Of the seven Earth-sized worlds orbiting the red dwarf star TRAPPIST-1, one planet in particular has attracted the attention of scientists. This planet orbits the star within the “Goldilocks zone” – a distance where water on its surface is theoretically possible, but only if the planet has an atmosphere. And where there is water, there might be life.
TRAPPIST-1e, an Earth-sized, habitable-zone exoplanet
Two recently scientific papers
While these reports are intriguing and show progress toward characterizing the nearest potentially earth-like exoplanet, Ranjan urges caution in a third paper, arguing that more rigorous studies are needed to determine whether TRAPPIST-1e has an atmosphere at all and whether preliminary hints of methane detected by James Webb are indeed signs of an atmosphere or have their origin with its host star.
The TRAPPIST system, so named after the survey that discovered it – “Transiting Planets and Planetesimals Small Telescope project” – is located about 39 light-years from Earth. It resembles a miniature version of our solar system. The star and all its planets would comfortably fit inside the orbit of planet Mercury. A “year” for any given TRAPPIST planet lasts mere days by Earth standards.
“The basic thesis for TRAPPIST-1e is this: If it has an atmosphere, it’s habitable,” said Ranjan, who is an assistant professor at LPL. “But right now, the first-order question must be, ‘Does an atmosphere even exist?’”
To answer this question, researchers aimed the space telescope’s powerful Near-Infrared Spectrograph, or NIRSpec, instrument at the TRAPPIST system as planet TRAPPIST-1e transited – or passed in front of – its host star. During a transit, starlight filters through the planet’s atmosphere – if there is one – and is partially absorbed, allowing astronomers to deduce what chemicals it may contain. With each additional transit, the atmospheric contents become clearer as more data is collected.
The four transits of TRAPPIST-1e studied by the team revealed hints of methane. However, because TRAPPIST-1e’s star is a so-called M dwarf, about one tenth the size of our sun and only slightly larger than Jupiter, its unique properties call for extra caution when interpreting data, Ranjan said.
“While the sun is a bright, yellow dwarf star, TRAPPIST-1 is an ultracool red dwarf, meaning it is significantly smaller, cooler and dimmer than our sun,” he explained. “Cool enough, in fact, to allow for gas molecules in its atmosphere. We reported hints of methane, but the question is, ‘is the methane attributable to molecules in the atmosphere of the planet or in the host star?’”
To rule on this question, Ranjan and colleagues simulated scenarios in which TRAPPIST-1e might have a methane-rich atmosphere and evaluated the probability for each of them. In the most likely scenario among the ones tested, the planet resembled Saturn’s methane-rich moon, Titan. However, the work showed that even that scenario was very unlikely.
“Based on our most recent work, we suggest that the previously reported tentative hint of an atmosphere is more likely to be ‘noise’ from the host star,” Ranjan said. “However, this does not mean that TRAPPIST-1e does not have an atmosphere – we just need more data.”
Ranjan pointed out that while James Webb is revolutionizing exoplanet science, the telescope was not originally designed to study small, Earth-like exoplanets.
“It was designed long before we knew such worlds existed, and we are fortunate that it can study them at all,” he said. “There is only a handful of Earth-sized planets in existence for which it could potentially ever measure any kind of detailed atmosphere composition.”
New answers could come from NASA’s Pandora mission, currently in development and slated for launch in early 2026. Led by Daniel Apai, professor of astronomy and planetary sciences at the U of A Steward Observatory, Pandora is a small satellite designed to characterize exoplanet atmospheres and their host stars. Pandora will monitor stars with potentially habitable planets before, during and after they transit in front of their host stars.
In addition, researchers hope that an ongoing, larger round of observations and new analytical techniques could finally tip the scale in one way or another. Currently, the collaboration is focusing on a technique known as dual transit: by observing the star when both TRAPPIST-1e, and TRAPPIST-1b, the innermost and airless planet of the system, pass in front of their star at the same time.
“These observations will allow us to separate what the star is doing from what is going on in the planet’s atmosphere – should it have one,” Ranjan said.
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