Astronomers become very good at hunting exoplanets with a little help from powerful terrestrial and space telescopes. We no longer find one planet here and there – we find entire solar systems. TRAPPIST-1 is of particular interest with its system of seven planets, discovered in 2016 and 2017. A new study confirmed that all these planets are as small and rocky as Earth and they are all surprisingly similar to 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 turn out to be rocky, and several were in the star’s habitable zone. TRAPPIST-1 is a red dwarf, so these potentially habitable planets are very close to the solar years measured on Earth days. All seven exoplanets are closer to TRAPPIST-1 than Mercury is to our sun.
Rome, led by Eric Agol of 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. In combination with orbital time, scientists have a better understanding of both the mass and the diameter of exoplanets. This means that we can also find the table – and here 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 the planetary composition varies greatly. We have never discovered a solar system that is this consistent. Here at home we have gas giants like Jupiter, which have a much lower density than Earth. Even among the rocky worlds in our solar system, there are noticeable variations in density. Mars is about 70 percent as dense as Earth, for example.
The team came up with three possible explanations for the lower density, each of which would change what these planets look like up close. The planets may have a similar composition to Earth, except for a lower iron content. If they had iron cores like Earth, they would just be a little smaller. Alternatively, iron can spread evenly across exoplanets with oxygen, essentially turning into large balls of rust without an iron core. The third possibility is a little more intriguing. 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 exactly the same for the planets to remain the same density.
It is possible to learn which of these options is correct soon. 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 big picture. The difference of 40 light-years 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 modern technology.