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In the quest for viable exoplanets, atmospheric characteristics are crucial in assessing whether a planet can maintain liquid water. Favorable candidates typically reside in the “Goldilocks zone,” a region that is neither excessively close nor too distant from their host star to permit liquid water. With the initiation of the James Webb Space Telescope (JWST), astronomers are gathering enhanced observations of exoplanet atmospheres that will assist in identifying which exoplanets are promising subjects for additional research.
In an open-access article released today in The Astrophysical Journal Letters, scientists utilized JWST to examine the atmosphere of the exoplanet TRAPPIST-1e, situated in the TRAPPIST-1 system. Although they have yet to uncover conclusive evidence about its composition — or if it possesses an atmosphere at all — they managed to eliminate several options.
“The premise is: If we presume that the planet is not devoid of air, can we limit different atmospheric scenarios? Do those scenarios still facilitate the presence of liquid water on the surface?” states Ana Glidden, a postdoctoral researcher in the MIT Department of Earth, Atmospheric and Planetary Sciences (EAPS) and the MIT Kavli Institute for Astrophysics and Space Research, as well as the primary author of the study. The responses they uncovered were affirmative.
The new findings dismiss a hydrogen-dominant atmosphere and impose stricter limitations on other atmospheric conditions typically generated through secondary processes, such as volcanic activity and outgassing from the planet’s interior. The data were consistent enough to still allow for the potential of a surface ocean.
“TRAPPIST-1e continues to be one of our most intriguing habitable-zone planets, and these fresh results bring us closer to understanding what type of world it is,” remarks Sara Seager, Class of 1941 Professor of Planetary Science at MIT and co-author of the research. “The evidence indicating a divergence from atmospheres similar to Venus and Mars sharpens our focus on the scenarios that remain viable.”
The study’s co-authors also comprise collaborators from the University of Arizona, Johns Hopkins University, the University of Michigan, the Space Telescope Science Institute, and members of the JWST-TST DREAMS Team.
Enhanced observations
Exoplanet atmospheres are examined utilizing a method called transmission spectroscopy. When a planet transits in front of its host star, the starlight traverses the planet’s atmosphere. Astronomers can identify which molecules are present in the atmosphere by observing how light varies across different wavelengths.
“Each molecule bears a spectral signature. You can match your observations against those signatures to discern which molecules might exist,” asserts Glidden.
JWST has a broader wavelength range and heightened spectral resolution compared to its predecessor, the Hubble Space Telescope, allowing for the observation of molecules like carbon dioxide and methane that are more prevalent in our own solar system. Nevertheless, the improved observations have also brought to light the issue of stellar contamination, where fluctuations in the host star’s temperature caused by factors like sunspots and solar flares complicate data interpretation.
“Stellar activity significantly disturbs the interpretation of planetary data because we can only observe a potential atmosphere through starlight,” Glidden explains. “It is difficult to distinguish which signals originate from the star versus those from the planet itself.”
Excluding atmospheric conditions
The researchers employed an innovative strategy to alleviate the impact of stellar activity, and, consequently, “any signal you can observe varying from one visit to another is most likely from the star, while anything that remains consistent across visits is likely from the planet,” Glidden notes.
They then compared the results against various atmospheric scenarios. They concluded that carbon dioxide-rich atmospheres, similar to those of Mars and Venus, are improbable, while a warm, nitrogen-rich atmosphere akin to Saturn’s moon Titan remains a possibility. The evidence, however, is insufficient to confirm the presence of any atmosphere, let alone identify a specific type of gas. Further, ongoing observations are already underway to help refine the possibilities.
“With our initial observations, we have demonstrated the advancements made with JWST. Our follow-up program will aid in further honing our understanding of one of our most promising habitable-zone planets,” states Glidden.
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