Seven Earth-sized planets circle one small star, and Webb is checking them for air
The TRAPPIST-1 system holds seven rocky planets around a star barely larger than Jupiter. Whether any of them kept an atmosphere is the question now being tested.
- Institutions
- NASA, European Southern Observatory, Spitzer Space Telescope
- Reported
- 2017
- Status
- Independently established and broadly accepted.
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What they found
About 40 light-years away, a cool red dwarf star named TRAPPIST-1 is orbited by seven planets close to Earth in size. Several sit in the range of distances where a planet could hold liquid water, if it has an atmosphere to hold it. The whole system is compact enough to fit comfortably inside the orbit of Mercury, so the planets circle their star in a matter of days rather than months.
How they found it
Each planet reveals itself by dimming the star slightly as it passes in front, like a moth crossing a streetlight. Because the star is small and dim, an Earth-sized planet blocks a relatively large fraction of its light, which is what makes such small worlds detectable here at all. Timing the dips gives the orbits, and the way the planets tug on each other gives their masses.
Why it matters
Small red stars are the most common kind in the galaxy, so whether their planets can hold on to atmospheres shapes how many habitable worlds there might be overall. TRAPPIST-1 is the best laboratory we have for that question, because it offers seven test cases around one star rather than one planet in isolation.
What this does not mean
Being Earth-sized and at the right distance is not the same as being habitable. Webb observations of the two innermost planets found no sign of a thick atmosphere, which is consistent with the concern that flares from an active red dwarf can strip a close-in planet bare. The outer planets have not been ruled either way. No biosignature has been detected in this system, and none has been claimed.