The TRAPPIST-1 star system is home to the largest batch of roughly Earth-size planets ever found outside our solar system. Discovered in 2016 some 40 light-years away, these seven rocky siblings offer a glimpse at the tremendous variety of planetary systems that likely fill the universe.
A study accepted by the Planetary Science Journal shows that the planets share similar densities. That could mean they all contain roughly the same ratio of materials thought to be common to rocky planets, such as iron, oxygen, magnesium and silicon. If so, then while the TRAPPIST-1 planets might be similar to each other, they appear to differ notably from Earth: They’re about 8% less dense than they would be if they had the same chemical composition as our planet.
These findings give astronomers new data that they’re using to try to pin down the precise composition of these planets, and compare them not just to Earth, but to all the rocky planets in our solar system, according to lead author Eric Agol, a University of Washington professor of astronomy.
“This is one of the most precise characterizations of a set of rocky exoplanets, which gave us high-confidence measurements of their diameters, densities and masses,” said Agol. “This is the information we needed to make hypotheses about their composition and understand how these planets differ from the rocky planets in our solar system.”
Since the initial detection in 2016 of the TRAPPIST-1 worlds, scientists have studied this planetary family with multiple space- and ground-based telescopes, including NASA’s now-retired Kepler Space Telescope and Spitzer Space Telescope. Spitzer alone provided over 1,000 hours of targeted observations of the system before being decommissioned in January 2020. Since they’re too small and faint to view directly, all seven exoplanets were found via the so-called transit method: looking for dips in the star’s brightness created when the planets cross in front of it.
Previous calculations had shown that the planets are roughly the size and mass of Earth and thus must also be rocky, or terrestrial – as opposed to gas-dominated worlds like Jupiter and Saturn. This new study offers the most precise density measurements to date for any group of exoplanets.
A planet’s density is determined not just by its composition, but also by its size: Gravity compresses the material a planet is made of, increasing the planet’s density. Uncompressed density adjusts for the effect of gravity, and can reveal how the composition of various planets compare.NASA/JPL-Caltech
“The night sky is full of planets, and it’s only been within the last 30 years that we’ve been able to start unraveling their mysteries,” said co-author Caroline Dorn of the University of Zurich. “The TRAPPIST-1 system is fascinating because around this one star we can learn about the diversity of rocky planets within a single system. And we can actually learn more about a planet by studying its neighbors as well, so this system is perfect for that.”
The team – which includes scientists based in the United States, Switzerland, France, the United Kingdom and Morocco – used observations of the starlight dips and precise measurements of the timing of the planets’ orbits to make detailed measurements of each planet’s mass and diameter, and from there to determine its density. Agol and UW co-authors Zachary Langford and Victoria Meadows, a professor of astronomy, analyzed data and performed computer simulations that constrained the orbits of the TRAPPIST-1 planets and calculated their densities.
With more precise measurements of an object’s density, we can know more about its composition. A baseball and a paperweight may be the same size, but the baseball is much lighter. Width and weight together reveal each object’s density, and from there it is possible to infer that the baseball is made of lighter materials, like string and leather, while the paperweight has a heavier composition, like glass or metal.
In our own solar system, the densities of the eight planets vary widely. The gas giants – Jupiter, Saturn, Uranus and Neptune – are larger, but much less dense than the four rocky planets. Earth, Venus and Mars have similar densities, but Mercury contains a much higher percentage of iron, so although it is the solar system’s smallest planet in diameter, Mercury has the second highest density of all eight planets.
The seven TRAPPIST-1 planets, on the other hand, all share a similar density, which makes the system quite different from our own. The difference in density between the TRAPPIST-1 planets and Earth, Venus and Mars, may seem small – about 8% – but it is significant on a planetary scale. For example, one way to explain the lower density is that the TRAPPIST-1 planets 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. The additional oxygen would decrease the planets’ densities. The surface of Mars gets its red tint from iron oxide, but like its three terrestrial siblings, it has a core composed of non-oxidized iron. By contrast, if the lower density of the TRAPPIST-1 planets were caused entirely by oxidized iron, then the planets would have to be rusty throughout and could not have iron cores.
Agol said the answer might be a combination of the two scenarios – less iron overall and some oxidized iron.
An artistic depiction of three possible interiors of the TRAPPIST-1 exoplanets. The more precisely scientists know the density of a planet, the more they can narrow down the range of possible interiors for that planet. All seven planets have very similar densities, so they likely have a similar compositions.NASA/JPL-Caltech
The team also looked into whether the surface of each 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 0.1% of Earth’s total mass. The three inner TRAPPIST-1 planets, positioned too close to their star for water to remain a liquid under most circumstances, would require hot, dense atmospheres like on Venus, where water could remain bound to the planet as steam. But 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, according to Agol.
When it launches, NASA’s James Webb Space Telescope should have the capabilities to probe this system further, including gathering more detailed information about the atmospheres of the seven TRAPPIST-1 worlds.
“There are many more questions to answer about TRAPPIST-1 and its worlds,” said Agol. “And in a way, answering them helps us understand our own solar system, too.”
Agol and Meadows are members of the NASA NExSS Virtual Planetary Laboratory team and the UW Astrobiology Program. Agol’s involvement in the study was funded by the National Science Foundation, NASA, the Guggenheim Foundation and the Virtual Planetary Laboratory.