TRAPPIST-1 Planets Have Remarkably Similar Densities, Study Shows
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The seven rocky exoplanets orbiting TRAPPIST-1, an ultracool dwarf star located 38.8 light-years away in the constellation Aquarius, are about 8% less dense than they would be if they had the same makeup as Earth, according to new research led by a University of Washington, Seattle astrophysicist.

Agol et al. measure the density of the seven TRAPPIST-1 planets. Image credit: NASA / JPL-Caltech.

“The densities of the eight planets in our own Solar System vary widely,” said lead author Professor Eric Agol from the University of Washington, Seattle, and colleagues.

“The puffy, gas-dominated giants — Jupiter, Saturn, Uranus, and Neptune — are larger but much less dense than the four terrestrial worlds because they’re composed mostly of lighter elements like hydrogen and helium.”

“Even the four terrestrial worlds show some variety in their densities, which are determined by both a planet’s composition and compression due to the gravity of the planet itself.”

“By subtracting the effect of gravity, we can calculate what’s known as a planet’s uncompressed density and potentially learn more about a planet’s composition.”

The researchers analyzed the complete set of transit-time measurements of the seven TRAPPIST-1 planets from NASA’s Spitzer Space Telescope, augmented by additional transits from the ground, the NASA/ESA Hubble Space Telescope, and NASA’s Kepler/K2 mission.

“Since we can’t see the planets directly, we analyze in detail the variations of the apparent brightness of their star as they transit it,” said co-author Dr. Michaël Gillon, an astrophysicist at the Université de Liège.

They found that the planets possess similar densities — the values differ by no more than 3%. This makes the system quite different from our own.

“The difference in density between the TRAPPIST-1 planets and Earth and Venus may seem small — about 8% — but it is significant on a planetary scale,” they said.

“For example, one way to explain 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%.”

“Alternatively, the iron in the TRAPPIST-1 planets might be infused with high levels of oxygen, forming iron oxide. The additional oxygen would decrease the planets’ densities.”

“By contrast, if the lower density of the TRAPPIST-1 planets were caused entirely by oxidized iron, the planets would have to be rusty throughout and could not have solid iron cores.”

“The answer might be a combination of the two scenarios — less iron overall and some oxidized iron,” Professor Agol said.

“Our new study greatly improved the precision of the densities of the planets, the measurements obtained indicating very similar compositions for these seven worlds,” said co-author Elsa Ducrot, a doctoral student at the Université de Liège.

“This could mean that they contain roughly the same proportion of materials that make up most rocky planets, such as iron, oxygen, magnesium and silicon, which make up our planet.”

The densities of the TRAPPIST-1 planets. Image credit: NASA / JPL-Caltech.

The scientists also looked into whether the surface of each TRAPPIST-1 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 account for about 5% of the total mass of the outer four planets. By comparison, water makes up less than one-tenth of 1% of Earth’s total mass.

Because they’re positioned too close to their star for water to remain a liquid under most circumstances, the three inner TRAPPIST-1 planets would require hot, dense atmospheres like Venus’, such that water could remain bound to the planet as steam.

“This explanation seems less likely because it would be a coincidence for all seven planets to have just enough water present to have such similar densities,” Professor Agol said.

“The TRAPPIST-1 system is fascinating because around this one star we can learn about the diversity of rocky planets within a single system,” said co-author Dr. Caroline Dorn, an astrophysicist at the University of Zurich.

“And we can actually learn more about a planet by studying its neighbors as well, so this system is perfect for that.”

The results were published in the Planetary Science Journal.

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Eric Agol et al. 2021. Refining the Transit-timing and Photometric Analysis of TRAPPIST-1: Masses, Radii, Densities, Dynamics, and Ephemerides. Planet. Sci. J 2, 1; doi: 10.3847/PSJ/abd022

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