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Researchers from U-M have contributed to the initial findings from JWST observations of a celestial body named TRAPPIST-1 e. It is orbiting its parent star within the ‘Goldilocks zone’—not excessively hot and not overly cold, but potentially just right for being able to support life.

Researchers from the University of Michigan, as part of an international collaboration, have disclosed that a rocky planet within a distant star’s “habitable zone” may possess an atmosphere. This suggests it might have the essential building blocks for supporting life as we understand it.

In two distinct studies published in the Astrophysical Journal Letters, scientists have illuminated new insights about an Earth-sized exoplanet located 40 light-years away where liquid water could theoretically be present on its surface. Such a scenario would only be feasible if an atmosphere is indeed around this planet, referred to as TRAPPIST-1 e. Nevertheless, uncertainties linger, leaving some enigma still encompassing this exoplanet outside of our solar system.

“We are observing two potential explanations. The most thrilling possibility is that TRAPPIST-1 e could harbor a so-called secondary atmosphere comprised of heavier gases like nitrogen. However, our preliminary observations do not yet dismiss the scenario of a barren rock without an atmosphere,” stated Ryan MacDonald, a contributor to the studies and NASA Sagan Fellow in the U-M Department of Astronomy. He is currently a lecturer at the University of St. Andrews in Scotland.

Nonetheless, thanks to the world’s largest space telescope, the JWST, scientists are now a step closer to unraveling this mystery definitively. The JWST mission is orchestrated by NASA, with backing from the European Space Agency and the Canadian Space Agency. The two studies were spearheaded by Néstor Espinoza from the Space Telescope Science Institute and Ana Glidden from the Kavli Institute for Astrophysics and Space Research at the Massachusetts Institute of Technology. The new study involved over 30 scientists from the U.S., the U.K., and India.

“In the years ahead, we will escalate from four JWST observations of TRAPPIST-1 e to nearly twenty,” MacDonald noted. “We finally possess the telescope and equipment to pursue habitable conditions in various star systems, marking today as one of the most exhilarating periods for astronomy.”

A Realm of (fewer) Opportunities

Among the seven Earth-sized worlds circling the red dwarf star TRAPPIST-1, planet e garners particular attention due to its orbit at a distance allowing conditions that are not excessively hot or cold to hinder water presence on its surface. Yet, this would only be viable if the planet possesses an atmosphere.

“TRAPPIST-1 e has long been regarded as one of the prime habitable zone planets to hunt for an atmosphere,” MacDonald explained. “However, when our observations arrived in 2023, we quickly recognized that the system’s red dwarf star was tainting our data, making the atmosphere search extremely complex.”

This is where the JWST becomes essential. Researchers directed the telescope’s advanced Near-Infrared Spectrograph, or NIRSpec, instrument at the system while planet e transited, or moved in front of, its star.

During transit, starlight that passes through the planet’s atmosphere, if present, will incur partial absorption. This phenomenon results in dips in the light spectrum that reach JWST, enabling astronomers to determine the chemical composition present. With each successive transit, JWST gathers additional data, clarifying the planetary atmospheric constituents.

“When JWST was under design, we only knew of a handful of planets orbiting a few stars. As time progressed, it became evident that JWST would serve as a powerful instrument in helping to ascertain their composition,” remarked Michael Meyer, a professor and chair of the U-M Department of Astronomy. Meyer was not directly involved in the studies but contributed to developing instruments and exoplanet science programs for more than two decades.

Numerous possibilities remain open for TRAPPIST-1 e, as only four transits have been scrutinized thus far. Nonetheless, researchers are confident that the planet does not retain its original or primary atmosphere. TRAPPIST-1 is a highly active star, frequently flaring, leading scientists to believe that any initial primary atmosphere, composed of the lightest elements, hydrogen and helium, would have been eroded by stellar radiation.

However, many planets, including Earth, can develop a secondary atmosphere with heavier elements and compounds after losing their primary atmosphere. According to the team associated with the new studies, based on existing data, the chances for planet e to possess a secondary atmosphere are about equal to the chances of it being a barren rock.

If a secondary atmosphere and liquid water are present on TRAPPIST-1 e, researchers argue that it would likely experience a greenhouse effect. In this environment, various gases, particularly carbon dioxide, help maintain atmospheric stability and warmth on the planet. Although Mars and Venus also have carbon-dioxide-rich atmospheres, what exists on TRAPPIST-1 e will significantly differ, noted Nikole Lewis, an associate professor at Cornell University and the subject lead for exoplanet transit spectroscopy on the JWST telescope scientific team.

“TRAPPIST-1 is a considerably different star compared to our sun, making the planetary system surrounding it equally unique, which challenges both our observational and theoretical premises,” Lewis remarked. Still, she added, “a slight greenhouse effect can have substantial impacts.”

Present measurements do not eliminate the possibility of sufficient carbon dioxide to sustain some water on the surface. Based on the team’s analysis, this water could manifest as a global ocean or cover a smaller expanse of the planet, situated where the star is in a constant state of noon, surrounded by ice.

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