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Webb finds evidence of an atmosphere on HD 3167 b, the coldest lava world yet to show one
University of Chicago researchers report Webb evidence of an atmosphere on HD 3167 b, a rocky planet with a 24-hour orbit. Planets that close to a star are expected to lose gas to stellar wind and high-energy light, so the result could help explain how rocky worlds keep their atmospheres.
The Scientist · Science desk
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What happened
- The finding is the first result from a Webb program led by Megan Weiner Mansfield, now at the University of Maryland, that is observing 10 ultra-hot lava worlds.
- The team measured the mid-infrared light lost as the planet passed behind its star, a secondary eclipse that gives the planet's dayside temperature.
- Five terrestrial planets have been found with atmospheres, HD 3167 b among them, and all five are ultra-hot.
- Most rocky planets Webb has examined around stars much smaller than the sun appear to be bare rock, according to first author Brandon Park Coy.
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Why it matters
- decision If planets like this one have a single cutoff temperature for keeping gas, it must sit at or below HD 3167 b's, so cooler lava worlds now carry the most information for locating it.
- constraint Atmosphere claims built on dayside temperature inherit an assumed surface reflectivity, so a different surface model could move a planet between the atmosphere and bare-rock columns.
- capability Direct looks at Earth-like planets for signs of life are out of reach, so lava worlds give Webb a case it can measure now of how rocky planets retain atmospheres.
A dayside temperature can stand in for gas because of where the heat goes. With no atmosphere, the side facing the star should be as hot as the planet's distance and surface reflectivity allow. An atmosphere carries some of that heat round to the night side [2]. Venus shows the limit, with almost no difference in surface temperature between its day and night sides [4]. The secondary-eclipse method rests on that logic [14].
The approach turns a hard problem, finding gas around a small, hot planet, into a temperature comparison. The comparison leans on a model. The bare-rock ceiling depends on how reflective the surface is [2], so the case for an atmosphere rests on assumptions about the rock as well as on the light Webb collected.
The result runs against the usual expectation. "What's so surprising is that the closer a rocky planet orbits its star, the harder it should be to have an atmosphere, because it's bombarded by stellar wind and gets more high-energy photons from the star. But it seems that many of these lava worlds do," said Edwin Kite, a University of Chicago associate professor of geophysical sciences and a co-author of the study [1].
Coy put it more strongly. He said the team has found evidence that "the majority of lava worlds -- similar in composition to Earth and Venus but much, much hotter -- might have atmospheres" [3]. The release does not say how many lava worlds have been observed in total, so that majority comes without a stated denominator. For scale, more than 6,300 exoplanets have been catalogued, most of them are not rocky, and only a small number of known rocky planets appear to have atmospheres [8].
HD 3167 b is a super-Earth 154 light-years away in Pisces [7]. Its value to Mansfield's program is its temperature: it is the coldest lava world found so far with evidence of an atmosphere [12]. "This is interesting because we're trying to understand the temperature transition between planets with and without atmospheres," Coy said [17]. Nine of the program's ten targets have yet to report [18]. One planet cannot place a transition. Whether this result should change which rocky worlds get Webb time outside the survey depends on what those nine show.
Kite was plain about the limits of the targets themselves. "These planets are too hot for life, but by studying them, we can say something about the processes that matter for other rocky worlds," he said [9]. Coy is a graduate student in Kite's group and first author of the paper in The Astrophysical Journal Letters [6]. He gave the longer motive: "We're interested in studying these kinds of planets because we think early Earth might have looked a lot like a lava world" [15].
What to watch
- Results from the other nine lava worlds in Mansfield's program, and whether any show a bare-rock dayside temperature that would bracket a cutoff.
- Any follow-up that identifies what HD 3167 b's atmosphere is made of, which would test the temperature-based inference directly.
- A published count of how many lava worlds have been observed in total, which would give Coy's 'majority' a denominator.