Webb finds a surprising atmosphere on a scorching lava world

For scientists searching for life beyond Earth, rocky planets with atmospheres are among the most promising places to investigate. An atmosphere is essential for maintaining liquid water on a planet's surface. Yet among the more than 6,300 exoplanets cataloged thus far, most are not rocky, and only a small number of known rocky planets appear to possess atmospheres. Now, researchers have added another world to that short list. In a study published in The Astrophysical Journal Letters, a team led by University of Chicago scientist Brandon Park Coy reports evidence of an atmosphere surrounding HD 3167 b, a rocky super-Earth located 154 light-years away in the constellation Pisces. The intensely hot planet, classified as a "lava world," completes an orbit around its star in only one Earth day. "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," explained Edwin Kite, UChicago associate professor of geophysical sciences and co-author of the study. "These planets are too hot for life, but by studying them, we can say something about the processes that matter for other rocky worlds." In the Q&A below,Coy, a graduate student in Kite's group and first author on the study, explains why atmospheres on lava worlds are scientifically valuable, how researchers detected evidence for this one, and what they hope to learn next. What was the goal of the survey that found this atmosphere-bearing exoplanet? One of the driving questions that the James Webb Space Telescope is being used to answer is whether planets orbiting stars much smaller than the sun have atmospheres. It appears that most of those terrestrial planets don't -- they're just bare rock. But surprisingly, we've found evidence that the majority of lava worlds -- similar in composition to Earth and Venus but much, much hotter -- might have atmospheres. Five terrestrial planets found with atmospheres, including the one described in this study, have been ultra-hot. This planet, however, is the coldest lava world found so far with evidence of an atmosphere. This is interesting because we're trying to understand the temperature transition between planets with and without atmospheres. The result is the first from a program led by Megan Weiner Mansfield, PhD'21, who's now at the University of Maryland. The goal of the program, which is looking at 10 ultra-hot lava worlds, is to see if there's a critical temperature over which you start seeing planets with atmospheres."We're interested in studying these kinds of planets because we think early Earth might have looked a lot like a lava world." -- Study author Brandon Park CoyHow can you tell if an exoplanet has an atmosphere? Right now, directly looking at Earth-like planets for signs of life is out of reach. Instead, we can use the Webb telescope to detect light in the mid-infrared wavelength range to estimate the temperature of exoplanets, which can tell us if they might have atmospheres. There are two primary ways that we study exoplanets. One is called transit, when the planet goes in front of the host star. Another called secondary eclipse, which is the method we used for this study, is when the planet passes behind its star, and we can measure how much light is lost. That difference tells us how much mid-infrared light comes from the planet itself, which essentially tells us how hot the planet is. If a planet doesn't have an atmosphere, its star-facing, day side should be as hot as theoretically possible based on how reflective its surface is and its distance from the star. But an atmosphere would help redistribute heat from the day side to the night side. We see this happen on Venus -- there's almost no difference in surface temperature between the day and night sides, or between the poles and the equator. If a planet has an atmosphere, it might also have clouds that reflect incoming starlight and cool the dayside. So if a planet's day side is cooler than the maximum possible, it likely has an atmosphere. The subject of this study, HD 3167 b, is noticeably cooler than its expected maximum, providing strong evidence that it has an atmosphere. What do you think HD 3167 b's atmosphere is made of? These lava worlds are so-named because the surface facing their star is likely melted rock. We think that HD 3167 b might have a silicate-rich composition, with a similar mixture of minerals that make up Earth's mantle. Before this study, we expected that any atmosphere on these ultra-hot planets would be composed of vaporized rock, but we're starting to see evidence that some might have heavier gases like carbon dioxide, carbon monoxide, or water in their atmospheres. The makeup of HD 3167 b's atmosphere is still up in the air. That's one reason we're so interested in getting new observations. Why is this result important? Because the four other lava worlds found to have atmospheres are in the ultra-hot regime, it raises questions about whether there's a critical transition temperature when silicate atmospheres start becoming very thick. Is there a temperature where these planets form silicate cloud decks that might reflect incoming radiation to space and cool its day side? This relatively cooler planet helps us better characterize and nail down this transition. Despite how inhospitable they are for life, we're also interested in studying these kinds of planets because we think early Earth might have looked a lot like a lava world. We think that very early in the solar system's history, when the terrestrial planets formed, they were extremely hot due to the energy from all of the planetesimal collisions. Earth had what's known as a magma ocean stage with an entirely liquid surface. This result gives us a window into studying what conditions may have been like in Earth's first couple of million years.