Astronomers Pin Down the Orbit of Exoplanet TRAPPIST-1h

The TRAPPIST-1 star system is interesting to say the least. This solar system is home to seven exoplanets, and astronomers have been watching it closely ever since the discovery was announced early this year. TRAPPIST-1 is particularly fascinating because it contains multiple planets that might be capable of supporting life. However, the outermost planet is not one of them. Astronomers have finally pinned down the orbit of TRAPPIST-1h and confirmed it’s far too chilly for life. Scientists have also confirmed many of the planets in the TRAPPIST-1 system have what’s known as “orbital resonance.”

TRAPPIST-1 is a mere 39 light years away, which is basically right next door in astronomical terms. It’s an extremely small, dim star with a mass just 8 percent that of our sun. However, all the planets around TRAPPIST-1 orbit very close — a year there is only a few Earth days. The close orbits mean that even planets around this cool star could have liquid water, and thus life as we know it.

Astronomers had a good handle on the orbits of the six inner TRAPPIST-1 planets, but TRAPPIST-1h proved difficult to track. Luckily, teams studying the system noticed that the six other planets were in orbital resonance. That means gravitational interaction among the planets has pulled them into stable orbits that are related by a ratio of two integers. In our solar system, the Jovian moons of Io, Europa, and Ganymede are in orbital resonance. For every one orbit of Ganymede, Europa completes two and Io has four.

While the ratio in the TRAPPIST-1 system is more complex than it is among Jupiter’s moons, astronomers were able to use it to narrow down the possible orbits of TRAPPIST-1h. Observations ruled out five of the six, and the final one turned out to be correct based on previous observations from NASA’s Kepler space telescope.

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With all seven orbits confirmed, astronomers can say for certain that TRAPPIST-1 is the largest example of orbital resonance. Based on TRAPPIST-1h’s location, it gets about as much energy as the dwarf planet Ceres in our solar system. So, not enough for life to exist right now. TRAPPIST-1 may have been warmer in the past, which could have made the outermost planet rather pleasant. The inner planets are still possible hosts to life today.

Astronomers hope to study TRAPPIST-1 in great detail to test theories about planetary formation and migration. They’ll also try to gather more data about the individual planets.

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