Back in 2016, the first exoplanet discovery using the Transiting Planets and Planetesimals Small Telescope (TRAPPIST) was made. With 3 planets found, and 4 more later discovered, the system was designated TRAPPIST-1. And the best part – all the planets’ sizes and masses are close to the Earth’s! Such an abundance of Earth-sized planets has never been found in one place before or since.
Now, a new study indicates that the planets in the system of TRAPPIST-1 have very similar densities. Once we know a planet’s density, it’s then possible to estimate what materials it might be composed of. It’s thought that these planets have a ratio of materials similar to that of the rocky planets we know (like Earth itself!). However, they are about 8% less dense than they would be if the ratio was the same as the Earth’s. So, that ratio must be a little different.
Each exoplanet was discovered using a method known as the transit method. When a planet moves in between its star and us, it blocks a tiny bit of the light that reaches us from that star. We can measure these dips in the level of light. If the same one turns up periodically, there might be a planet there! Simply by measuring the orbital period in this way, scientists can calculate a bunch of things about the planet. They can find its size, mass, and even get insights into the climate.
Possible Explanations for TRAPPIST-1
There are a number of reasons why the TRAPPIST-1 planets could be less dense than expected. Firstly, they could be similar to Earth but simply containing less iron. Or, that iron could be infused with oxygen on the planet, forming iron oxide (rust), a lighter material that iron itself. But if this was the case, the planets would have to be full of rust, rather than having a solid iron core like planets in our solar system. Another option is that each planet’s surface is covered in water, though this would require the innermost planets to have hot, dense atmospheres to stop it boiling off. It’s likely that the real reason is some combination of the three.
What do you think the TRAPPIST-1 system will turn out to be?