James Webb Space Telescope’s first spectrum of TRAPPIST-1 planet


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This artistic representation of the TRAPPIST-1 red dwarf star shows its highly active nature. The star appears to have numerous starspots (cool areas on its surface, similar to sunspots) and flares. Exoplanet TRAPPIST-1b, the closest planet to the system’s central star, can be seen in the foreground with no apparent atmosphere. The exoplanet TRAPPIST-1g, one of the planets in the system’s habitable zone, can be seen in the background to the right of the star. The TRAPPIST-1 system consists of seven Earth-sized exoplanets. Credit: Benoît Gougeon, Université de Montréal

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This artistic representation of the TRAPPIST-1 red dwarf star shows its highly active nature. The star appears to have numerous starspots (cool areas on its surface, similar to sunspots) and flares. Exoplanet TRAPPIST-1b, the closest planet to the system’s central star, can be seen in the foreground with no apparent atmosphere. The exoplanet TRAPPIST-1g, one of the planets in the system’s habitable zone, can be seen in the background to the right of the star. The TRAPPIST-1 system consists of seven Earth-sized exoplanets. Credit: Benoît Gougeon, Université de Montréal

In a solar system called TRAPPIST-1, seven Earth-sized planets orbit a cool star 40 light-years from the Sun.

Astronomers obtained new data from the James Webb Space Telescope (JWST) on TRAPPIST-1b, the closest planet to its star in the TRAPPIST-1 solar system. These new observations provide insight into how its star may influence observations of exoplanets in the habitable zone of cool stars. In the habitable zone, liquid water may still exist on the surface of an orbiting planet.

The team, which included University of Michigan astronomer and NASA Sagan Fellow Ryan McDonald, published their study in the journal The Astrophysical Journal Letters,

“Our observations showed no signs of an atmosphere around TRAPPIST-1 b. This tells us that the planet may be bare rock, have clouds in the atmosphere, or may have a very heavy molecule like carbon dioxide that makes up the atmosphere. Makes it much smaller to put in,” MacDonald said. “But what we see is that the star is the biggest influence dominating our observations, and it will do exactly the same for the other planets in the system.”

Much of the team’s investigation focused on how much they could learn about the star’s influence on observations of the planets of the TRAPPIST-1 system.

“If we can’t figure out how to deal with the star now, it will be very hard to see any atmospheric signals when we look at the planets in the habitable zone – TRAPPIST-1 d, e and f –.” ” McDonald said.

A promising exoplanetary system

TRAPPIST-1, a star much smaller and cooler than our Sun located about 40 light-years from Earth, has attracted the attention of scientists and space enthusiasts since the discovery of its seven Earth-sized exoplanets in 2017. These worlds, tightly packed with three of them within the habitable zone around their star, have raised hopes of finding potentially habitable environments beyond our solar system.

The study, led by Olivia Lim of the Trottier Institute for Research on Exoplanets at the University of Montreal, used a technique called transmission spectroscopy to gain important insights into the properties of TRAPPIST-1b. By analyzing the light from the central star after it passes through the exoplanet’s atmosphere during transit, astronomers can see the unique fingerprint left by the molecules and atoms found within that atmosphere.

“These observations were made with the NIRISS instrument on JWST, which was built over a period of nearly 20 years by an international collaboration led by René Doyon at the University of Montreal under the auspices of the Canadian Space Agency,” said UM professor Michael Meyer. ” of astronomy. “It was an honor to be a part of this collaboration and it was extremely exciting to see the results coming from this unique capability of NIRIS featuring diverse worlds around nearby stars.”

Know your star, know your planet

The main conclusion of the study was the significant impact of stellar activity and contamination when trying to determine the nature of an exoplanet. Stellar contamination refers to the effect of the star’s own features, such as dark areas called spots and bright areas called faculae, on measurements of the exoplanet’s atmosphere.

The team found strong evidence that stellar contamination plays an important role in shaping the transmission spectra of TRAPPIST-1 b and possibly other planets in the system. The activity of the central star can produce “ghost signals” that can fool an observer into thinking they have detected a particular molecule in the exoplanet’s atmosphere.

This result underlines the importance of considering stellar contamination when planning future observations of all exoplanetary systems. This is especially true for systems like TRAPPIST-1, as it is centered around a red dwarf star that can be particularly active with starspots and frequent flare events.

“In addition to pollution from star spots and faculae, we observed a stellar flare, an unexpected phenomenon during which a star appears brighter for several minutes to hours,” Lim said. “This brightness affected our measurements of the amount of light blocked by the planet. Such signatures of stellar activity are difficult to model but we need to take them into account to ensure we can interpret the data correctly.”

MacDonald played a key role in modeling the impact of the star and discovering the atmosphere in the team’s observations, running a series of millions of models to explore the full range of properties of cool starspots, hot star active regions and planetary atmospheres that Could explain the JWST observations astronomers were looking at.

No significant atmosphere on TRAPPIST-1B

While all seven of the TRAPPIST-1 planets are attractive candidates in the search for Earth-sized exoplanets with atmospheres, TRAPPIST-1 b’s proximity to its star means it finds itself more exposed than its siblings. Found in harsh conditions. It receives four times more radiation from the Sun than Earth and its surface temperature ranges between 120 and 220 degrees Celsius.

However, if TRAPPIST-1 b had an atmosphere, it would be easiest to detect and characterize all the targets in the system. Since TRAPPIST-1 b is the closest planet to its star and thus the hottest planet in the system, its transit creates a strong signal. All of these factors make TRAPPIST-1 an important, yet challenging, target for observation.

To take into account the effect of stellar contamination, the team conducted two independent atmospheric retrievals, a technique for determining the type of atmosphere present on TRAPPIST-1b based on observations. In the first approach, stellar contamination was removed from the data before analysis. In the second approach, pioneered by MacDonald, stellar contamination and planetary atmospheres were modeled and fitted together.

In both cases, the results indicated that the spectra of TRAPPIST-1 b could match the stellar contamination modeled well alone. This suggests that there is no evidence of any significant atmosphere on the planet. Such a result remains very valuable, because it tells astronomers what types of atmospheres are inconsistent with the observed data.

Based on their collected JWST observations, Lim and his team explored a series of atmospheric models for TRAPPIST-1 b, examining different possible compositions and scenarios. They found that a cloud-free, hydrogen-rich atmosphere was ruled out with high confidence. This means that there appears to be no obvious, extended atmosphere around TRAPPIST-1b.

However, the data could not confidently exclude thin atmospheres, such as those composed of pure water, carbon dioxide or methane, nor an atmosphere similar to that of Titan, Saturn’s moon and the only moon in the Solar System with a significant atmosphere. These results, the first spectrum of a TRAPPIST-1 planet, are generally consistent with previous JWST observations of TRAPPIST-1 b observed in the same color with the MIRI instrument.

As astronomers continue to explore other rocky planets across the vastness of space, these findings will inform future observing programs on JWST and other telescopes, contributing to a broader understanding of exoplanetary atmospheres and their potential habitability.

more information:
Olivia Lim et al, Atmospheric Reconnaissance of TRAPPIST-1B with JWST/NIRISS: Evidence for Strong Stellar Contamination in Transmission Spectra, The Astrophysical Journal Letters (2023). DOI: 10.3847/2041-8213/acf7c4

Journal Information:
Astrophysical Journal Letters


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