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The 7 planets of TRAPPIST-1 have surprisingly similar properties

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Astronomers become very good at hunting for exoplanets with a little help from powerful Earth and space-based telescopes. We no longer find a planet here and there – we discover entire solar systems. TRAPPIST-1 has been of particular interest with its seven-planet system 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.

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

The breadth 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 timing, scientists have a better grasp of both the mass and diameter of exoplanets. This means that we can also find the masses – and here 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 that astronomers might expect, but the composition of the planet varies widely. We have never found a solar system that is 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 remarkable variation in density. Mars is e.g. About 70 percent as close as Earth.

The team has come up with three possible explanations for the lower density, each of which will change how these planets look up close. The planets may have a similar composition to Earth with the exception of lower iron content. If they have iron cores like Earth, they would just be a little smaller. Alternatively, the iron could be spread uniformly through the exoplanets with oxygen and essentially become large rust spheres without the iron cores. The third option is a little more exciting. 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 be able to learn which of these options is the right one before long. The TRAPPIST-1 system is a popular destination for astronomers because there are so many planets to study everything in one place. It’s close, too, at least in the grand scheme of things. The 40-light gap will not be a problem for instruments like the upcoming Webb Space Telescope, but it will take hundreds of thousands of years to travel there with current technology.

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