TRAPPIST-1 contains the largest group of roughly Earth-sized planets ever discovered in one
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 located like thisclose to their star that they fit into the orbit of Mercury were discovered using the transit method: scientists cannot see planets directly (they are too small and dim compared to the star), so they look for dips in the star's brightness that occur when planets cross in front of her.
Repeated observations of the fall of a starlight, combined with measurements of the planets' orbital times, allowed astronomers to estimate the masses and diameters of the planets, which in turn were used to calculate their densities. Previous calculations determined that the planets were roughly equal in size and mass to Earth and therefore should also be rocky or terrestrial, as opposed to gas-dominated planets such as Jupiter and Saturn.
The more accurately scientists know the density of the planet, themore 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 planets of TRAPPIST-1 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 - iron onplanets TRAPPIST-1 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 the TRAPPIST-1 planets were entirely caused by oxidized iron, the planets would have to be completely rusty and could not have solid iron cores.
The team also examined whether the surface couldeach planet be covered with 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 ownAs a star, in order for water to remain liquid in most cases, the three inner planets of TRAPPIST-1 require a hot and dense atmosphere, similar to Venus, 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.
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