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Webb narrows atmospheric possibilities for Earth-sized exoplanet TRAPPIST-1 d

The exoplanet TRAPPIST-1 d intrigues astronomers looking for possibly habitable worlds beyond our solar system because it is similar in size to Earth, rocky, and resides in an area around its star where liquid water on its surface is theoretically possible. But according to a new study using data from the NASA/ESA/CSA James Webb Space Telescope, it does not have an Earth-like atmosphere.

A protective atmosphere, a friendly Sun, and lots of liquid water — Earth is a special place. Using the unprecedented capabilities of the Webb, astronomers are on a mission to determine just how special, and rare, our home planet is. Can this temperate environment exist elsewhere, even around a different type of star? The TRAPPIST-1 system provides a tantalizing opportunity to explore this question, as it contains seven Earth-sized worlds orbiting the most common type of star in the galaxy: a red dwarf.

“Ultimately, we want to know if something like the environment we enjoy on Earth can exist elsewhere, and under what conditions. While the James Webb Space Telescope is giving us the ability to explore this question in Earth-sized planets for the first time, at this point we can rule out TRAPPIST-1 d from a list of potential Earth twins or cousins,”

said Caroline Piaulet-Ghorayeb of the University of Chicago and Trottier Institute for Research on Exoplanets (IREx) at Université de Montréal, lead author of the study published in The Astrophysical Journal.

Planet TRAPPIST-1 d

The TRAPPIST-1 system is located 40 light-years away and was revealed as the record-holder for most Earth-sized rocky planets around a single star in 2017, thanks to data from NASA’s retired Spitzer Space Telescope and other observatories. Due to that star being a dim, relatively cold red dwarf, the “habitable zone” – where the planet’s temperature may be just right, such that liquid surface water is possible – lies much closer to the star than in our solar system. TRAPPIST-1 d, the third planet from the red dwarf star, lies on the cusp of that temperate zone, yet its distance to its star is only 2 percent of Earth’s distance from the Sun. TRAPPIST-1 d completes an entire orbit around its star, its year, in only four Earth days.

Webb’s NIRSpec (Near-Infrared Spectrograph) instrument did not detect molecules from TRAPPIST-1 d that are common in Earth’s atmosphere, like water, methane, or carbon dioxide. However, Piaulet-Ghorayeb outlined several possibilities for the exoplanet that remain open for follow-up study.

“There are a few potential reasons why we don’t detect an atmosphere around TRAPPIST-1 d. It could have an extremely thin atmosphere that is difficult to detect, somewhat like Mars. Alternatively, it could have very thick, high-altitude clouds that are blocking our detection of specific atmospheric signatures — something more like Venus. Or, it could be a barren rock, with no atmosphere at all,” Piaulet-Ghorayeb said.

The star TRAPPIST-1

No matter what the case may be for TRAPPIST-1 d, it’s tough being a planet in orbit around a red dwarf star. TRAPPIST-1, the host star of the system, is known to be volatile, often releasing flares of high-energy radiation with the potential to strip off the atmospheres of its small planets, especially those orbiting most closely. Nevertheless, scientists are motivated to seek signs of atmospheres on the TRAPPIST-1 planets because red dwarf stars are the most common stars in our galaxy. If planets can hold on to an atmosphere here, under waves of harsh stellar radiation, they could, as the saying goes, make it anywhere.

“Webb’s sensitive infrared instruments are allowing us to delve into the atmospheres of these smaller, colder planets for the first time,” said Björn Benneke of IREx at Université de Montréal, a co-author of the study. “We’re really just getting started using Webb to look for atmospheres on Earth-sized planets, and to define the line between planets that can hold onto an atmosphere, and those that cannot.”

The outer TRAPPIST-1 planets

Webb observations of the outer TRAPPIST-1 planets are ongoing, which hold both potential and peril. On the one hand, Benneke said, planets e, f, g, and h may have better chances of having atmospheres because they are further away from the energetic eruptions of their host star. However, their distance and colder environment will make atmospheric signatures more difficult to detect, even with Webb’s infrared instruments.

“All hope is not lost for atmospheres around the TRAPPIST-1 planets,” Piaulet-Ghorayeb said. “While we didn’t find a big, bold atmospheric signature at planet d, there is still potential for the outer planets to be holding onto a lot of water and other atmospheric components.”

“Our detective work is just beginning. While TRAPPIST-1 d may prove a barren rock illuminated by a cruel red star, the outer planets TRAPPIST-1e, f, g, and h, may yet possess thick atmospheres,” added Ryan MacDonald, a co-author of the paper, now at the University of St Andrews in the United Kingdom, and previously at the University of Michigan. “Thanks to Webb we now know that TRAPPIST-1 d is a far cry from a hospitable world. We’re learning that the Earth is even more special in the cosmos.”

