Scientific Method / Science & Exploration

Cloudy with a chance of exoplanets

The atmospheres of two exoplanets are full of salty clouds.


While design work is continuing on a space-based telescope that has the potential to image nearby exoplanets, research is still being done with existing hardware. And thanks to a bit of time on the Hubble Space Telescope, scientists have managed to image the atmosphere of two nearby exoplanets. In both cases, the best explanation for the data they've gathered is that the atmospheres contain clouds, although they're unlikely to be anything like the clouds we see on Earth. The exoplanets and their clouds are the subject of papers published by Nature on New Year's Day.

The planets in question, GJ 436b and GJ 1214b, orbit close in to nearby dwarf stars. Both of these factors—the proximity and the small size of the star—mean that the planets create a much larger signal when they pass in between the star and Earth. That's helpful, because these are not especially large as planets are reckoned; one is about the size of Neptune, the second is a Super-Earth.

As the planets pass in front of their host star, some of the star's light gets blocked out by the mass of the planet itself. But a small fraction ends up passing through the atmosphere on its way to Earth. If the atmosphere contains a distinct mix of chemicals, then we should be able to detect their signatures by the light that gets absorbed as it passes through the atmosphere. This signature, called a spectrum, should be able to tell us about the atmosphere's composition.

This signal is too small to pick up during a single passage of the planet, but researchers on both papers used a new imaging mode on the Hubble, along with multiple orbits' worth of imaging, to capture a series of images of the exoplanets' transits. By combining those, they were able to obtain a portion of the atmosphere's spectrum from the near-infrared region.

And the spectrum obtained for both exoplanets is remarkably boring. Rather than containing hints of any particular chemical, which would absorb light at specific wavelengths, both planets' observed spectrum is completely flat.

A boring spectrum, though, doesn't mean boring results. In the case of GJ 436b, the hot Neptune, there are two possible explanations for the lack of observed features in the spectrum: either the planet has an atmosphere that's nearly devoid of hydrogen, or it's covered in a layer of high clouds. Right now, the error bars of their measurements encompass models of both of these options, but they say that some additional observation time will allow them to rule one or the other out.

In the case of Super-Earth GJ 1214b, however, the authors already have enough observations to rule out the hydrogen-poor atmosphere. That leaves clouds as the only viable explanation for the spectrum that they see.

These clouds are almost certainly not the water vapor clouds that we see on Earth. Both exoplanets are much closer to their host stars, with correspondingly hotter atmospheres. As a result, both papers suggest that the clouds are likely to be composed of what we would call salts in their solid form: "Zinc sulphide and potassium chloride are both plausible candidates for the composition of the cloud." On the Super-Earth, the authors also indicate that a Titan-like atmosphere, where UV light has triggered chemical reactions that create complex hydrocarbons, is also a possibility.

As long as a planet is relatively nearby and the star it orbits is small, the techniques used here should be effective, which means we can probably expect some additional exoplanets to be examined in the near future. But the authors of one of the papers also point out that we can learn even more from the light reflected and emitted by a planet when it's passing behind its host star. And the James Webb Space Telescope, along with some next-generation ground-based instruments, should have the ability to resolve that.

So by the end of the decade, we could have a much greater sense of what distant planets look like.

Nature, 2013. DOI: 10.1038/nature12887, 10.1038/nature12888  (About DOIs).

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