The Coolest Lava World Yet May Have an Atmosphere — And It Could Reveal What Early Earth Looked Like

When scientists search for potentially habitable worlds beyond our Solar System, they often look for rocky planets with atmospheres. Earth is the obvious example: its atmosphere helps regulate temperature and allows liquid water to exist on the surface.

But rocky planets with atmospheres appear to be surprisingly difficult to find.

Now, astronomers have discovered evidence of an atmosphere around an unusual world that is anything but habitable. Located about 154 light-years away in the constellation Pisces, the planet HD 3167 b is a rocky “lava world” so close to its star that it completes an entire orbit in just one Earth day.

What makes it especially interesting is that it is reportedly the coolest lava world yet found with evidence of an atmosphere.

The discovery, led by University of Chicago scientist Brandon Park Coy, was published in The Astrophysical Journal Letters. Researchers say this strange, extremely hot planet could help answer a much bigger question: How did rocky planets like Earth develop their atmospheres in the first place?

A Planet Covered in Extreme Heat

HD 3167 b belongs to a class of planets known as lava worlds. These planets orbit extremely close to their stars, receiving enormous amounts of radiation and heat.

Because of this intense heating, the star-facing side of a lava world can become hot enough for rocks to melt. Scientists believe the surface of HD 3167 b may contain molten or partially molten rock.

The planet is also a super-Earth, meaning it is larger than Earth but still considered a rocky planet rather than a gas giant.

At first glance, a planet this close to its star might seem like one of the least likely places to find an atmosphere.

That is exactly what makes the discovery surprising.

“The closer a rocky planet orbits its star, the harder it should be to have an atmosphere,” explained University of Chicago geophysicist Edwin Kite, a co-author of the study. Close-in planets are constantly exposed to powerful stellar winds and high-energy radiation that can strip gases away.

Yet observations are increasingly suggesting that some lava worlds can retain atmospheres.

How Can Scientists Detect an Atmosphere So Far Away?

Astronomers cannot simply take a picture of HD 3167 b and see its atmosphere. Instead, they use changes in light to study the planet.

For this research, scientists used the James Webb Space Telescope (JWST) and a technique called a secondary eclipse.

A secondary eclipse happens when a planet moves behind its star from Earth's point of view.

Before the planet disappears behind the star, telescopes detect light coming from both the star and the planet. Once the planet passes behind the star, only the star's light reaches us.

The tiny difference between these measurements reveals how much infrared radiation was coming from the planet.

That information gives scientists an estimate of the planet's temperature.

And temperature can provide an important clue about an atmosphere.

The Temperature Clue

Without an atmosphere, the dayside of a rocky planet very close to its star would be expected to become extremely hot. The exact maximum temperature depends on factors such as the planet's distance from its star and how much light its surface reflects.

But an atmosphere can change this picture.

A thick atmosphere can move heat away from the dayside and distribute it toward the nightside. Clouds can also reflect some incoming starlight back into space, reducing how much energy reaches the surface.

Earth's atmosphere does something similar on a much milder scale by moving heat around the planet.

Venus provides an even more dramatic example. Its thick atmosphere helps keep temperatures relatively similar across different regions of the planet.

HD 3167 b appears to be cooler than scientists would expect if it had no atmosphere.

That lower-than-expected temperature provides evidence that the planet may possess an atmosphere capable of affecting its energy balance.

What Is the Atmosphere Made Of?

This is where the mystery becomes even more interesting.

Scientists currently do not know exactly what HD 3167 b's atmosphere contains.

Because its surface is likely extremely hot, researchers initially expected an atmosphere made largely from vaporized rock.

At such temperatures, minerals from the surface can evaporate and form gases. This could create an atmosphere rich in silicon- and oxygen-bearing compounds.

However, recent observations of other lava worlds have raised the possibility that their atmospheres could contain heavier molecules such as carbon dioxide, carbon monoxide or even water.

For HD 3167 b, researchers still need more observations to determine the exact chemical composition.

Future JWST observations could help identify specific molecules by looking for their characteristic signatures in the planet's infrared light.

Is There a Temperature Limit for Atmospheres?

One of the biggest scientific questions behind this research is whether there is a particular temperature at which rocky planets begin to lose or develop very different types of atmospheres.

The team is studying 10 ultra-hot lava worlds as part of a larger survey led by Megan Weiner Mansfield of the University of Maryland.

By comparing planets with different temperatures, scientists hope to identify a possible transition point.

Perhaps extremely hot rocky planets develop thick atmospheres made from vaporized silicate rock.

Those atmospheres could also produce clouds that reflect some of the star's energy back into space, helping cool the planet's dayside.

HD 3167 b is particularly useful because it sits at a relatively cooler point in this population.

Studying it could therefore help researchers understand how atmospheric behavior changes as lava worlds become hotter or cooler.

Why Study a Planet That Cannot Support Life?

HD 3167 b is far too extreme to be considered a comfortable home for life as we know it.

So why spend valuable telescope time studying it?

Because these planets may act as natural laboratories for planetary evolution.

Scientists believe that Earth itself may have passed through a lava-world-like phase billions of years ago.

When the planets of our Solar System were forming, enormous numbers of planetary building blocks called planetesimals collided with one another. These impacts released tremendous amounts of energy.

As a result, the young Earth may have been hot enough to develop a global magma ocean, with much of its surface temporarily molten.

The conditions on HD 3167 b are not identical to those of early Earth. However, studying an existing rocky planet with an extremely hot surface gives scientists an opportunity to investigate processes that may have occurred during Earth's earliest history.

A Window Into Earth's Deep Past

The discovery of an atmosphere around the coolest known lava world with atmospheric evidence is therefore about more than an exotic planet 154 light-years away.

It could help scientists understand how rocky planets hold onto gases, how atmospheres form, how heat moves around young worlds and how planetary surfaces interact with their atmospheres.

Those processes are important not only for lava worlds but also for understanding planets that eventually become more Earth-like.

As Brandon Park Coy and his colleagues continue observing HD 3167 b, the key question will be what its atmosphere is actually made of.

If future observations can identify its gases and clouds, this scorching world could provide an unexpected glimpse into the conditions that existed when Earth was still a young, molten planet.

In other words, one of the hottest and most hostile planets we know may help scientists understand the earliest chapter of Earth's story.

Reference: Brandon Park Coy et al., Evidence for an Atmosphere on the Ultra-short-period Super-Earth HD 3167 b, The Astrophysical Journal Letters (2026), DOI: 10.3847/2041-8213/ae7f23.