News Releases | Research | Science
22 January 2021
Artist’s depiction of the TRAPPIST-1 star and his seven worlds.NASA / JPL-Caltech / R. Seer (IPAC)
The TRAPPIST-1 galaxy is home to the largest number of Earth-sized planets ever found outside our solar system. These seven rocky siblings, discovered about 40 light-years away, offer a glimpse into the vast array of planetary systems that are likely to fill the universe.
A study accepted by the Planetary Science Journal shows that the planets have similar densities. This could mean that they all contain about the same ratio as is commonly considered for rocky planets, such as iron, oxygen, magnesium, and silicon. If this is the case, the TRAPPIST-1 planets appear to be similar to each other, but they look remarkable from Earth: they are about 8% less dense than if they had the same chemical composition as our planet.
These findings give astronomers new data that they use to try to determine the exact composition of these planets, and compare them not only with the earth, but with all the rocky planets in our solar system, according to lead author Eric Agol, University of Washington professor of astronomy.
“This is one of the most precise features of a set of rocky exoplanets, which has given us the confidence of their diameters, densities and masses,” Agol said. “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 discovery of the TRAPPIST-1 worlds in 2016, scientists have studied this planetary family with multiple space and ground telescopes, including the now retired Kepler Space Telescope and Spitzer Space Telescope. Spitzer alone provided more than 1,000 hours of targeted observations of the system before it went live in January 2020. Because they are too small and faint to see directly, all seven exoplanets have been found using the so-called transit method: look for dips in the star’s brightness created when the planets cross in front of it.
Previous calculations have shown that the planets are approximately the size and mass of the earth and therefore must be rocky or nice – in contrast to gas-dominated worlds such as Jupiter and Saturn. This new study provides 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 that makes up a planet, which increases the density of the planet. Uncompressed density adjusts to the effect of gravity and can show how the composition of different planets compares.NASA / JPL-Caltech
“The night sky is full of planets and it is only in the last thirty years that we have 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 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 in the United States, Switzerland, France, the United Kingdom and Morocco – used observations of the starlight dips and accurate measurements of the timing of the planets’ orbits to make detailed measurements of mass and diameter. of each planet. 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 limited the orbits of the TRAPPIST-1 planets and calculated their density.
With more accurate measurements of the density of an object, 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 the density of each object, and from this it can be deduced that the baseball is made of lighter materials, such as rope and leather, while the paper weight has a heavier composition, such as glass or metal.
In our own solar system, the density of the eight planets varies greatly. 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, although it is the smallest planet in the diameter of the solar system, Mercury has the second highest density of all eight planets.
The seven TRAPPIST-1 planets, on the other hand, all have similar densities, making the system completely 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. One way to explain the lower density, for example, 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 can be applied in the TRAPPIST-1 planets with high oxygen levels, which form iron oxide or rust. The additional oxygen will reduce the density of the planets. The surface of Mars gets its red tint of iron oxide, but like its three earthly siblings, it has a core made up of non-oxidized iron. In 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 an iron core.
Agol said the answer could be a combination of the two scenarios – less iron in general 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 limit the variety of possible interiors for that planet. All seven planets have very similar densities, so they probably have a similar composition.NASA / JPL-Caltech
The team also looked at 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 make up about 5% of the total mass of the outer four planets. By comparison, water makes up less than 0.1% of the total mass of the earth. The three inner TRAPPIST-1 planets, positioned too close to their star to hold water under most conditions, require a warm, dense atmosphere like on Venus, where water can remain bound to the planet as steam. But this statement seems less likely, because it would be coincidental for all seven planets to have, according to Agol, just enough water to have such similar densities.
When it launches, NASA’s James Webb Space Telescope should have the capability to further explore this system, including more detailed information about the atmosphere of the seven TRAPPIST-1 worlds.
“There are still many questions to answer about TRAPPIST-1 and its worlds,” Agol said. “And in a way, it also helps us answer our own solar system if we answer it.”
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.
For more information, contact Agol at [email protected].
Process from a story by NASA’s Jet Propulsion Laboratory.
Tag (s): Astronomy & Astrophysics • College of Arts & Sciences • Department of Astronomy • Eric Agol • Planetary Science