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Astronomers are getting very good at chasing exoplanets with a little help from powerful telescopes from land and space. We no longer find a planet here and there – we discover 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 of these planets are small and rocky like Earth, and they are all surprisingly similar to each other. .
The TRAPPIST-1 system was initially spotted using the TRAPPIST telescope in Chile. At the time, astronomers believed that all planets would turn out to be rocky, and several are in the star’s habitable zone. TRAPPIST-1 is a red dwarf, so these potentially habitable planets are all very close to solar years measured in Earth days. The seven exoplanets are closer to TRAPPIST-1 than Mercury is to our sun.
The train led by Erik Agol at the University of Washington was able to estimate the masses of all the planets by observing them as they passed in front of the star. Combined with orbital synchronization, scientists have better control over the mass and diameter of exoplanets. This means that we can find mass as well – and this is where things get weird. They are all eight percent less dense than they would be if they had the same makeup as Earth.
This number is easily within the range expected by astronomers, but the planetary composition varies a lot. We never found a solar system that is coherent. At home, we have gas giants like Jupiter which have a much lower density than Earth. Even among the rocky worlds of our solar system, there is a noticeable variation in density. Mars is about 70% as dense as Earth, for example.
The team came up with three possible explanations for the lower density, each of which would change how these planets look up close. The planets could be similar in composition to Earth, except for a lower iron content. If they have iron cores like Earth, they would be just a little smaller. Alternatively, the iron could be evenly distributed throughout the exoplanets with oxygen, essentially becoming large rust balls with no iron core. The third possibility is a little 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 should be such that the planets have the same density.
We may soon find out which of these options is the right one. The TRAPPIST-1 system is a popular target for astronomers because there are so many planets to study in one place. It’s also close, at least in the grand scheme of things. The 40 light-year gap won’t be a problem for instruments like the next Webb Space Telescope, but it would take hundreds of thousands of years to get there with current technology.
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