New climate models of small star TRAPPIST 1’s seven intriguing worlds

Modeling unfamiliar atmospheres.


In a new study, scientists are modeling atmospheres, not just assuming that the things we see in the solar system will look the same way around another star. They even have provided updated climate models for the seven planets around the star TRAPPIST-1.

TRAPPIST-1, 39 light-years or about 235 trillion miles away, is about as small as a star can be and still be a star. A relatively cool “M dwarf” star — the most common type in the universe — it has about 9 percent the mass of the sun and about 12 percent its radius. TRAPPIST-1 has a radius only a little bigger than the planet Jupiter, though it is much greater in mass.

All seven of TRAPPIST-1’s planets are about the size of Earth, and three of them — planets labeled e, f, and g — are believed to be in their habitable zone, that swath of space around a star where a rocky planet could have liquid water on its surface, thus giving life a chance. TRAPPIST-1 d rides the inner edge of the habitable zone, while farther out, TRAPPIST-1 h orbits just past that zone’s outer edge.

Through this work, scientists wanted to know what these different atmospheres could look like. They believe that their work could help them study planets around stars unlike our sun and better use the limited, expensive resources of the James Webb Space Telescope.

Scientists found that to an extremely hot, bright early stellar phase, each of the seven of the star’s worlds may have developed like Venus, with any early oceans they may have had evaporating and leaving thick, appalling climates. Be that as it may, one planet, TRAPPIST-1 e, could be an Earthlike ocean world worth further investigation, as past research additionally has demonstrated.

The UW doctoral student said, “This is a whole sequence of planets that can give us insight into the evolution of planets, particularly around a star that’s very different from ours, with different light coming off of it. It’s just a gold mine.”

“Previous papers have modeled TRAPPIST-1 worlds, but he and this research team tried to do the most rigorous physical modeling that we could in terms of radiation and chemistry — trying to get the physics and chemistry as right as possible.”

The group’s radiation and science models make spectral or wavelength marks for every conceivable atmospheric gas, empowering observers to better predict where to search for such gases in exoplanet airs.

Lincowski said, “When traces of gases are actually detected by the Webb telescope or others, someday, astronomers will use the observed bumps and wiggles in the spectra to infer which gases are present — and compare that to work like ours to say something about the planet’s composition, environment and perhaps its evolutionary history.”

“People are used to thinking about a planet’s habitability around stars similar to the sun. But M dwarf stars are very different, so you really have to think about the chemical effects on the atmosphere(s) and how that chemistry affects the climate.”

When scientists combined terrestrial climate modeling with photochemistry models, their model indicated that:

  • TRAPPIST-1 b, the closest to the star, is a blazing world too hot even for clouds of sulfuric acid, as on Venus, to form.
  • Planets c and d receive slightly more energy from their star than Venus and Earth do from the sun and could be Venus-like, with a dense, uninhabitable atmosphere.
  • TRAPPIST-1 e is the most likely of the seven to host liquid water on a temperate surface and would be an excellent choice for further study with habitability in mind.
  • The outer planets f, g, and h could be Venus-like or frozen, depending on how much water formed on the planet during its evolution.

Lincowski said, “In actuality, any or all of TRAPPIST-1’s planets could be Venus-like, with any water or oceans long burned away. When water evaporates from a planet’s surface, ultraviolet light from the star breaks apart the water molecules, releasing hydrogen, which is the lightest element and can escape a planet’s gravity.”

“This could leave behind a lot of oxygen, which could remain in the atmosphere and irreversibly remove water from the planet. Such a planet may have a thick oxygen atmosphere — but not one generated by life, and different from anything yet observed.”

“This may be possible if these planets had more water initially than Earth, Venus or Mars. 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. In this case, it could have a climate similar to Earth.”

Co-author Jacob Lustig-Yaeger, a UW astronomy doctoral student, said, “Before we knew of this planetary system, estimates for the detectability of atmospheres for Earth-sized planets were looking much more difficult.”

“The star being so small will make the signatures of gases (like carbon dioxide) in the planet’s atmospheres more pronounced in telescope data.”

“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.”

Victoria Meadows, professor of astronomy and director of the UW’s Astrobiology Program, said, “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. This paper suggests that we may soon be able to search for potentially detectable signs of these processes on alien worlds.”

Scientists have published their study in Astrophysical Journal.

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