Planets of TRAPPIST-1: Complex atmospheres, probably lots of water
We've now developed a healthy-sized catalog of planets orbiting in the habitable zone of distant stars. But we don't have the slightest idea whether any of them are actually habitable. That's largely because, at these distances, it's extremely difficult to get any sense of what the planets are made of and what their atmospheres are like. And the greenhouse potential of the atmosphere can make the difference between a frozen world like Mars and an out-of-control hothouse like Venus.
But at least in the case of one nearby star, scientists are slowly narrowing down the options. TRAPPIST-1 has at least seven planets, all small enough to be Earth-like, with several inside the star's habitable zone. In two papers released this week, teams of scientists have narrowed down what their atmospheres might look like and provided a greater sense of their composition. The results suggest that at least one planet has the potential to be a watery world.
In the air
The first study, which appears in Nature Astronomy, looks at the atmospheres of several of the planets, but not directly. Instead, it relies on the Hubble to observe the star's light as a planet passes in front of it. A tiny fraction of the photons will have passed through the planet's atmosphere on their way to Earth. Any colors of light that are absorbed or scattered by the gases in the atmosphere will be missing from that fraction, making it possible to infer the atmosphere's composition.