Webb telescope sees world that could reek of burnt matches and rotten eggs

Astronomers have found a world outside the solar system that could be a stink bomb, with air that smells of rotten eggs and burnt matches. 

The exoplanet would owe its signature fragrance to sulfur dioxide and hydrogen sulfide, colorless gasses known for their stenches, according to a new study. Those chemicals in the planet's atmosphere could hint at the planet having a molten or volcanic surface. 

Using the James Webb Space Telescope, an international cooperation of NASA and the European and Canadian space agencies, a team of scientists peered at L 98-59 d, a so-called super Earth that's a bit larger and heavier than Earth. This potentially rocky world orbits a red dwarf star about 35 light-years away in the constellation Volans. 

If the team's findings, published in The Astrophysical Journal Letters on Nov. 1, can be confirmed in follow-up studies, it will hold the record as the smallest-known exoplanet with an atmosphere.

"This discovery was surprising, as it stands out in stark contrast to the atmospheres of rocky planets in our own solar system, where water vapor and carbon dioxide are much more prevalent," said Agnibha Banerjee, lead author of the study, in The Conversation, a nonprofit news site written by scholars and researchers. 

Red dwarfs, sometimes referred to as M-type, are the most common stars within the Milky Way, yet nobody knows whether planets closely orbiting them can hold onto atmospheres, Néstor Espinoza, a co-author of the paper, previously told Mashable. Though the host stars aren't as hot as the sun, nearby circling worlds, such as L 98-59 d, would be subjected to episodes of violent stellar radiation.

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But the new exoplanet findings are tenuous at best. The team used transmission spectroscopy to collect data for the study. This technique uses starlight from the host star that has filtered through a planet's atmosphere. Molecules within the atmosphere absorb certain light wavelengths, or colors, so by splitting the light into its basic parts — like a rainbow — astronomers can detect what light segments are missing to discern the molecular makeup of an atmosphere.

So far the team has only had the chance to analyze the planet's atmosphere once, from a single time it crossed in front of the star. The team "only slightly prefers the atmosphere" explanation over the possibility that the data was contaminated by stellar spots, Banerjee said in a post on X, formerly known as Twitter.

Most astronomers agree that detecting atmospheres is crucial in the search for habitable worlds. NASA has playfully called Earth's own atmosphere its "security blanket": Without it, the type of life flourishing here wouldn't exist. This cocoon holds oxygen in the air and filters out harmful ultraviolet radiation from the sun, all while keeping the world warm. Furthermore, it creates pressure that allows liquid water to pool on the surface.

Earth's atmosphere is filled with nitrogen, oxygen, and traces of water vapor. Venus has a thick atmosphere of carbon dioxide, and Mars has a thin atmosphere of carbon dioxide. Based on the terrestrial worlds humans are familiar with, no one expected to find a similar type of planet with a sulfur-rich atmosphere. 

If the team's results are confirmed, the exoplanet wouldn't seem to be a good fit for life. But astronomers say its potential volcanic activity is intriguing. One idea posited is that the exoplanet is similar to Jupiter's moon Io, the most volcanic world in the solar system. Jupiter's gravity squeezes Io's core as the moon moves closer, then slackens as it moves farther away. This swelling and contracting causes Io's interior to heat up, triggering tidal volcanism. 

Espinoza is heading up the implementation of a massive Webb telescope study of rocky worlds outside the solar system, specifically to discover if planets orbiting closely to small cool stars could have air. The campaign, first reported by Mashable, will take a closer look at a dozen nearby-ish planets over the next two years.

As for L 98-59 d, the Webb data so far implies large amounts of hydrogen sulfide in the planet's atmosphere. 

"So large that in an atmosphere like ours, it could give rise to you even losing your sense of smell!" Espinoza said in an email. "So probably not nice."

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