There is a planet 154 light-years away, in the constellation Pisces, where a year lasts a single Earth day. Its surface is hot enough to melt rock. By every rule astronomers thought they knew, it should be a naked ball of magma — any atmosphere long since blasted away by stellar wind and high-energy radiation.
It has an atmosphere anyway.
The planet is HD 3167 b, a rocky super-Earth orbiting a K-type star so closely that it completes a full lap in about 24 hours. And according to new research trending through the science press on September 30, it is now the coldest lava world found to date with evidence of an atmosphere. “Coldest” is doing a lot of heavy lifting in that sentence — this is still a world of molten rock — but in the physics of atmospheres, relative cold changes everything. Or so we thought. HD 3167 b just voted otherwise.
How do you weigh air on a world of lava?
You cannot exactly send a weather balloon. The team used the James Webb Space Telescope and a technique called the secondary eclipse method: they measured the tiny dip in light when the planet slips behind its star, disappearing from view. By comparing the system’s brightness with and without the planet’s contribution, astronomers can work out how hot the planet’s day side is.
Here is the part that made researchers sit up: the day side was cooler than expected. On an airless rock, heat has nowhere to go — the day side broils while the night side freezes. But HD 3167 b’s dayside temperature suggested heat was being carried around to the night side. Something was moving that heat. The best explanation, and the one the team landed on, is an atmosphere redistributing warmth around the globe — the same kind of heat-spreading blanket effect we see on Venus.
The study was led by University of Chicago graduate student Brandon Park Coy, working with Edwin Kite, and published in The Astrophysical Journal Letters. It is the first result from a broader program led by Megan Weiner Mansfield of the University of Maryland, which is surveying ten ultra-hot lava worlds. One world in, and the survey has already broken a record. Not a bad start.
Why this one matters more than most
Of the more than 6,300 exoplanets catalogued so far, only a handful are rocky worlds with evidence of atmospheres. Gas giants are easy to study; small rocky planets are faint, their atmospheres whisper-thin against the glare of their stars. Every confirmed rocky atmosphere is precious data.
But HD 3167 b carries an extra layer of meaning, and it is the one Coy himself highlighted: “We’re interested in studying these kinds of planets because we think early Earth might have looked a lot like a lava world.”
Read that again. Four and a half billion years ago, our own planet may have been a magma-ocean world not unlike HD 3167 b — and yet here we are, with oceans, forests, and someone writing about it on a Wednesday morning. Understanding how a lava world can cling to an atmosphere is, in a very real sense, studying the opening chapter of our own story. The question is not just “what is that planet like?” It is “how did a place like that become a place like this?”
That is the quiet thrill running underneath the exoplanet field right now. Each of these scorched rocks is a time machine pointed at Earth’s infancy.
What comes next
The Mansfield survey still has nine lava worlds to go, and HD 3167 b has set the tone: expect surprises. A few things to watch:
Confirmation and composition. “Evidence of an atmosphere” is the careful phrasing of good science. Follow-up observations will try to pin down what that atmosphere is actually made of — and how thick it is. Different gases tell different stories about where the air came from: outgassed from the interior, or delivered from elsewhere.
The wind-stripping puzzle. Close-orbiting rocky planets face a brutal environment. Stellar wind and high-energy photons should strip atmospheres fast. HD 3167 b is holding on anyway, which means our models of atmospheric escape need updating — or the planet has a way of replenishing its air that we have not figured out yet. Either answer is interesting.
The early-Earth connection. Every lava world with an atmosphere is another data point for models of how Earth kept its own air through the magma-ocean era. The same spirit of patient, ambitious engineering that is teaching aircraft to fly themselves past highway traffic is at work here — except the vehicle is a space telescope, and the destination is deep time.
There is something deeply optimistic about this kind of discovery. The universe keeps handing us worlds that should not exist according to the old rules, and each one forces the rules to get better. HD 3167 b is 154 light-years of molten rock with a wisp of atmosphere it has no business keeping — and it may be holding a mirror up to the planet we live on.
Not bad for a Wednesday. It has been a week for looking up — whether at the future of flight over New York or at a lava world 154 light-years out — and the view keeps getting better.
