JWST provides new insights into a distant star and its exoplanets

A team of astronomers led by the University of Montréal (UdeM) has recently made significant advancements in understanding the mysterious TRAPPIST-1 exoplanetary system, which was first discovered in 2016 and has sparked speculation regarding its potential to host human life.

Published in the Astrophysical Journal Letters, the findings by astronomers at UdeM’s Trottier Institute for Research on Exoplanets (iREx) and colleagues in Canada, the U.K., and the U.S. have shed light on the complex interplay between stellar activity and exoplanet characteristics, underscoring the importance of parent stars in studying exoplanets.

Potentially habitable planets

Located about 40 light-years away from Earth, TRAPPIST-1 is a star much smaller and cooler than our sun. 

The star has captured the attention of scientists and space enthusiasts alike with its seven Earth-sized exoplanets, three of which are within its habitable zone, fueling hopes of finding potentially habitable environments beyond our solar system.

Focus of the study 

The research, led by iREx doctoral student Olivia Lim, employed the powerful James Webb Space Telescope (JWST) to observe TRAPPIST-1 b, the exoplanet orbiting closest to the system’s star. 

Their observations were collected as part of the largest Canadian-led General Observers (GO) program during the JWST’s first year of operations, which also included observations of three other planets in the system, TRAPPIST-1 c, g, and h, using the Canadian-made NIRISS instrument aboard the JWST.

First spectroscopic observations

“These are the very first spectroscopic observations of any TRAPPIST-1 planet obtained by the JWST, and we’ve been waiting for them for years,” said Lim, the GO program’s principal investigator.

The experts used transmission spectroscopy to peer deeper into this distant world. By analyzing the central star’s light after it has passed through the exoplanet’s atmosphere during a transit, astronomers can identify the unique fingerprint left behind by the molecules and atoms found within that atmosphere.

“This is just a small subset of many more observations of this unique planetary system yet to come and to be analyzed,” said co-author René Doyon, the principal investigator of the NIRISS instrument. “These first observations highlight the power of NIRISS and the JWST in general to probe the thin atmospheres around rocky planets.”

Stellar contamination 

A key finding of the study was the significant role of stellar activity and contamination in determining the nature of an exoplanet. 

Stellar contamination refers to the influence of the star’s own features, such as dark spots and bright faculae, on the measurements of the exoplanet’s atmosphere. 

The scientists found compelling evidence that stellar contamination plays a crucial role in shaping the transmission spectra of TRAPPIST-1 b and, likely, the other planets in the system. 

This contamination can create “ghost signals” that may fool the observer into thinking they have detected a particular molecule in the exoplanet’s atmosphere.

Signatures of stellar activity

“In addition to the contamination from stellar spots and faculae, we saw a stellar flare, an unpredictable event during which the star looks brighter for several minutes or hours,” said Lim. 

“This flare affected our measurement of the amount of light blocked by the planet. Such signatures of stellar activity are difficult to model but we need to account for them to ensure that we interpret the data correctly.”

Study implications 

These findings highlight the importance of considering stellar contamination when planning future observations of all exoplanetary systems, particularly for systems like TRAPPIST-1, which is centered around a red dwarf star known for being particularly active with starspots and frequent flare events.

Based on the collected JWST observations, Lim and her team explored a range of atmospheric models for TRAPPIST-1 b, examining various possible compositions and scenarios. They found they could confidently rule out the existence of cloud-free, hydrogen-rich atmospheres, indicating that there appears to be no clear, extended atmosphere around TRAPPIST-1 b. 

However, the data could not confidently exclude thinner atmospheres composed of pure water, carbon dioxide, or methane, nor an atmosphere similar to that of Titan, a moon of Saturn with its own atmosphere.

These results are consistent with previous JWST observations of TRAPPIST-1 b and confirm that Canada’s NIRISS instrument is a highly performing, sensitive tool able to probe atmospheres on Earth-sized exoplanets at impressive levels.

Image Credit: Benoît Gougeon, Université de Montréal

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