A new study has found that the seven planets orbiting the nearby ultracool dwarf star TRAPPIST-1 are made primarily of rock, and some could potentially contain more water than Earth. The densities of the planets, now known with much more precision than before, suggest that some of them could have up to 5 percent of their mass in the form of water, about 250 times more than Earth's oceans. The hottest planets closest to their parent star are likely to have dense, smoky atmospheres, and the most distant ones are likely to have icy surfaces. In terms of size, density and the amount of radiation it receives from its star, the fourth planet is the most similar to Earth. It appears to be the rockiest planet of the seven and has the potential to host liquid water.
Planets around the faint red star TRAPPIST-1, just 40 light-years from Earth, were first detected by the TRAPPIST-Sur telescope at ESO's La Silla Observatory in 2016. More observations were made the following year Using ground-based telescopes including NASA's Very Large Telescope and Spitzer Space Telescope revealed that there were no fewer than seven planets in the system, each about the same size as Earth. They are called TRAPPIST-1b,c,d,e,f,g and h, as the distance from the central star increases.
More observations have now been made, both from ground-based telescopes, including the almost complete SPECULOOS facility at ESO's Paranal Observatory, and from NASA's Spitzer Space Telescope and the Kepler Space Telescope. A team of scientists led by Simon Grimm from the University of Bern in Switzerland has applied highly complex computer modeling methods to all available data and determined the densities of the planets with much greater precision than was possible before.
Simon Grimm explains how the masses meet: “The planets in TRAPPIST-1 are so close together that they gravitationally interfere with each other, so the times they pass in front of the star change slightly. These changes depend on the planets' masses, their distances, and other orbital parameters. computer, we simulate the orbits of the planets until the calculated transits agree with the observed values and, therefore, we derive the planetary masses.”
Team member Eric Agol comments on the meaning: “For some time, one of the goals of exoplanet studies has been to investigate the composition of planets similar to Earth in size and temperature. The discovery of TRAPPIST-1 and the capabilities of the ESO facilities in Chile and the Space Telescope NASA's Spitzer in orbit has made this possible, giving us our first idea of what Earth-sized exoplanets are made of!”
Measurements of the densities, when combined with models of the planets' composition, strongly suggest that TRAPPIST-1's seven planets are not barren rocky worlds. They appear to contain significant amounts of volatile material, probably water, in some cases representing up to 5% of the planet's mass: an enormous amount; By comparison, the Earth is only about 0.02% water by mass!
“Densities, while important clues about the composition of planets, say nothing about habitability. However, our study is an important step forward as we continue to explore whether these planets could support life.” said Brice-Olivier Demory, co-author from the University of Bern.
TRAPPIST-1b and c, the innermost planets, likely have rocky cores and are surrounded by atmospheres much thicker than Earth's. Meanwhile, TRAPPIST-1d is the lightest of the planets, at about 30 percent the mass of Earth. Scientists are not sure if it has a large atmosphere, an ocean or an ice sheet.
Scientists were surprised that TRAPPIST-1e is the only planet in the system slightly denser than Earth, suggesting it may have a denser iron core and does not necessarily have an atmosphere, ocean or thick ice shell. . It is mysterious that TRAPPIST-1e appears to have a much rockier composition than the rest of the planets. In terms of size, density, and the amount of radiation it receives from its star, this is the planet most similar to Earth.
TRAPPIST-1f, g and h are far enough from the host star that water can freeze into ice across their surfaces. If they have thin atmospheres, they are unlikely to contain the heavy molecules we find on Earth, such as carbon dioxide.
“It is interesting that the densest planets are not those closest to the star, and that the coldest planets cannot support thick atmospheres,” says Caroline Dorn, a co-author of the study based at the University of Zurich, Switzerland.
The TRAPPIST-1 system will continue to be the subject of intense scrutiny in the future with many installations on the ground and in space, including the ESO Extremely Large Telescope and the NASA/ESA/CSA James Webb Space Telescope.
Astronomers are also working hard to look for more planets around faint red stars like TRAPPIST-1. As team member Michaël Gillon explains: “This result highlights the enormous interest in exploring nearby ultracool dwarf stars, like TRAPPIST-1, for transiting terrestrial planets. This is exactly the goal of SPECULOOS, our new search for exoplanets that is about to start operating at the Paranal Observatory of ESO in Chile”.
Image credit: ESO/M. Kornmesser
Explanation from: https://www.eso.org/public/news/eso1805/
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