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The seven TRAPPIST-1 planets have similar densities – the values do not differ by more than 3%. It makes the system very different from ours. The difference in density between the TRAPPIST-1 planets and Earth and Venus may seem small – around 8% – but it is significant on a planetary scale. For example, one way to explain why the TRAPPIST-1 planets are less dense is that they are similar in composition to Earth, but with a lower percentage of iron – around 21% compared to Earth’s 32%, according to the study.
Alternatively, the iron in the TRAPPIST-1 planets could be infused with high levels of oxygen, forming iron oxide or rust. The additional oxygen would decrease the densities of the planets. The surface of Mars gets its red hue from iron oxide, but like its three terrestrial siblings, it has a core made up of unoxidized iron. On the other hand, if the lower density of the TRAPPIST-1 planets was caused entirely by oxidized iron, the planets would have to be rusty and could not have solid iron cores.
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 overall and some oxidized iron.
The team also investigated whether the surface of each planet could be covered with water, which is even lighter than rust and would change the planet’s overall density. If this were the case, water would have to make up about 5% of the total mass of the four outer planets. In comparison, water is less than a tenth of 1% of the total mass of the Earth.
Because they are placed too close to their star for water to remain liquid under most circumstances, the three inner planets TRAPPIST-1 would require hot, dense atmospheres like that of Venus, so that the water could stay linked to the planet in the form of vapor. But Agol says that explanation seems less likely because it would be a coincidence for the seven planets to have just enough water to have similar densities.
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