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January 22, 2021
Artistic representation of the star TRAPPIST-1 and its seven worlds.NASA / JPL-Caltech / R. Injured (IPAC)
The TRAPPIST-1 star system is home to the largest cluster of roughly Earth-sized planets ever found outside of our solar system. Discovered in 2016 some 40 light years away, these seven rocky brothers and sisters offer a glimpse into the wide variety of planetary systems that likely fill the universe.
A study accepted by the Planetary Science Journal shows that planets share similar densities. This could mean that they all contain roughly the same ratio of materials considered common to rocky planets, such as iron, oxygen, magnesium, and silicon. If so, while the TRAPPIST-1 planets might be similar to each other, they appear to differ markedly from Earth: they are about 8% less dense than they would be if they had the same chemical makeup as our planet.
These findings give astronomers new data they are using to try to figure out the precise makeup of these planets and compare them not only to Earth, but to all of the rocky planets in our solar system, according to lead author Eric Agol. , a Professor of Astronomy at the University of Washington.
“This is one of the most accurate characterizations of a set of rocky exoplanets, which has given us highly reliable measurements of their diameters, densities and masses,” Agol said. “This is the information we needed to make assumptions 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 several space and terrestrial telescopes, including the Kepler Space Telescope and NASA’s Spitzer Space Telescope. On its own, Spitzer provided over 1,000 hours of targeted observations of the system before it was decommissioned in January 2020. As they are too small and too faint to be seen directly, the seven exoplanets were found via the method. known as transit: looking for hollows in the star. luminosity created when the planets cross in front of it.
Previous calculations had shown that the planets were roughly the size and mass of Earth and therefore must also be rocky or terrestrial – as opposed to gas-dominated worlds like Jupiter and Saturn. This new study offers the most accurate density measurements to date for any group of exoplanets.
The density of a planet is determined not only by its composition, but also by its size: gravity compresses the material of a planet, increasing the density of the planet. The uncompressed density adjusts to the effect of gravity and can reveal how the makeup of various planets compare.NASA / JPL-Caltech
“The night sky is full of planets, and it’s only in the last 30 years that we’ve been able to begin to unravel their mysteries,” said co-author Caroline Dorn of the University of Zurich. “The TRAPPIST-1 system is fascinating because around this 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, United Kingdom and Morocco – used observations of starlight troughs and precise measurements of the synchronization of the planets’ orbits. to make detailed measurements of the mass and diameter of each planet, and from there to determine its density. Agol and UW co-authors Zachary Langford and Victoria Meadows, professor of astronomy, analyzed data and performed computer simulations that constrained the orbits of TRAPPIST-1 planets and calculated their densities.
With more precise measurements of an object’s density, we can learn more about its composition. A baseball and a clipboard can be the same size, but the baseball is much lighter. The width and weight together reveal the density of each object, and from this it is possible to deduce that the baseball is made of lighter materials, like twine and leather, while the paperweight has a heavier composition, such as 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 eight planets.
The seven TRAPPIST-1 planets, on the other hand, all share a similar density, making the system very different from ours. The difference in density between the TRAPPIST-1 planets and the Earth, Venus and Mars, may seem small – around 8% – but it is significant on a planetary scale. For example, one way to explain the lower density is that the TRAPPIST-1 planets are similar in composition to Earth, but with a lower percentage of iron – around 21% compared to Earth’s 32%, according to l ‘study.
Alternatively, the iron in the TRAPPIST-1 planets could be infused with high levels of oxygen, forming iron oxide or rust. The additional oxygen would decrease the densities of the planets. The surface of Mars gets its red hue from iron oxide, but like its three terrestrial siblings, it has a core made up of unoxidized iron. On the other hand, if the lower density of the TRAPPIST-1 planets was caused entirely by oxidized iron, then the planets should be rusty and could not have iron cores.
Agol said the answer could be a combination of the two scenarios – less iron overall and some oxidized iron.
An artistic representation of three possible interiors of 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. The seven planets have very similar densities, so they probably have a similar makeup.NASA / JPL-Caltech
The team also investigated whether the surface of each planet could be covered with water, which is even lighter than rust and would change the planet’s overall density. If this were the case, water would have to make up about 5% of the total mass of the four outer planets. In comparison, water represents less than 0.1% of the total mass of the Earth. The three inner planets TRAPPIST-1, positioned too close to their star for water to remain liquid under most circumstances, would require hot and 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 the seven planets to have just enough water to have similar densities, according to Agol.
When launched, NASA’s James Webb Space Telescope is expected to have the capabilities to probe this system further, including collecting more detailed information about the atmospheres of the seven TRAPPIST-1 worlds.
“There are a lot more questions to be answered about TRAPPIST-1 and its worlds,” Agol said. “And in a way, answering them also helps us understand our own solar system.”
Agol and Meadows are members of the NASA NExSS Virtual Planetary Laboratory team and the UW Astrobiology Program. Agol’s participation in the study was funded by the National Science Foundation, NASA, the Guggenheim Foundation, and the Virtual Planetary Laboratory.
For more information, contact Agol at [email protected].
Adapted from a story by NASA’s Jet Propulsion Laboratory.
Tag (s): astronomy & astrophysics • College of Arts and Sciences • Department of Astronomy • Eric Agol • planetary science
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