Scientists Find the Coldest Known Lava Planet — and It Has an Atmosphere

September 30 (IT Home) — Scientists have discovered a new planet that overturns every expectation. It is a lava planet, and yet it is also very cold. In fact, it is the coldest lava planet with an atmosphere that humans have ever observed.

The researchers observed HD 3167 b, a rocky super-Earth whose surface is covered in lava, orbiting a star 154 light-years from Earth. The planet’s orbit lies extremely close to its star: on this world, a single “year” — one complete trip around the star — lasts only as long as a day on Earth. It is not only a rocky planet wrapped in lava, but also the smallest lava planet researchers have found to date. Even so, the planet has managed to hold on to its atmosphere.

“What is very surprising is that the closer a rocky planet orbits its star, the harder it should be to keep an atmosphere, because it is constantly battered by stellar wind and receives enormous numbers of high-energy photons from the star,” said Edwin Kite, an associate professor of geophysical sciences at the University of Chicago and a co-author of the study, in a statement. “But it now appears that many of these lava planets do in fact retain atmospheres.”

Since the first exoplanet — a planet outside our solar system — was confirmed more than thirty years ago, scientists have discovered more than 6,300 worlds of every kind. The variety is astonishing, ranging from gas giants like hot Jupiters to small Neptune-like bodies. Among them are a handful of “lava planets”: rocky worlds so hot that their surfaces have melted into molten rock.

For this new study, the research team used the Mid-Infrared Instrument (MIRI) aboard the James Webb Space Telescope. The instrument can capture a planet’s mid-infrared signal, helping researchers estimate its temperature and probe whether an atmosphere is present. The team observed the planet during a “secondary eclipse” — the moment when the planet passes behind its star — which allowed them to capture changes in the mid-infrared light.

The planet is tidally locked, which is very common among lava planets. In other words, one side always faces the star while the other is permanently shrouded in darkness. Yet the newly discovered world differs slightly from its peers. Brandon Park Coy, a graduate student in Kite’s research group and the first author of the study, noted in the same statement that if the planet had no atmosphere, its dayside temperature, given how close it sits to its star, “would reach the theoretical maximum.” The observations, however, showed otherwise. Coy added that what the team actually observed was a planet with an unusually low temperature that still retains an atmosphere — a planet “clearly below the highest temperature theoretically possible.”

The researchers concluded that the planet must have an atmosphere, which reflects the star’s light and carries heat around to the planet’s night side, cooling the dayside. So although it is still hot enough to have a molten surface, it is the coolest of all the lava planets known to have an atmosphere.

Strangely, although this planet’s atmosphere was inferred precisely from its temperature, scientists had previously found atmospheres on extremely hot lava planets. That raises a series of new questions: how exactly do such planets form atmospheres, and what range of temperatures allows them to do so?

The new finding is part of a larger research programme devoted to answering those questions. Led by University of Maryland astronomer Megan Weiner Mansfield, the project will observe ten different lava planets in order to work out the temperature conditions under which such planets typically form atmospheres.

The discovery also lends further support to a view held by a growing number of scientists: that the great majority of lava planets probably have atmospheres. By studying these distant and strange alien worlds, exoplanet science keeps pushing back the boundaries of what we know — and it also helps us better understand Earth itself, and how life might arise and persist in the universe.

“These planets are far too hot to support life, but by studying them we can deepen our understanding of the evolutionary processes that matter for other rocky planets,” Kite said.

Coy added: “Even though these worlds are completely unsuited to life, we are still interested in studying them, because we think the early Earth may have looked very much like a lava planet. In the very earliest days of the solar system, when the terrestrial planets had just formed, the enormous energy released by countless impacts made them extremely hot. Earth was then in its magma-ocean stage, with its entire surface molten. This research also opens a window onto the environmental conditions of Earth’s first few million years.”

The research has been formally published in The Astrophysical Journal Letters.