Webb scientists wonder if this planet lost then rebuilt an atmosphere

As scientists continue to study a distant star system with seven rocky exoplanets, the odds of finding a livable world there seem to be getting longer.

Early results on the fourth planet from the TRAPPIST-1 star suggest it lost its original atmosphere to space long ago, and it’s unclear if it ever built a new one. Researchers used NASA's James Webb Space Telescope for their observations.

But the planet, TRAPPIST-1e, could still have enough greenhouse gases surrounding it to allow water to stick — maybe as a global ocean or one that covers the side that always faces its red dwarf star.

Among the seven worlds orbiting the star 40 light-years away, planet E has stood out as one of the best bets for life because it may sit in a so-called habitable zone, where it's neither too hot nor too cold for liquid water, so long as there's an atmosphere.

"TRAPPIST-1 is a very different star from our sun, and so the planetary system around it is also very different, which challenges both our observational and theoretical assumptions," said Nikole Lewis, an astronomy professor at Cornell University involved in the study, in a statement.

Two scientific papers on the initial findings appear in Astrophysical Journal Letters. They come just weeks after the results of its neighboring planet, TRAPPIST-1d, were published, also offering a grim perspective on whether life could be possible there.

The TRAPPIST-1 system was discovered about eight years ago with NASA’s now-retired Spitzer Space Telescope. The planets, all close to Earth’s size, quickly became a top target in the search for worlds that could support life.

Astronomers especially wanted to know if the TRAPPIST planets have atmospheres, because red dwarf stars are the most common type in the Milky Way. If planets in such harsh environments can hold onto air despite blasts of radiation, that would suggest many other worlds might as well.

The planet E results were based on just four of 19 planned observations. Publishing early may let other scientists see the team's approach, build upon it, and contribute to the interpretation of the findings. 

As the scientists continue their study, they also will be testing a new method: comparing signals from TRAPPIST-1e to TRAPPIST-1b, which seems to be little more than an airless, bare rock. 

For the first observations, the team used a technique called transmission spectroscopy. When a planet passes in front of its star, some starlight filters through its atmosphere, if one exists. Different gases absorb different colors of light, so scientists can spot missing chunks in the spectrum to see what chemicals are there. The technique does have its challenges, though, sometimes revealing confusing results because of sunspots and other types of stellar activity. 

Scientists are trying to determine whether any of of the rocky worlds orbiting TRAPPIST-1, a red dwarf star about 40 light-years away from Earth, could have an atmosphere. Credit: NASA / JPL-Caltech illustration

Based on that original data, the team doesn't think planet E has a hydrogen-helium atmosphere; probably, it was destroyed by strong stellar flares from its feisty star. But planets can develop subsequent atmospheres, just as Earth did billions of years ago. The researchers have wondered whether planet E could have done the same.

To get to the bottom of that, the team has devised a new strategy. They plan to time their observations so that planets B and E cross the star at the same time to compare the two sets of data. In this scenario, planet B becomes the control: Anything seen in both planets' chemical fingerprints is therefore probably coming from the star; anything seen only when planet E crosses is probably from its own air. 

A big question for Webb is whether red dwarfs — also called M-type stars — can host planets with atmospheres. To tackle that, Webb scientists have begun a large survey of rocky planets, first reported by Mashable. But instead of transmission spectroscopy, it will use the "secondary eclipse" method, which also sidesteps some of the problems caused by star activity.

"We are really still in the early stages of learning what kind of amazing science we can do with Webb," said Ana Glidden, one of the lead authors from MIT, in a statement. "We’re in a new age of exploration that’s very exciting to be a part of."