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TRAPPIST-1 planets
This illustration shows the seven Earth-size planets of TRAPPIST-1, an exoplanet system about 39 light-years away. The image shows the relative sizes of planets b through h, from left to right, but does not represent their orbits to scale. (NASA / JPL-Caltech Illustration)

If you had to pick a place to set up shop amid the seven planets in the TRAPPIST-1 star system, 39 light-years from Earth, the fourth rock from that alien sun is the best place to start.

That Earth-sized world, known as TRAPPIST-1 e, came out on top in a recent round of exoplanetary climate modeling, detailed in a paper published Nov. 1 by the Astrophysical Journal.

Not that anyone’s planning on setting up shop there soon: Unless there’s a breakthrough that allows us to travel at a significant fraction of the speed of light, it would take hundreds of thousands of years to get to TRAPPIST-1. But the climate modeling methods developed for the TRAPPIST-1 system could help scientists decide which planets to target first with telescopes capable of analyzing alien atmospheres.

“We are modeling unfamiliar atmospheres, not just assuming that the things we see in the solar system will look the same way around another star,” study lead author Andrew Lincowski, a doctoral student at the University of Washington, explained in a news release issued today. “We conducted this research to show what these different types of atmospheres could look like.”

TRAPPIST-1 took the spotlight last year when astronomers reported detecting seven roughly Earth-sized planets, six of which seemed to be rocky planets like our own. If these planets were in our own solar system, they’d all be cooking within the orbit of Mercury. But because TRAPPIST-1 is a faint red dwarf, scientists said three of the planets are inside the star’s habitable zone, where surface temperatures could allow for liquid water.

The newly published research, which takes pains to include the effects of atmospheric photochemistry, throws a bit of a wet blanket on hopes for habitability. Lincowski and his colleagues say there’s a good chance that all of the planets could be like Venus, where strong ultraviolet radiation has broken apart water molecules and dried up any oceans that ever existed.

TRAPPIST-1’s closest-in known planet, called TRAPPIST-1 b, definitely suffered that hellish fate: The climate model suggests the planet is so hot it couldn’t even harbor the sulfuric acid clouds that surround Venus. A bit farther out, the c and d planets are likely to be Venus-like, with a dry surface and a dense, inhospitable atmosphere.

The three farthest-out planets — f, g and h — would be Venus-like or frozen, depending on how much water formed on those worlds during their evolution. Although TRAPPIST-1 f and TRAPPIST-1 g might be technically within the habitable zone, the climate modeling suggests that their atmospheres consist mainly of carbon dioxide. Thus, they can’t have Earthlike environments, the researchers say.

That leaves TRAPPIST-1 e. Like the closer-in planets, the “e” planet would have been battered by ultraviolet light. But its size and orbital location leave the door open for a goodly amount of water to exist throughout the planet’s infancy. What’s more, the atmosphere could have an Earthlike oxygen content..

“If planet TRAPPIST-1 e did not lose all of its water during this phase, today it could be a water world, completely covered by a global ocean,” Lincowski said. “In this case, it could have a climate similar to Earth.”

The climate model used by the UW-led team meshes well with earlier results produced using a different model for the d, e and f planets.

As next-generation telescopes swing into operation, astronomers will be able to check predictions from computer models against actual data about exoplanetary atmospheres. “Our work informs the scientific community of what we might expect to see for the TRAPPIST-1 planets with the upcoming James Webb Space Telescope,” said study co-author Jacob Lustig-Yaeger, a UW doctoral student in astronomy.

Those observations, in turn, should lead to improvements in the computer modeling for long-term planetary processes.

“The processes that shape the evolution of a terrestrial planet are critical to whether or not it can be habitable, as well as our ability to interpret possible signs of life,” said study co-author Victoria Meadows, director of UW’s Astrobiology Program and principal investigator for the NASA Astrobiology Institute’s Virtual Planetary Laboratory. “This paper suggests that we may soon be able to search for potentially detectable signs of these processes on alien worlds.”

In addition to Lincowski, Lustig-Yaeger and Meadows, the authors of the paper in the Astrophysical Journal, titled “Evolved Climates and Observational Discriminants for the TRAPPIST-1 Planetary System,” include David Crisp, Tyler Robinson, Rodrigo Luger and Giada Arney.

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