Astronomers have discovered an unusual rocky planet that fits two seemingly incompatible descriptions at once: it is a scorching lava world, yet at the same time the coolest known lava world with an atmosphere. The planet HD 3167 b is located 154 light-years from Earth and completes an orbit around its star in just one Earth day.

Observations using the James Webb Space Telescope showed that the planet is noticeably cooler than it should be without an atmosphere. According to the researchers, the atmosphere helps redistribute heat between the planet’s day and night sides. The results were published in The Astrophysical Journal Letters.

A year on this planet lasts just one day

HD 3167 b is a rocky super-Earth and the smallest lava world known so far. The planet orbits extremely close to its star, so one full revolution around it takes about 24 hours.

Because of this proximity, HD 3167 b is likely tidally locked: one side always faces the star, while the other remains in perpetual darkness.

Such conditions should lead to extreme heating. If the planet had no atmosphere, its dayside would reach the highest temperature possible for this system.

But observations showed otherwise.

Why the planet turned out to be unexpectedly cool

Researchers observed HD 3167 b using the James Webb telescope’s MIRI instrument, which operates in the mid-infrared range. Scientists studied the planet during a secondary eclipse — the moment when it passes behind its star.

By comparing the system’s infrared emission before and during such an eclipse, astronomers were able to estimate the planet’s temperature.

It turned out that HD 3167 b is significantly cooler than the expected maximum.

According to the study’s lead author, Brandon Park Coy, if there were no atmosphere, the planet’s dayside would be as hot as physically possible given its position relative to the star.

But the observed temperature points to the presence of an atmosphere.

The atmosphere transports heat to the night side

The researchers suggest that HD 3167 b’s atmosphere reflects some of the star’s radiation and helps redistribute energy across the planet.

Some of the heat received by the dayside may be carried toward the side locked in permanent night. As a result, the surface facing the star is cooler than it would be on a completely airless lava world.

At the same time, the planet remains hot enough for its surface to be covered with molten rock. That is why HD 3167 b remains both a lava world and the coolest known member of this class with detected signs of an atmosphere.

This is especially interesting to researchers because such close proximity to the star would seem to make an atmosphere on this kind of planet extremely unstable. Strong stellar wind and high-energy radiation should gradually strip away its gaseous envelope.

“The closer a rocky planet is to its star, the harder it is for it to hold on to an atmosphere,” explains study co-author Edwin Kite of the University of Chicago. However, observations of lava worlds show that atmospheres can survive on them even under extreme conditions.

Lava worlds may reveal clues about the young Earth

HD 3167 b is far too hot for life. But that is exactly why it is of interest to scientists.

Astronomers suggest that planets like this can serve as a kind of model for studying the early stages of rocky planet formation.

When Earth was still forming, collisions between planetesimals released enormous amounts of energy. The surface of the young planet may for a time have turned into a global magma ocean.

“We think that at the very earliest stage of the Solar System’s history, when the terrestrial planets were forming, they were extremely hot because of the energy released by planetesimal collisions,” says Coy.

Earth also went through a magma ocean stage, when its surface was almost completely molten.

Therefore, observing a modern lava planet makes it possible to study processes that may have occurred on Earth in the first millions of years of its existence.

Scientists are testing where lava-world atmospheres reach their limit

The discovery of HD 3167 b became part of a larger program devoted to the atmospheres of lava planets. Researchers are studying ten such worlds to determine at what temperatures rocky planets are able to retain or form gaseous envelopes.

The paradox of HD 3167 b is especially important for this task: its temperature is lower than expected, and the presence of an atmosphere, judging by the observations, helps explain this difference.

Moreover, astronomers had previously already found signs of atmospheres on even hotter lava worlds. This raises a new question: how do atmospheres form and survive on planets that receive such intense radiation from their stars?

As James Webb studies more and more such objects, scientists will be able to compare them with one another and determine how common atmospheres are among lava worlds.

These planets themselves are uninhabitable, but they may turn out to be a kind of window into Earth’s past. As Kite notes, studying such extreme worlds helps us understand processes that matter for the evolution of much more temperate rocky planets — including our own.