Astronomers used the Hubble Space Telescope to learn more about the Trappist system by studying the planets' atmospheres as they passed in front of their star, appearing as a dark, traveling dot from the observer's point of view.
Unlike the planets in the Earth's solar system, the planets in the TRAPPIST-1 system are more tightly knit, meaning a human on any of the planets would be able to see the other in the sky.
This artist's impression shows several of TRAPPIST-1 planets. But because the star is faint, its habitable zone - the swath of space around it just right to allow an orbiting rocky planet to sustain water on its surface - lies closer in, so orbiting planets could still, in theory, hold liquid water.
Dr Amaury Triaud, from the University of Birmingham, a leading member of the global team, said: "Of the seven planets, and of all the exoplanets that have been identified so far, Trappist-1e is the most resembling Earth, when we consider the amount of energy a planet receives from its star, and its density, which reflects its internal composition". Researchers determined that all of the planets are mostly made of rock. The planets' densities suggest that some could have up to 5% of their mass in water - which is 250 times more than the oceans on Earth.
Because conventional telescopes aren't powerful enough to get useful information about the planets directly, astronomers have to rely on other methods.
Having a better idea of each planet's radius and mass, as well as new information about the star itself, should give scientists a greater understanding of each planet's density.
One reason that scientists have to believe that the TRAPPIST-1 system may play host to life is the concept of "panspermia". On planets further from the star, a layer of ice may cover the surface, and on the second planet from the star, a thick atmosphere of water vapor is likely present.
The density of a planet doesn't have any direct impact on whether or not it's habitable.
"Densities, while important clues to the planets' compositions, do not say anything about habitability", said Brice-Olivier Demory, study co-author from the University of Bern.
In 2016, Hubble observations also did not find evidence for hydrogen atmospheres in c and d. It may have a denser iron core and does not necessarily have a thick atmosphere, ocean or ice layer. If they have thin atmospheres, they would be unlikely to contain the heavy molecules that we find on Earth, such as carbon dioxide.
"The presence of puffy, hydrogen-dominated atmospheres would have indicated that these planets are more likely gaseous worlds like Neptune", says Julien de Wit, lead author of the study.
This graph presents known properties of the seven TRAPPIST-1 exoplanets (labeled b thorugh h), showing how they stack up to the inner rocky worlds in our own solar system. This is called a transit. The Spitzer Space Telescope, the Kepler Space Telescope, and the SPECULOOS (Search for habitable Planets EClipsing ULtra-cOOl Stars) facility at ESO's Paranal Observatory were all used in the study. The amount by which the starlight dims is related to the radius of the planet. Because the TRAPPIST-1 planets are all squashed in so closely, their gravitational pulls can mess with each other's orbits. It's a "35-dimensional problem", according to the researchers, because it's based on the variations in the timing of each planet's orbit.
The next step in exploring the TRAPPIST system will be with the help of the soon-to-be-launched James Webb Space Telescope. The image does not show the planets' orbits to scale, but highlights possibilities for how the surfaces of these intriguing worlds might look. But in multi-planet systems like TRAPPIST-1, there is a way.
"Our conceptions of what these planets look like today may change dramatically over time", said Robert Hurt, senior visualization scientist at the Spitzer Science Center.
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