The seven planets of TRAPPIST-1 have similar intensities – the values vary by no more than 3%. This makes the system very different from ours. The difference in density between TRAPPIST-1, Earth and Venus might seem small – about 8% – but it is significant on a planetary scale. For example, one way to explain why TRAPPIST-1 planets are less dense is that they have a composition similar to Earth, but with less iron – about 21% compared to 32% for Earth, according to the study.
Alternatively, the iron in TRAPPIST-1 planets may be leaked with high levels of oxygen, forming iron oxide or rust. The additional oxygen will reduce the planets’ density. The surface of Mars gets a red pigment from iron oxide, but like its three siblings on Earth, it has a core made up of non-oxidized iron. In contrast, if the low densities of TRAPPIST-1 planets were entirely due to oxidized iron, then the planets must be rusty all the time and could not have solid iron cores.
The answer could be a combination of the two scenarios – less iron overall and some oxidized iron, said Eric Ajol, an astrophysicist at the University of Washington and lead author of the new study.
The team also looked at whether the surface of each planet could be covered with water, which is lighter than rust and that would change the planet’s overall density. If this were the case, then the water would have to account for about 5% of the total mass of the four exoplanets. By comparison, water makes up less than 1 tenth of 1% of Earth’s total mass.
Because they are located so close to their star that water remains liquid in most conditions, the three inner planets TRAPPIST-1 require dense and hot atmospheres like Venus, so that the water can remain attached to the planet as vapor. But Ajol says that this explanation appears less likely because it would be a coincidence that all seven planets contain enough water to have such a similar density.
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