TRAPPIST-1’s Planets May Have Shockingly Similar Compositions, Says NASA


The nearby TRAPPIST-1 star system’s seven rocky planets have compositions that appear to be both low in density and maybe even shockingly similar, an international team of astronomers reported this week.

New, more precise observational mass estimates of these rocky low-mass planets orbiting the cool red dwarf star TRAPPIST-1, make it clear that this alien solar system is radically different from our own. 

“These planets could have accreted directly from the protoplanetary disk around a much cooler star than the Sun,” Erik Asphaug, a planetary geophysicist at the University of Arizona, who was not part of the study, told me.

The international team of researchers used 1000 hours of observing time on NASA’s now decommissioned Spitzer Space Telescope to make their observations. Their findings, reported in the Planetary Science Journal, make it clear that this bizarre alien solar system —- located nearly 40 light years away in the constellation of Aquarius —- formed under very different circumstances than our own solar system.

If these planets did accrete directly from their star’s protoplanetary disk, rather than by a more random and violent, repeated accretion process, Asphaug says he would not be surprised that they ended up lower density.

That’s because the kind of giant impact accretion that we believe made Earth, Venus, Mars, Mercury and the Moon was a wasteful process and disposed of a fraction of the lower-density materials in each collision (a few percent to tens of percent), says Asphaug.

“The TRAPPIST-1 system would in this sense maybe resemble the progenitor bodies from which the Earth and Venus were ultimately built, being less dense because they never suffered these giant impacts,” said Asphaug.

Even if the individual terrestrial planets from TRAPPIST-1 and the Solar System look very similar in terms of mass and size, the full system architecture is very different, Simon Grimm, one of the paper’s co-authors and an astrophysicist at the University of Bern, told me. The new mass estimates show that the densities of all seven planets are much more similar than we thought before, he says.

As for their actual compositions?

The fact that these planets are less dense than the terrestrial planets of our solar system indicates a different composition to be sure, says Asphaug. Whether this is because of less iron available to condense to the core, or more water mixed into the crust and mantle remains unclear, he says.

NASA says that it could be that the TRAPPIST planets have a composition similar to Earth’s —- composed of about the same ratios of iron, oxygen, magnesium and silicon as our own planet.  The crucial difference appears to be that the TRAPPIST planets simply lack Earth’s abundance of iron.  Or it may be that the iron in the TRAPPIST planets are simply infused with higher levels of oxygen than Earth, creating iron oxide (or rust).  This hypothetical additional oxygen, says NASA, would also act to decrease the planets’ densities.

Even so, the team reports that the TRAPPIST’s system’s three innermost planets are severely lacking in water. TRAPPIST-1b, TRAPPIST-1c and TRAPPIST-1d are unlikely to harbor water (in any phase) at all, Martin Turbet, an astrophysicist at the University of Geneva and a co-author of the study, told me.

“For these three [inner] planets, the prospects for habitability are therefore not very good,” said Turbet.

But Turbet says that even though the inner three planets may be biological wastelands, three of the outer four planets of this system still offer some hope of habitability.

NASA’S forthcoming James Webb Space Telescope (JWST) will likely be able to observe the atmosphere of these planets and bring considerable progress in determining their potential habitability, says Turbet.



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