Coldest Lava Exoplanet Detected

 Coldest lava exoplanet detected to hold an atmosphere, up till now in the universe

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. Exoplanets continue to challenge our understanding of planetary formation, evolution, and whether they might be able to support life as we know it. Scientists often use planets in our solar system as analogs for exoplanets, studying exoplanets with extremely short orbital periods forces scientists to constantly rethink our understanding of what planets can withstand during their lifetimes. One such type of exoplanet which has entered the foray in recent years are “lava world” exoplanets, and especially lava exoplanets that orbit dangerously close to their stars. But while it has been long known that lava worlds on with higher temperatures have atmospheres, “colder” lava worlds have been found to lack atmospheres.

Now, an internation team of researchers might have inched our way to solving that conundrum, as they examine an Earth-sized lava world orbiting its star in less than one Earth day. In a study, 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.”  Researchers included from the US, UK, China and Spain present new evidence that an unusually cold lava world might challenge our understanding of lava worlds due to its potential possession of an atmosphere.

For the study, the researchers used NASA’s James Webb Space Telescope (JWST) to observe HD 3167 b, which is located about 154 light-years from Earth, has a radius and mass of about 1.6 and 4.8 of Earth, respectively, and orbits its K-type star in about 0.96 Earth days. K-type stars are both smaller and cooler than our Sun but are estimates to comprise the largest population of stars in the Milky Way Galaxy. While “hotter” lava worlds have been confirmed to have atmospheres since the extreme temperatures melt away the surface, resulting in this becoming a gas and enveloping the planet, “colder” lava worlds are known for being bare rocks which lack atmospheres. 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 question comes to mind. 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. However, despite HD 3167 b being on the colder end of the temperature spectrum, the researchers found that it could possess an atmosphere. They made this discovery through observing the exoplanet’s secondary eclipse, which is when it is observed as it passes behind its star. This differs from a transit, which occurs when it passes in front of its star. When JWST observed HD 3167 b’s secondary eclipse, the exoplanet’s surface was found to be much cooler than previously hypothesized, with the researchers noting this could mean its heat from the dayside is redistributed to the night side. The note of dayside and night side is important since exoplanets orbiting this close to their stars are tidally locked, meaning one side always faces its star. The researchers note how this study could challenge our understanding of lava worlds, specifically regarding their ability to retain and maintain atmospheres based on the surface temperature and composition.

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. "As suggested, lava worlds have been studied dating back to the 2000s, but their formation and evolution differ greatly from our solar system’s own lava world, Jupiter’s moon, Io. While Io’s volcanic activity, which includes hundreds of active volcanoes, is caused by tidal heating from the gravitational interactions between the much larger Jupiter and the other Galilean moons, lava exoplanets result from the searing temperatures literally melting their surfaces, and this material evaporating to form 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. This 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. 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. This will tell us if a planet has an atmosphere with less ambiguity than the temperature inference 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.

The survey focuses on 10 planets, and this is only the first result. Researchers are also looking at observations from TESS, a planet-finding satellite that works in the visible light range. This helps to 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. These lava worlds are so-named because the surface facing their star is likely melted rock. HD 3167 b might have a silicate-rich composition, with a similar mixture of minerals which make up Earth’s mantle. Before this study, researchers expected that any atmosphere on these ultra-hot planets would be composed of vaporized rock, but they are starting to see evidence that some might have heavier gases like CO2, CO, or H2O 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. What new insights into lava exoplanets will researchers make in the coming years and decades? Only time will tell. 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. These planets are a pathway to colder Earth-like, potentially habitable worlds.