Seven rocky planets orbit the nearby dwarf star TRAPPIST-1, providing a unique opportunity to search for atmospheres on small planets outside the Solar System. Thanks to the recent launch of the NASA/ESA/CSA James Webb Space Telescope, possible atmospheric constituents such as carbon dioxide are now detectable. In the new research, astronomers used the new data from Webb’s Mid-Infrared Instrument (MIRI) to calculate the amount of heat energy coming from the rocky exoplanet TRAPPIST-1c.
With a dayside temperature of roughly 380 K (107 degrees Celsius, 225 degrees Fahrenheit), TRAPPIST-1c is now the coolest rocky exoplanet ever characterized based on thermal emission. Image credit: NASA / ESA / CSA / Joseph 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, TRAPPIST-1b, c and d, lay in the star’s habitable zone, meaning they may harbor suitable conditions for life.
“We want to know if rocky planets have atmospheres or not,” said Sebastian Zieba, a graduate student at the Max Planck Institute for Astronomy.
“In the past, we could only really study planets with thick, hydrogen-rich atmospheres.”
“With Webb we can finally start to search for atmospheres dominated by oxygen, nitrogen, and carbon dioxide.”
“TRAPPIST-1c is interesting because it’s basically a Venus twin,” said Dr. Laura Kreidberg, also from the Max Planck Institute for Astronomy.
“It’s about the same size as Venus and receives a similar amount of radiation from its host star as Venus gets from the Sun.”
“We thought it could have a thick carbon dioxide atmosphere like Venus.”
In their study, the astronomers used Webb’s MIRI instrument to observe the TRAPPIST-1 system on four separate occasions as the planet moved behind the star, a phenomenon known as a secondary eclipse.
By comparing the brightness when the planet is behind the star (starlight only) to the brightness when the planet is beside the star (light from the star and planet combined), they were able to calculate the amount of mid-infrared light with wavelengths of 15 microns given off by the dayside of the planet.
“Our results are consistent with the planet being a bare rock with no atmosphere, or the planet having a really thin carbon dioxide atmosphere (thinner than on Earth or even Mars) with no clouds,” Zieba said.
“If the planet had a thick carbon dioxide atmosphere, we would have observed a really shallow secondary eclipse, or none at all.”
“This is because carbon dioxide would be absorbing all of the 15-micron light, so we wouldn’t detect any coming from the planet.”
The data also show that it is unlikely the planet is a true Venus analog with a thick carbon dioxide atmosphere and sulfuric acid clouds.
The absence of a thick atmosphere suggests that the planet may have formed with relatively little water.
If the cooler, more temperate TRAPPIST-1 planets formed under similar conditions, they too may have started with little of the water and other components necessary to make a planet habitable.
The sensitivity required to distinguish between various atmospheric scenarios on such a small planet so far away is truly remarkable.
The decrease in brightness that Webb detected during the secondary eclipse was just 0.04%: equivalent to looking at a display of 10,000 tiny light bulbs and noticing that just four have gone out.
“It is extraordinary that we can measure this,” Dr. Kreidberg said.
“There have been questions for decades now about whether rocky planets can keep atmospheres.”
“Webb’s ability really brings us into a regime where we can start to compare exoplanet systems to our solar system in a way that we never have before.”
The findings were published this month in the journal Nature.
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S. Zieba et al. No thick carbon dioxide atmosphere on the rocky exoplanet TRAPPIST-1c. Nature, published online June 19, 2023; doi: 10.1038/s41586-023-06232-z
