By Tristan Bove
Medill Reports, Dec. 16, 2023
Place a planet close enough to a star and the extreme temperatures will start to melt its surface. Turn the heat up high enough, and it could reveal the internal secrets deep below the surface of rocky planets like our own Earth.
With next-generation equipment like NASA’s James Webb Space Telescope allowing people to peer deeper into the dark recesses of space than ever before, scientists like Raymond Pierrehumbert are zooming in on lava worlds. These blazing planets get their names from the high temperatures created by their extremely close orbits around host stars, where conditions are hot enough to melt a planet’s rocky surface into rivers and oceans of pure magma.
Such a hostile environment may not seem like a helpful candidate to learn about the inner workings of habitable planets like Earth. To the trained eye, however, a glance through the fire and brimstone is shedding light not only on how our planet was formed, but also if life as we know it can exist on other worlds – or maybe even life as we don’t know it.
“Even though lava worlds themselves are insanely uninhabitable, it’s one of the few ways we have of telling what rocky planets are made out of,” said Pierrehumbert, a physicist at the University of Oxford. He spoke about the inhabitable climate of lava worlds at the 2023 Comer Climate Conference, an annual fall exchange in southwestern Wisconsin of scientists confronting catastrophic changes in the Earth’s climate.
Pierrehumbert uses the Webb telescope to identify and monitor around 20 lava world candidates, some of which orbit stars up to 100 light years away from the Earth. Webb helps scientists determine the atmospheric conditions of exoplanets outside our Solar System with the help of its powerful spectrometers, instruments that can analyze how starlight bounces off faraway planets. How a planet absorbs and reflects that light can tell observers a lot about what gasses envelop it, a crucial step to finding other worlds that could conceivably play host to life.
But even the most powerful spectrometers cannot pierce all the mysteries of rocky planets, worlds composed primarily of rock, metal and water like the Earth and the most suitable candidates for life. The mineral makeup of these planets is hidden below their surface and invisible to Webb’s probing glare, but visualizing it would help scientists understand where in the universe life can emerge, according to Pierrehumbert.
Looking at lava worlds helps get around this problem. When rock turns to magma, gases like sodium vapor and silicon monoxide can escape and lets scientists view the internal makeup of rocky planets. With Webb’s powerful tools, scientists can effectively create a weather report for worlds light years away from our solar system, and what materials went into making these alien planetary environments.
“Even though lava worlds are too hot to bear life, they still tell us something about how rocky planets are formed and what their chemical composition is,” Pierrehumbert said. “We [study] lava worlds to understand the basic planetary formation process.”
Beacons of light in the darkness
The lava world candidates Pierrehumbert and his team have identified are relatively few and far between, but their incandescent brightness can be hard to miss with the right equipment.
“Usually planets have to be closer to us in order to be able to tell anything about their climate. But lava planets are so hot you can actually push that out a good bit farther,” Pierrehumbert said. “Even though lava planets aren’t that common, they’re easier to characterize so we have a bigger search volume.”
To be a lava world, a planet must orbit close enough to its host star to become tidally locked. Think of how the Moon always orbits the Earth with the same side facing us. Tidally locked planets have permanent “day” sides facing their star and “night” sides looking out into space. On most worlds, this leads to dramatic temperature differences, with barren deserts covering a planet’s day side and perpetually frozen wastelands on its night side. On day sides, temperatures can get hot enough to break rock into magma and release the gas and mineral signatures Pierrehumbert and his team are looking for.
But lava world candidates also require an atmosphere for scientists to observe them, something that is far from a guarantee on tidally locked planets. Orbiting so close to a star makes a planet vulnerable to powerful solar rays, which can obliterate an atmosphere over time and render a planet unable to retain gasses. Even if a planet’s day side has been turned to magma, the absence of atmosphere will cause the gasses that make up a planet’s interior to simply escape into outer space before they can be traced.
