Coolest lava world yet with signs of an atmosphere offers clues to early Earth

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 6,300 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 scientist 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,” 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 of the study, describes why atmospheres on so-called lava worlds matter, how they found this one and next steps.

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 coolest 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 above 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,” said Coy.

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 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 a transit, when the planet goes in front of its host star. The other is called a secondary eclipse, which is the method we used for this study. It occurs 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 dayside 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 dayside to the nightside. 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 dayside 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 their star-facing surfaces are likely melted rock. 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. 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 into space and cool their daysides? 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.

Who’s behind this story?

Lisa Lock

BA art history, MA material culture. Former museum editor, paramedic, and transplant coordinator. Editing for Science X since 2021.

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Robert Egan

Bachelor’s in mathematical biology, Master’s in creative writing. Well-traveled with unique perspectives on science and language.

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