The 7 Planets of TRAPPIST-1 Have Surprisingly Similar Properties

Astronomers are getting very good at hunting for exoplanets with a little help from powerful ground and space-based telescopes. We’re no longer finding one planet here and there — we’re discovering entire solar systems. TRAPPIST-1 has been of particular interest with its system of seven planets, discovered in 2016 and 2017. A new study has confirmed that all these planets are small and rocky like Earth, and they’re all surprisingly similar to each other.

The TRAPPIST-1 system was originally spotted using the TRAPPIST telescope in Chile. At the time, astronomers believed all the planets would turn out to be rocky, and several are in the habitable zone of the star. TRAPPIST-1 is a red dwarf, so those potentially habitable planets are all very close with solar years measured in Earth days. All seven exoplanets are closer to TRAPPIST-1 than Mercury is to our sun.

The ream led by Erik Agol at the University of Washington was able to estimate the masses of all the planets by observing them as they transited in front of the star. Combined with orbital timing, the scientists have a better handle on both the mass and diameter of the exoplanets. That means we can also find the mass — and here’s where things get weird. They are all eight percent less dense than they would be if they had the same composition as Earth.

This number is easily within the range astronomers would expect, but planetary composition varies a lot. We’ve never found a solar system that’s this consistent. Here at home, we have gas giants like Jupiter that have a much lower density than Earth. Even among rocky worlds in our solar system, there is notable variation in density. Mars is about 70 percent as dense as Earth, for example.

The team has come up with three possible explanations for the lower density, each of which would change what these planets look like up close. The planets might have a similar composition as Earth, with the exception of lower iron content. If they have iron cores like Earth, they’d just be a little smaller. Alternatively, the iron could be spread uniformly throughout the exoplanets with oxygen, essentially becoming big balls of rust with no iron core. The third possibility is a bit more intriguing. The lower density could also be explained by deep oceans covering the four outer planets. This is less likely because the water content would have to be just such to leave the planets with the same density.

We might learn which of these options is the right one before long. The TRAPPIST-1 system is a popular target for astronomers because there are so many planets to study all in one place. It’s also close, at least in the grand scheme of things. The 40-light-year gap will be no problem for instruments like the upcoming Webb Space Telescope, but it would take hundreds of thousands of years to travel there with current technology.

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