A new study published by the Planetary Science Journal shows that the planets TRAPPIST-1 have very similar densities. They all probably contain roughly the same ratio of materials such as iron, oxygen, magnesium, and silicon. But if this is the case, this ratio should be markedly different from that of the Earth: TRAPPIST-1 planets are about 8% less dense than Earth.
TRAPPIST-1 is home to the largest group of planets, roughly the size of Earth, ever found in a single star system. These seven rocky planets, located about 40 light-years from Earth, are examples of the vast variety of planetary systems that populate the universe.
Some of these planets have been known since 2016, when scientists announced that they had discovered three planets around the star TRAPPIST-1 with the Transiting Planets and Planetesimals (TRAPPIST) small telescope in Chile. Follow-up observations by NASA’s Spitzer Space Telescope, in collaboration with ground-based telescopes, confirmed the presence of two original planets and discovered five more.
All seven TRAPPIST-1 planets that are so close to their star that they fit into the orbit of Mercury were discovered using the transit method: scientists cannot see the planets directly (they are too small and dim compared to the star), so they search dips in the brightness of a star arising from the intersection of planets in front of it.
Repeated observations of the incidence of starlight, combined with measurements of the orbital times of planets, allowed astronomers to estimate the masses and diameters of the planets, which, in turn, were used to calculate their density. Previous calculations determined that the planets are roughly equal in size and mass to the Earth and therefore must also be rocky or terrestrial, unlike gas-dominated planets such as Jupiter and Saturn.
The more accurately scientists know the density of a planet, the more restrictions they can impose on its composition. The density of the eight planets in our solar system varies greatly. The gas-dominated giants – Jupiter, Saturn, Uranus, and Neptune – are large but much less dense than the four terrestrial worlds because they are made up mostly of lighter elements like hydrogen and helium. But even they show some variation in density, which is determined by both the composition of the planet and the contraction due to the gravity of the planet itself. By subtracting the effect of gravity, scientists can calculate the planet’s so-called uncompressed density and learn more about its composition.
The seven TRAPPIST-1 planets have similar densities – the values differ by no more than 3%. This distinguishes this system from ours. The density difference between the planets TRAPPIST-1 and Earth and Venus may seem small – about 8%, but on a planetary scale it is significant. For example, one way to explain why TRAPPIST-1 planets are less dense is that they have the same composition as Earth, but with a lower percentage of iron – about 21% compared to 32% on Earth.
As an alternative theory, the iron on the TRAPPIST-1 planets can be saturated with oxygen, forming iron oxide or rust. The extra oxygen would decrease the density of the planets. The surface of Mars takes on a red tint due to iron oxide, but has a core composed of unoxidized iron. If the lower density of TRAPPIST-1 planets were completely caused by oxidized iron, the planets would have to be completely rusty and could not have solid iron cores.
The team also examined whether each planet’s surface could be covered in water, which is even lighter than rust, and which can change the overall density of the planet. If this were the case, water would have to account for about 5% of the total mass of the four outer planets. By comparison, water makes up less than one tenth of one percent of the total mass of the Earth.
Because they are too close to their star for water to remain liquid in most cases, TRAPPIST-1’s three inner planets require a hot and dense Venus-like atmosphere so that water can remain bound to the planet as vapor. This explanation seems less likely because it would be a coincidence if there was enough water on all seven planets to have such the same density.