Illustration of a planet silhouetted in front of a star. The star shows a large eruption on one side and more wisps of red coming from its southern hemisphere. Two more planets appear in the background. — The James Webb Space Telescope narrows atmospheric possibilities for Earth-sized rocky exoplanet TRAPPIST-1 d. This artist’s concept depicts planet TRAPPIST-1 d passing in front of its turbulent star, with other members of the closely packed system shown in the background.
The TRAPPIST-1 system is intriguing to scientists for a few reasons. Not only does the system have seven Earth-sized rocky worlds, but its star is a red dwarf, the most common type of star in the Milky Way galaxy. If an Earth-sized world can maintain an atmosphere here, and thus have the potential for liquid surface water, the chance of finding similar worlds throughout the galaxy is much higher. In studying the TRAPPIST-1 planets, scientists are determining the best methods for separating starlight from potential atmospheric signatures in data from the NASA/ESA/CSA James Webb Space Telescope. The star TRAPPIST-1’s variability, with frequent flares, provides a challenging testing ground for these methods.
Credit: NASA, ESA, CSA, J. Olmsted (STScI)

Bibliographic information:

Caroline Piaulet-Ghorayeb et al., ApJ 989 181 2025, DOI: 10.3847/1538-4357/adf207

Press release from ESA Webb.

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Hubble finds water vapour in the atmosphere of GJ 9827d, a small exoplanet

Astronomers using the NASA/ESA Hubble Space Telescope observed the smallest exoplanet where water vapour has been detected in its atmosphere, GJ 9827d. At only approximately twice Earth’s diameter, the planet GJ 9827d could be an example of potential planets with water-rich atmospheres elsewhere in our galaxy.

“This would be the first time that we can directly show through an atmospheric detection that these planets with water-rich atmospheres can actually exist around other stars,” said team member Björn Benneke of the Université de Montréal. “This is an important step toward determining the prevalence and diversity of atmospheres on rocky planets.”

However, it remains too early to tell whether Hubble spectroscopically measured a small amount of water vapour in a puffy hydrogen-rich atmosphere, or if the planet’s atmosphere is mostly made of water, left behind after a primaeval hydrogen/helium atmosphere evapourated under stellar radiation.

“Our observing programme was designed specifically with the goal of not only detecting the molecules in the planet’s atmosphere, but of actually looking specifically for water vapour. Either result would be exciting, whether water vapour is dominant or just a tiny species in a hydrogen-dominant atmosphere,” said the science paper’s lead author, Pierre-Alexis Roy of the Université de Montréal.

“Until now, we had not been able to directly detect the atmosphere of such a small planet. And we’re slowly getting into this regime now,” added Benneke. “At some point, as we study smaller planets, there must be a transition where there’s no more hydrogen on these small worlds, and they have atmospheres more like Venus (which is dominated by carbon dioxide).”

Because the planet is as hot as Venus at roughly 425 degrees Celsius, it definitely would be an inhospitable, steamy world if the atmosphere were predominantly water vapour.

At present the team is left with two possibilities. The planet is still clinging to a hydrogen-rich envelope laced with water, making it a mini-Neptune. Alternatively, it could be a warmer version of Jupiter’s moon Europa, which has twice as much water as Earth beneath its crust.

“The planet GJ 9827d could be half water, half rock. And there would be a lot of water vapour on top of some smaller rocky body,” said Benneke.

If the planet has a residual water-rich atmosphere, then it must have formed farther away from its host star, where the temperature is cold and water is available in the form of ice, than its present location. In this scenario, the planet would have then migrated closer to the star and received more radiation. The hydrogen was then heated and escaped, or is still in the process of escaping, the planet’s weak gravity. The alternative theory is that the planet formed close to the hot star, with a trace of water in its atmosphere.

The Hubble programme observed the planet during 11 transits — events in which the planet crossed in front of its star — that were spaced out over three years. During transits, starlight is filtered through the planet’s atmosphere and carries the spectral fingerprint of water molecules. If there are clouds on the planet, they are low enough in the atmosphere that they don’t completely hide Hubble’s view of the atmosphere, and Hubble is able to probe water vapour above the clouds.

Hubble’s discovery opens the door to studying the planet in more detail. It’s a good target for the NASA/ESA/CSA James Webb Space Telescope to do infrared spectroscopy to look for other atmospheric molecules.

GJ 9827d was discovered by NASA’s Kepler Space Telescope in 2017. It completes an orbit around a red dwarf star every 6.2 days. The star, GJ 9827, lies 97 light-years from Earth in the constellation Pisces.

Occupying the upper half of this illustration is a foreground exoplanet, partly in shadow, with subtle blue and white atmospheric features along the crescent closest to the star. The planet appears above a red dwarf star, which is represented by a smaller reddish-white, mottled globe at the bottom left. Two other planets in this system are to the left and right of the red dwarf star. The planet to the star’s left is tiny, appears fully lit, and is closest to the star. The second planet is slightly larger, but appears farther away, about midway between the star and the foreground exoplanet. It is in shadow, with only the crescent facing the star bathed in light. — This is an artist’s conception of the exoplanet GJ 9827d, the smallest exoplanet where water vapour has been detected in its atmosphere. The planet could be an example of potential planets with water-rich atmospheres elsewhere in our galaxy. It is a rocky world, only about twice Earth’s diameter. It orbits the red dwarf star GJ 9827. Two inner planets in the system are on the left. The background stars are plotted as they would be seen to the unaided eye looking back toward our Sun, which itself is too faint to be seen. The blue star at upper right is Regulus, the yellow star at bottom centre is Denebola, and the blue star at bottom right is Spica. The constellation Leo is on the left, and Virgo is on the right. Both constellations are distorted from our Earth-bound view from 97 light-years away.
Credit: NASA, ESA, Leah Hustak and Ralf Crawford (STScI)

Press release from ESA Hubble.

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