We have now developed a catalog of healthy planets orbiting in the habitable zones of distant stars. But we have no idea if they are actually habitable. That’s largely because at these distances it’s extremely difficult to get any idea of what the planets are made of and what their atmospheres look like. And the greenhouse potential of the atmosphere can make the difference between a frozen world like Mars and an uncontrollable greenhouse like Venus.
But in the case of one nearby star, scientists are slowly narrowing the options. TRAPPIST-1 has at least seven planets, all small enough to resemble Earth, with several within the star’s habitable zone. In two papers released this week, teams of scientists narrowed down what their atmosphere might look like and gave a better idea 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 Natural Astronomy, looks at the atmospheres of different planets, but not directly. Instead, it relies on the Hubble to sense the star’s light as a planet passes in front of it. A small fraction of the photons will have passed through the planet’s atmosphere on their way to Earth. Any colors of light absorbed or scattered by the gases in the atmosphere will be missing in that fraction, making it possible to deduce the composition of the atmosphere.
It is possible, but not easy. In this case, the planets were observed as the Hubble Space Telescope orbited the South Atlantic anomaly, where Earth’s radiation belts bend down to meet its orbit. Images taken during this period have more radiation-induced noise and lower resolution, as the telescope shuts down its fine-point hardware.
While the results don’t tell us what’s in the atmosphere, they do tell us what probably won’t be there: lots of hydrogen. The spectrum of the atmospheres of these planets is relatively uncharacteristic, while hydrogen would absorb at a number of different wavelengths covered by the Hubble data. It’s possible to get something that looks like this with a lot of clouds, but there’s no obvious way to generate the aerosols needed for clouds in a hydrogen-rich atmosphere. Previous work had come to this conclusion for the inner planets; the new data excludes hydrogen for planets d, e, and f. That leaves only TRAPPIST-1 g for further investigation.
That doesn’t tell us what’s in these realms. For many of the planets, a variety of compositions match the data. For b and c, for example, the authors say the options include “atmospheres dominated by water, nitrogen, or carbon dioxide; tenuous atmospheres composed of a variety of chemical species, and atmospheres dominated by aerosols.” But excluding hydrogen is important for two reasons. First, it’s likely that most planets start out with hydrogen-rich atmospheres, so this suggests that these planets evolved a bit. The second is that hydrogen is a potent greenhouse gas and thus would have a strong impact on global temperatures.
To the ground
Separately, a team of researchers examined the composition of the planets. This is done through a combination of observations. The transits of the planets, combined with their distance from the star, give us a measure of their size. Given their mass, we can calculate their density, which tells us a lot about their likely composition.
To get the mass, you normally have to look at how much each planet orbits the star as it orbits. But that wasn’t possible with TRAPPIST-1, because the star’s light is so dim. Instead, researchers have looked at how much the planets are pulling together. This is reflected in small variations in how long a job lasts; sometimes a planet is dragged along by its neighbors, sometimes it is dragged back.
For two planets, using these changes to calculate a planet’s mass is simple physics. For a system with seven planets, the calculations become quite dramatically complex and there may be several plausible solutions. To deal with TRAPPIST-1, the authors packed the key properties of the planets into a single vector and ran the vector through a genetic algorithm that tested several permutations of the vector, then recombined the most successful. The result was an optimized solution that included the masses of all planets.
The resulting masses were then used to derive the likely composition. TRAPPIST-1 c and e appear to be rocky bodies, but the rest (b, d, f, g, and h) all appear to have somewhat less dense components. These can be thick atmospheres, water or an icy shell. This non-rocky portion of their mass is usually less than five percent — which may not seem like much, but is much more than Earth’s oceans and atmosphere combined.
Combined with data on their position relative to the star, the authors conclude that the non-rocky material in b is almost certainly in the atmosphere — if it has water, it will be in the form of water vapor, which would neatly explain its low density. Planet D is close to the limit for a runaway greenhouse, so it could easily have a similar water-rich atmosphere. Alternatively, d is one of the planets likely to be cooked by induction heating of the star’s magnetic field, so the greenhouse gases may not be needed to cook down the oceans.
Planets f, g and h could all be ocean worlds or covered in icy shells, depending on the amount of greenhouse gases in their atmosphere.
So while we don’t have a complete picture of these planets yet, one is slowly coming into view, and the data puts some limitations on how we interpret any further information we may gain. Our next chance at major refinements, however, may have to wait for the James Webb Space Telescope’s launch, scheduled for next year.
Natural Astronomy2018. DOI: 10.1038/s41550-017-0374-z (About DOIs).
Astronomy and Astrophysics2018. DOI not yet available.
List image by ESO/M. kornmesser