Is the TRAPPIST-1 System Hiding a Gas Giant?

The answer may tell us how our own gas giants formed.


Early this year, a group of astronomers discovered seven Earth-like planets orbiting a single nearby star, TRAPPIST-1. These seven planets might be a good place to search for extraterrestrial life, especially because three of them lie within their star's habitable zone. But for all of the unique features of the TRAPPIST-1 system, it's missing something that nearly every other planetary system we've studied seems to have: a gas giant.

Our solar system has several large gas giants, and almost every other star system where we've spotted planets seems to have at least one as well. But so far, we haven't found one orbiting TRAPPIST-1. The lack of a gas giant suggests that either the large planets in the system are strangely hard to spot or there's some process in the TRAPPIST-1 system that prevents gas giants from forming.

A group of astronomers from the Carnegie Institution for Science are working to find out whether TRAPPIST-1 has a gas giant or not. Either way, we're going to learn something about our own solar system in the process.

"A number of other star systems that include Earth-sized planets and super-Earths are also home to at least one gas giant," said lead author Alan Boss. "So, asking whether these seven planets have gas giant siblings with longer-period orbits is an important question."

The astronomers have already used the Carnegie Institution's du Pont telescope to study TRAPPIST-1 in an attempt to detect the presence of a gas giant. While they didn't find one, they now know that if TRAPPIST-1 does have a gas giant, it must be less than 4.6 times Jupiter's mass if it's in a 1-year orbit, or less than 1.6 times Jupiter's mass if it's in a 5-year orbit.

Astronomers already know how big a planet must be in order to become a gas giant, which means we know exactly what we're looking for. The Carnegie astronomers are hoping that a more powerful telescope will provide a definite answer to the question. If they do manage to find a gas giant, it could tell us more about how the gas giants in our own solar system formed.

Currently, scientists have developed two possible theories for how planets like Jupiter and Saturn first formed. The first—called core accretion theory—states that the gas giants first formed a large rocky core which became large enough to attract much of the solar system's gas. The competing theory—disk instability theory—claims that the gas giants formed when the disk of gas orbiting our star formed spiral arms, which collapsed into planets without the need for a solid core.

It's tough to say which theory is correct, but only one of them—disk instability—predicts that TRAPPIST-1 would have its own gas giants. If we do end up finding a gas giant orbiting TRAPPIST-1, it could spell the end for core accretion theories. On the other hand, if TRAPPIST is missing a gas giant, it's more evidence that core accretion could be correct.

Either way, extraterrestrial life isn't the only interesting thing we could find in the TRAPPIST-1 system.

Source: Carnegie Institution for Science

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