloudy Forecast on a Brown Dwarf Near You

Exoplanets aren’t the only alien worlds with clouds. Brown dwarfs can have bad weather too.

Luhman 16AB was discovered last year, displacing Wolf 359 as the third-closest system to Earth.
NASA / JPL / Gemini Observatory / AURA / NSF
Adam Burgasser (University of California, San Diego) reported a weather forecast for Luhman 16B, one of a newly discovered pair of brown dwarfs just 6.6 light-years from Earth. Observations from an international monitoring campaign spanning four continents showed that 15–25% of this world’s visible surface is covered in a massive, hurricane-force storm. That makes the Great Red Spot, which covers only 1% of Jupiter, look puny in comparison.

How did astronomers make out a storm on a brown dwarf they can barely see? After all, Luhman 16A and B are not only dim, they’re separated by only 3 Earth-Sun distances in their orbits, making them difficult to resolve in telescopes. But Burgasser’s team was able to deduce a surprising amount by observing the changes over time in the brightness and color of the two dwarfs, including the size, temperature, and wind speed in the storm on Luhman 16B.

Brown dwarfs, sometimes called "failed stars" for their inability to sustain fusion, have bad weather too, with thick cloud decks of sand and iron vapor. By tracking the changes in brown dwarfs' brightness and spectra, astronomers can watch storms form and fade as the brown dwarf spins in its orbit.
NASA / JPL-Caltech / Univ. of Western Ontario / Stony Brook Univ.
(And in case you’re wondering, there aren’t likely to be any pink potash clouds here — previous near-infrared observations from Spitzer have shown the thick cloud decks are likely made of sand and iron vapor.)

And that’s just one brown dwarf. Surveys are capturing many more. Aren Heinze (Stony Brook University) announced the results of a monitoring campaign of 44 brown dwarfs, about half of which changed in brightness over the 21 hours of observation. Two varied regularly, suggesting stable Great Red Spot-like features rotating in and out of view, but the others varied more erratically, like Luhman 16B, suggesting that massive cloud formations can form and fade within hours. Why some clouds should be stable and others changing within hours is still an open question.

This is only the beginning of a new age — with the launch of the Transiting Exoplanet Survey Satellite (scheduled for 2017) and the James Webb Space Telescope (scheduled for October 2018), we’ll see a whole lot more about weather on other worlds.

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