Webb eclipse measurements suggest HD 3167 b, the coolest known lava world, holds an atmosphere

For a bare rock, the hottest the star-facing side can get depends on two things: the planet's distance from its star and how reflective its surface is ^[5]. An atmosphere pulls the reading below that ceiling by moving heat to the nightside. Venus does this so thoroughly that its day and night surfaces are almost the same temperature ^[6]. Clouds would pull the reading down too, by reflecting starlight before it reaches the ground ^[7].

The result is only as good as that ceiling. If the real surface is more reflective than the model assumes, the true maximum is lower and part of the shortfall goes away ^[5]. HD 3167 b "is noticeably cooler than its expected maximum, providing strong evidence that it has an atmosphere," Coy said ^[8]. The Q&A does not give the measured temperature, the predicted maximum or the uncertainty on the gap between them.

Coy's answer on composition is hedged, and it starts with the rock. Lava worlds get their name because their star-facing surfaces are likely melted rock ^[19]. "We think that HD 3167 b might have a silicate-rich composition, with a mixture of minerals similar to those that make up Earth's mantle," said Coy, a graduate student in Kite's group at the University of Chicago and the paper's first author ^[2][9].

The sample behind the surprise is small. More than 6,300 exoplanets have been catalogued. Most are not rocky, and only a handful of the rocky ones appear to have atmospheres ^[12]. Coy said five terrestrial planets found with atmospheres, this one included, have been ultra-hot ^[14]. Webb observations of rocky planets around stars much smaller than the sun suggest most of those are bare rock ^[13]. Edwin Kite, an associate professor of geophysical sciences and a co-author, said the pattern runs against 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," Kite said ^[10].

The two groups are sorted by different things. The bare planets are grouped by their host stars; the planets with atmospheres are grouped by their own heat ^[13][14]. With five cases, the link between heat and atmosphere is a correlation. It could still come from some other property these planets share. Coy said the new planet's temperature is what makes it useful: "This is interesting because we're trying to understand the temperature transition between planets with and without atmospheres" ^[17].

Kite was direct about where HD 3167 b sits in the search for life. "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 ^[11]. Coy gave a second reason. "We're interested in studying these kinds of planets because we think early Earth might have looked a lot like a lava world," he said ^[16].