Astronomers are very much looking for exoplanets with a little help from powerful ground and space-based telescopes. We no longer find one planet here and there – we discover entire solar systems. TRAPPIST-1 was of particular interest with its seven-planet system discovered in 2016 and 2017. A new study has confirmed that all these planets are small and rocky like the earth, and that they all look amazing on each other.
The TRAPPIST-1 system was originally spotted using the TRAPPIST telescope in Chile. At the time, astronomers believed that all the planets would appear 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 to 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 departed before the star. Combined with the timing of the orbit, the scientists handled the mass and diameter of the exoplanets better. This means that we can also find the masses – and this is where things get weird. They are all eight percent less dense than they would be if they had the same composition as the earth.
This number is easily within the range that astronomers would expect, but its planetary composition varies widely. We have never found a solar system that is 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 a notable variation in density. Mars, for example, is about 70 percent as dense as Earth.
The team devised three possible explanations for the lower density, each of which would change what these planets look like up close. The planets have a similar composition to Earth, with the exception of a lower iron content. If they have iron cores like the earth, they will only be a little smaller. Alternatively, the iron with oxygen can be uniformly distributed through the exoplanets, becoming in fact large rust spheres without iron core. The third possibility is a little more interesting. The lower density can also be explained by deep oceans covering the four outer planets. This is less likely because the water content must be just like that to leave the planets with the same density.
We may soon find out which of these options is the right option. The TRAPPIST-1 system is a popular target for astronomers because there are so many planets to study all in one place. It’s close too, at least in the grand scheme of things. The 40-light-year gap is no 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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