The seven TRAPPIST-1 planets have similar densities – the values differ by no more than 3%. This makes the system completely different from our own. The difference in density between the TRAPPIST-1 planets and the Earth and Venus may seem small – about 8% – but it is significant on a planetary scale. One way to explain, for example, why the TRAPPIST-1 planets are less dense is that they have a similar composition to Earth, but with a lower percentage of iron – about 21% compared to Earth’s 32%, according to the study.
Alternatively, the iron can be applied in the TRAPPIST-1 planets with high oxygen levels, which form iron oxide or rust. The additional oxygen will reduce the density of the planets. The surface of Mars gets its red tint of iron oxide, but like its three earthly siblings, it has a core made up of non-oxidized iron. Conversely, if the lower density of the TRAPPIST-1 planets is caused entirely by oxidized iron, the planets will have to be rusted throughout and have no solid iron core.
Eric Agol, an astrophysicist at the University of Washington and lead author of the new study, said the answer could be a combination of the two scenarios – less iron in general and slightly oxidized iron.
The team also looked at whether the surface of each planet could be covered with water, which is even lighter than rust and which would change the planet’s overall density. If that were the case, water would have to make up about 5% of the total mass of the outer four planets. By comparison, water makes up less than a tenth of 1% of the earth’s total mass.
Because in most circumstances they are placed too close to their star for water to remain liquid, the three inner TRAPPIST-1 planets need a warm, dense atmosphere like Venus so that water can remain bound to the planet as steam. However, Agol says this statement seems less likely, as it would be coincidental for all seven planets to offer just enough water to have such similar densities.