TRAPPIST-1 is a nearby system of seven Earth-sized, temperate, rocky exoplanets transiting a Jupiter-sized ultracool red dwarf, ideally suited for in-depth atmospheric studies. Using the NIRISS instrument aboard the NASA/ESA/CSA James Webb Space Telescope, astronomers have now captured the first transmission spectra of TRAPPIST-1b, the innermost planet in the TRAPPIST-1 system.
An artist’s impression of the rocky planet TRAPPIST-1b. Image credit: NASA / ESA / CSA / J. Olmsted, STScI.
TRAPPIST-1 is an ultracool M-dwarf star located 38.8 light-years away in the constellation of Aquarius.
Also known as K2-112 or TIC 278892590, the star is barely larger than Jupiter and has just 8% of our Sun’s mass.
In February 2017, astronomers announced that the star hosts at least seven planets: TRAPPIST-1b, c, d, e, f, g and h.
All these planets are similar in size to Earth and Venus, or slightly smaller, and have very short orbital periods: 1.51, 2.42, 4.04, 6.06, 9.21, 12.35 and 20 days, respectively.
Three of these planets lay in the star’s habitable zone, meaning they may harbor suitable conditions for life.
TRAPPIST-1b, the innermost planet, has an orbital distance about one hundredth that of Earth’s and receives about four times the amount of energy that Earth gets from the Sun.
Although it is not within the habitable zone, observations of the planet can provide important information about its sibling planets, as well as those of other red-dwarf systems.
In the new research, Université de Montréal doctoral student Olivia Lim and her colleagues employed Webb to observe TRAPPIST-1b.
The planet was observed during two transits — the moment when the planet passes in front of its star – using Webb’s NIRISS instrument.
“These are the very first spectroscopic observations of any TRAPPIST-1 planet obtained by Webb, and we’ve been waiting for them for years,” said Lim, the lead author of a paper published in the Astrophysical Journal Letters.
The astronomers used the technique of transmission spectroscopy to peer deeper into the distant world.
By analyzing the central star’s light after it has passed through the exoplanet’s atmosphere during a transit, astronomers can see 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 NIRISS principal investigator Dr. René Doyon, also from the Université de Montréal.
“These first observations highlight the power of NIRISS and Webb in general to probe the thin atmospheres around rocky planets.”
The team’s key finding was just how significant stellar activity and contamination are when trying to determine 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 authors found compelling evidence that stellar contamination plays a crucial role in shaping the transmission spectra of TRAPPIST-1b and, likely, the other planets in the system.
The central star’s activity can create ‘ghost signals’ that may fool the observer into thinking they have detected a particular molecule in the exoplanet’s atmosphere.
This result underscores the importance of considering stellar contamination when planning future observations of all exoplanetary systems.
This is especially true for systems like TRAPPIST-1, since the system is centered around a red dwarf which can be particularly active with starspots and frequent flare events.
“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,” Lim said.
“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.”
Based on the new Webb observations, the researchers explored a range of atmospheric models for TRAPPIST-1b, examining various possible compositions and scenarios.
They found they could confidently rule out the existence of cloud-free, hydrogen-rich atmospheres — in other words, there appears to be no clear, extended atmosphere around TRAPPIST-1b.
However, the data could not confidently exclude thinner atmospheres, such as those composed of pure water, carbon dioxide, or methane, nor an atmosphere similar to that of Titan, a moon of Saturn and the only moon in the Solar System with its own atmosphere.
These results are generally consistent with previous photometric observations of TRAPPIST-1b with Webb’s MIRI instrument.
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Olivia Lim et al. 2023. Atmospheric Reconnaissance of TRAPPIST-1b with JWST/NIRISS: Evidence for Strong Stellar Contamination in the Transmission Spectra. ApJL 955, L22; doi: 10.3847/2041-8213/acf7c4
