U-M researchers have helped publish the first results from JWST observations of a planet called TRAPPIST-1 e. It's orbiting its host star in the 'Goldilocks zone'-not too hot and not too cold, but potentially just right for being capable of supporting life.
Studies:
JWST-TST DREAMS: NIRSpec/PRISM Transmission Spectroscopy of the Habitable Zone Planet TRAPPIST-1 e (DOI: 10.3847/2041-8213/adf42e)
JWST-TST DREAMS: Secondary Atmosphere Constraints for the Habitable Zone Planet TRAPPIST-1 e (10.3847/2041-8213/adf62e)
University of Michigan researchers in an international team revealed that a rocky planet in a distant star's "habitable zone" may have an atmosphere. Potentially, this means it could have the requisite ingredients for supporting life as we know it.
In two separate studies published in the Astrophysical Journal Letters, researchers have shed new light on an Earth-sized exoplanet 40 light-years away where liquid water might exist on its surface. That would only be possible if an atmosphere is present on this planet, dubbed TRAPPIST-1 e. Still, there's enough uncertainty that some mystery still shrouds this exoplanet, a planet outside our solar system.
"We are seeing two possible explanations. The most exciting possibility is that TRAPPIST-1 e could have a so-called secondary atmosphere containing heavy gases like nitrogen. But our initial observations cannot yet rule out a bare rock with no atmosphere," said Ryan MacDonald, who worked on the studies as a NASA Sagan Fellow in the U-M Department of Astronomy. He's now a lecturer at the University of St. Andrews in Scotland.
Still, scientists are now one step closer to cracking the case conclusively thanks to the world's largest telescope in space, the JWST. The JWST mission is led by NASA with support from the European Space Agency and the Canadian Space Agency. The two studies were led by Néstor Espinoza of the Space Telescope Science Institute and Ana Glidden of the Massachusetts Institute of Technology's Kavli Institute for Astrophysics and Space Research. More than 30 scientists from the U.S., the U.K. and India were involved in the new study.
"In the coming years we will go from four JWST observations of TRAPPIST-1 e to nearly twenty," MacDonald said. "We finally have the telescope and tools to search for habitable conditions in other star systems, which makes today one of the most exciting times for astronomy."
World of (fewer) possibilities
Of the seven Earth-sized worlds orbiting the red dwarf star TRAPPIST-1, planet e is of particular interest because it orbits the star at a distance where it's not too hot or too cold to prohibit water on its surface. Still, that would only be possible if the planet has an atmosphere.
"TRAPPIST-1 e has long been considered one of the best habitable zone planets to search for an atmosphere," MacDonald said. "But when our observations came down in 2023, we quickly realized that the system's red dwarf star was contaminating our data in ways that made the search for an atmosphere extremely challenging."
That's where the JWST comes in. Researchers aimed the telescope's powerful Near-Infrared Spectrograph, or NIRSpec, instrument at the system as planet e transited, or passed in front of, its star.
During transit, starlight that passes through the planet's atmosphere, if there is one, will be partially absorbed. That creates dips in the light spectrum that reaches JWST and these tell astronomers what chemicals are found there. With each additional transit, JWST collects more data and the planet's atmospheric contents will become clearer.
"When JWST was being designed, we only knew of a handful of planets around a few stars. Over time, it became clear that JWST would be a powerful tool to help figure out their composition," said Michael Meyer, professor and chair of the U-M Department of Astronomy. Meyer was not directly involved in either study but helped develop instrumentation and exoplanet science programs for more than two decades.
Multiple possibilities remain open for TRAPPIST-1 e because only four transits have been analyzed so far. But researchers are confident that the planet does not still have its original, or primary, atmosphere. TRAPPIST-1 is a very active star, with frequent flares, so researchers believe that any earlier primary atmosphere, made of the lightest elements, hydrogen and helium, would have been stripped off by stellar radiation.
However, many planets, including Earth, build up a secondary atmosphere with heavier elements and compounds after losing their primary atmosphere. The team behind the new studies says, based on current data, there are about equal odds for planet e having a secondary atmosphere and not having one-that is, being a bare rock.
If there is a secondary atmosphere and liquid water on TRAPPIST-1 e, the researchers say it would be accompanied by a greenhouse effect. Here, various gases, particularly carbon dioxide, keep the atmosphere stable and the planet warm. Although Mars and Venus have carbon-dioxide rich atmospheres, too, whatever is on TRAPPIST-1 e is going to look much different, said Nikole Lewis, associate professor at Cornell University and the subject lead for exoplanet transit spectroscopy on the JWST telescope scientist team.
"TRAPPIST-1 is a very different star from our sun, and so the planetary system around it is also very different, which challenges both our observational and theoretical assumptions," Lewis said. Still, she added, "a little greenhouse effect goes a long way,"
Current measurements do not rule out adequate carbon dioxide to sustain some water on the surface. According to the team's analysis, the water could take the form of a global ocean or cover a smaller area of the planet where the star is at perpetual noon, surrounded by ice.
