Precise measurements reveal that the exoplanets have remarkably similar densities, which provides clues about their composition.
The red dwarf star TRAPPIST-1 is home to the largest group of roughly Earth-size planets ever found in a single stellar system.Located about 40 light-years away, these seven rocky siblings provide an example of the tremendous variety of planetary systems that likely fill the universe.
A new study published today in the Planetary Science Journal shows that the TRAPPIST-1 planets have remarkably similar densities.That could mean they all contain about the same ratio of materials thought to compose most rocky planets, like iron, oxygen, magnesium, and silicon.
But if this is the case, that ratio must be notably different than Earth’s: The TRAPPIST-1 planets are about 8% less dense than they would be if they had the same makeup as our home planet.
Based on that conclusion, the paper authors hypothesized a few different mixtures of ingredients could give the TRAPPIST-1 planets the measured density.
Some of these planets have been known since 2016, when scientists announced that they’d found three planets around the TRAPPIST-1 star using the Transiting Planets and Planetesimals Small Telescope (TRAPPIST) in Chile.Subsequent observations by NASA’s now-retired Spitzer Space Telescope, in collaboration with ground-based telescopes, confirmed two of the original planets and discovered five more.
Click on this interactive visualization to explore the TRAPPIST-1 planets in their orbit around a small, faint red dwarf star.All seven TRAPPIST-1 planets, which are so close to their star that they would fit within the orbit of Mercury, were found via the transit method: Scientists can’t see the planets directly (they’re too small and faint relative to the star), so they look for dips in the star’s brightness created when the planets cross in front of it.Previous calculations determined that the planets are roughly the size and mass of Earth and thus must also be rocky, or terrestrial – as opposed to gas-dominated, like Jupiter and Saturn.
The new paper offers the most precise density measurements yet for any group of exoplanets – planets beyond our solar system.
The densities of the eight planets in our own solar system vary widely.Uncompressed density adjusts for the effect of gravity and can reveal how the composition of various planets compare.
The seven TRAPPIST-1 planets possess similar densities – the values differ by no more than 3%.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.
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%, according to the study.
Alternatively, the iron in the TRAPPIST-1 planets might be infused with high levels of oxygen, forming iron oxide, or rust.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.
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.But Agol says 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.
All seven planets have very similar densities, so they likely have a similar compositions.
“The TRAPPIST-1 system is fascinating because around this one star we can learn about the diversity of rocky planets within a single system.
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