Cloudy with a chance of exoplanets

NASA & ESA

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's 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.