[University of Chicago] In the search for extraterrestrial life, it makes sense to first look for rocky planets with an atmosphere, like Earth. Without an atmosphere, a planet can’t have surface water. But of the more than 6300 exoplanets cataloged thus far, the vast majority are not rocky, and only a handful of the rocky worlds appear to have an atmosphere.
In a study published in The Astrophysical Journal Letters, a group led by University of Chicago graduate student Brandon Park Coy reports another: a rocky super-Earth 154 light-years away in the constellation Pisces. Named HD 3167 b, this very hot “lava world” zips around its host star in just 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,” said 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.”
Coy, a graduate student in Kite’s group and first author on the study, describes why atmospheres on so-called lava worlds matter, how they found this one, and next steps in the Q&A below.
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.
How 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 James Webb 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.
Why use the secondary eclipse method rather than the more common transit method? When using the transit method, we take advantage of the fact that specific gases in the atmosphere are really absorptive at certain wavelengths. For example, there is one wavelength where CO2 basically absorbs all the incoming light. When we look at a planet with a lot of CO2 in its atmosphere during its transit, it will look larger in specific wavelengths because CO2 is blocking incoming light. This method works really well for planets like Jupiter that have extended atmospheres. But for planets like Venus and Earth, the atmosphere is a lot smaller, so the CO2 signal is tiny and extremely difficult to see. The secondary eclipse method works much better for planets with smaller atmospheres.
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.
What’s next?
The survey focuses on 10 planets, and this is only the first result. I’m also looking at observations from TESS, a planet-finding satellite that works in the visible light range. That helps us see the light being reflected off the planet, which can offer insight into whether a planet has clouds and provide more details about its atmosphere.
Further down the line, 20–30 years from now, the Habitable Worlds Observatory will change how we search for signatures of life on Earth-like planets. HWO will work in the infrared/optical/ultraviolet light range and will be looking at the reflected light spectrum of planets, where you can see evidence for ozone, water, carbon dioxide, or methane. That will tell us if a planet has an atmosphere with less ambiguity than the temperature inference method.
Even though lava worlds are not habitable, they are going to be the best targets for the James Webb Space Telescope to figure out what their atmospheres are made of. That’s why I’m really excited to study these planets more–they’re a pathway to colder Earth-like, potentially habitable worlds.
Citation: “Evidence for an Atmosphere on the Ultra-short-period Super-Earth HD 3167 b.” Brandon Park Coy et al, 2026, ApJL, 1005 L77.
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