So for a planet to be considered a viable lava world candidate, it must be tidally locked and possess an atmosphere. If these boxes are checked, atmospheric wind patterns can transport gases released on its superhot day side over to the colder night side. Once gases move to a lava world’s night side, the relatively mild temperatures cool them down, where mixtures can combine to form quartz, the same mineral that makes up beach sand here on Earth. The end result is a fiery planet with highly reflective quartz clouds that rain or snow sand down on the surface.
The main gas Pierrehumbert is looking for is silicon monoxide, which can break off relatively easily from the silicate rock that rocky planets are primarily made of. Silicon monoxide is the team’s prime target because it is “the most abundant thing” Webb can observe on lava worlds at the moment, Pierrehumbert said.
“If we can’t even see the [silicon monoxide], it’s going to be hopeless to see other things,” he added.
Eventually, if Pierrehumbert and his team can detect abundant silicon monoxide, it could help score more observation time to hunt for rarer, more telling elements, such as magnesium or calcium. Data on these chemicals’ concentrations would then help scientists test against theories of how the Earth was formed.
The team has already found evidence of another, unknown gas floating around lava worlds. Pierrehumbert has seen signs of thicker atmospheres slightly moderating temperatures between the planets’ day and night sides, he announced when presenting recent findings at the Comer conference. Pierrehumbert’s early guess is that the culprit could be carbon monoxide, a compound lighter than Earth’s air and deadly to human and most other animal life. Carbon monoxide has been known to escape from atmospheres such as the one surrounding Mars. Pierrehumbert’s team is still analyzing the findings, but the results could help refine models of what gasses are most likely to escape from atmospheres on rocky planets.
Window to other worlds
Many of Pierrehumbert’s observations would have scarcely been possible without access to the James Webb Space Telescope, the first telescope designed specifically with the goal to observe exoplanets and alien atmospheres in granular detail.
“Webb has been revolutionary,” Pierrehumbert said. “Before Webb, we really had very little capability to characterize the atmospheres of small planets.”
Webb’s long list of accomplishments since its launch on Christmas Day 2021 include detecting the most distant black hole identified to date and one of the most detailed images of Neptune’s elusive rings. Webb’s equipment can collect light from much further away than past generations of telescopes, and scientists have relied on its enormous mirrors and sensitive instruments to categorize faraway exoplanets in greater detail than ever before using infrared astronomy.
Webb is designed to read light as heat radiation, or infrared, which allows scientists to see celestial bodies and events at extreme distances. The Hubble telescope, often referred to as Webb’s predecessor, could only capture light in the visible spectrum, but as light travels over cosmic distances, its wavelength is gradually stretched to a point where it becomes invisible to us—a process known as redshift. Where the human eye cannot see, Webb is able to spot the infrared signatures of redshifted light.
Data collected with Webb will likely be the “only game in town” supporting Pierrehumbert’s research for the next decade or so as far as space-based observations of exoplanets go, but new ground-based telescopes could also help soon enough. The desert plains of northern Chile, long host to some of science’s most advanced space-observation equipment, will soon welcome two flagships of the next generation of ground-based telescopes. The European Southern Observatory’s Extremely Large Telescope, expected to begin observations in 2028, and the Carnegie Institution for Science’s Giant Magellan Telescope, slated to be operational by the early 2030s, will provide scientists with even more tools to pull the curtain on the secrets of lava worlds.
Ground-based telescopes can be a disadvantage for exoplanet observations, as the Earth’s atmosphere scatters some of the incoming light from other stars, but there are huge upsides too. Unlike in space, there are virtually no limits on the size of mirrors with which scientists can build telescopes while abundant energy supply means measurements can be taken with extremely high precision, and images snapped with granular detail and resolution.
“You can do a lot of things that you can’t do with the sort of spectrometers that fly in space,” Pierrehumbert said. “The new ground-based telescopes and the new spectrometers should make it easier for us to go after things like sodium, magnesium, calcium and all these smaller, less abundant species.”