The James Webb Space Telescope has confirmed its first exoplanet, a rocky Earth-size planet, and attempted to take the measure of its atmosphere.

Illustration of a planet and its star on a black background. The planet is large, in the foreground at the centre, and the star is smaller, in the background and also at the centre. The planet is rocky. The top quarter of the planet (the side facing the star) is lit, while the rest is in shadow. The star is bright yellowish-white, with no clear features.
this illustration reflects the conclusion that the exoplanet LHS 475 b is rocky and almost precisely the same size as Earth.
NASA / ESA / CSA / L. Hustak (STScI)

The planet formerly known as TOI 910.01, now LHS 475b, is real, according to new observations by the James Webb Space Telescope. Although roughly Earth-like in size, this world is nevertheless completely uninhabitable, roasting in its two-day orbit around its middle-aged red dwarf star.

NASA’s TESS mission first pinpointed this planet as a candidate worth follow-up, detecting the blip as the world crossed its star. In the follow-up Webb observations on August 31st and September 4th, the telescope caught two such transits, in which the planet blocked a mere 0.1% of the star’s light for 40 minutes.

“These pristine data help to validate and confirm the discovery of this Earth-size exoplanet,” says Jacob Lustig-Yaeger (Johns Hopkins University), who presented the as-yet unpublished results at the 241st meeting of the American Astronomical Society in Seattle, Washington.

“Our ultimate goal here was to search for the atmosphere of this planet,” he adds. To do this, the team relied on a technique known as transmission spectroscopy. The team basically measures the planet’s size at different wavelengths. When there are particles in the atmosphere absorbing the star’s light at particular wavelengths, the planet will appear bigger at those wavelengths; at wavelengths for which there’s no atmospheric absorption, the planet will appear smaller.

Drake Deming (University of Maryland), who was not involved with the study, says the team made the right observations for the job: “They have picked the best spectral region — the 4.3-micron carbon dioxide absorption line has great intrinsic strength, and that’s the most sensitive probe.”

The graphic shows the transmission spectrum of the rocky exoplanet LHS 475 b. The data points are plotted as white circles with grey error bars on a graph of the amount of light blocked in percent on the vertical axis versus wavelength of light in microns on the horizontal axis. A straight green line represents a best-fit model. A curvy red line represents a methane model, and a slightly less curvy purple line represents a carbon dioxide model.
The transmission spectrum of LHS 475b, shown here, shows that the data (white dots) are consistent with a featureless spectrum representative of a planet that has no atmosphere (yellow line). The purple line represents a pure carbon dioxide atmosphere and is indistinguishable from a flat line at the current level of precision. The green line represents a pure methane atmosphere, which is not favoured since methane, if present, would be expected to block more starlight at 3.3 microns.
NASA / ESA / CSA / L. Hustak (STScI) / K. Stevenson / J. Lustig-Yaeger / E. May (Johns Hopkins University Applied Physics Laboratory) / G. Fu (Johns Hopkins University) / S. Moran (University of Arizona)

Such observations are difficult at best, and in this case inconclusive. Lustig-Yaeger reported that while they can match the data with an atmosphere dominated by carbon dioxide, those data are also consistent with a completely airless world. Zero atmosphere for a planet several hundred degrees warmer than Earth wouldn’t be a great surprise, especially around the type of star known for its atmosphere-stripping flares.

Further observations with Webb will help disentangle these possibilities, and Lustig-Yaeger is excited for upcoming time already scheduled to look at a third transit.

Whether this world has air or not, perhaps the more important take-away is what the current data mean for future observations. “Our measurements meet the sensitivity requirements to be able to detect the atmospheres of Earth-size planets,” Lustig-Yaeger notes.

Those planets probably won’t be habitable either, Deming points out. “I think that eventually carbon dioxide in a rocky planet’s atmosphere will be detected using this technique,” he says. “But in my opinion the best prospects are hotter than LHS 475b, which is already well above a habitable temperature.” 

Future observations of both types of worlds, airless and not, will help shed light on what conditions cause a planet to lose its atmosphere. “We’re interested in where this dividing line might be between planets with and without atmospheres,” Lustig-Yaeger says. “It’ll be really interesting, I think, to learn about the processes that make these very different stars and their planetary systems different from our own solar system.”


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