Taking a direct picture of a planet around another star is one of the hardest feats in astronomy. A planet is a faint speck lost in the overwhelming glare of its sun, like trying to photograph a firefly next to a searchlight. The few dozen exoplanets we have managed to image directly nearly all share one quality that makes them possible to see: they are young, and still glowing with the heat of their birth, radiating at more than a thousand degrees. Then, in 2024, the James Webb Space Telescope photographed one that broke the pattern entirely, a planet so cold it hovers near the freezing point of water.
The world is called Epsilon Indi Ab, and at about twelve light-years away it is practically a neighbor. It is the coldest exoplanet ever directly imaged, and studying it has opened a window onto a kind of world we had never been able to photograph before: a mature, frigid giant, more like the planets of our own solar system than the hot youngsters usually caught on camera.
Why cold planets are hard to see
Directly imaging a planet means separating its faint light from the blinding light of its star. This is easiest when the planet is bright in its own right, and young planets are: freshly formed, they still glow with leftover heat, shining at temperatures well above a thousand kelvin. That warmth makes them stand out. But it also makes them unrepresentative. Most planets in the universe are old and cold, their birth heat long since faded. Those are the ones we could almost never image, because they are simply too dim.
Epsilon Indi Ab changed that. Using the mid-infrared instrument on JWST, astronomers captured a mature super-Jupiter, several times the mass of Jupiter, radiating at an effective temperature of only about 275 kelvin, roughly two degrees Celsius, barely above freezing. Nothing we had ever imaged directly came close to being that cold. It was a picture of a genuinely old planet, resembling the giants in our own backyard.
Nearly every planet we had photographed was a glowing newborn. This one was cold enough to freeze water, and we saw it anyway.
An atmosphere like home
Because Epsilon Indi Ab is so cold, its atmosphere behaves more like those of Jupiter and Saturn than like the scorching worlds usually studied. And when JWST examined its chemistry, it found a surprise. The mid-infrared instrument detected less ammonia in the upper atmosphere than models predicted there should be. Something was removing ammonia from view.
The team's best explanation was clouds, specifically a thick, patchy layer of water-ice clouds sitting high in the atmosphere, not unlike the wispy cirrus clouds of Earth. Such clouds could hide the ammonia beneath them, accounting for the shortfall. It was a striking result: high clouds of water ice, drifting through the sky of a giant planet a dozen light-years away, inferred from the light of a world seen only as a single point.
A neighbor worth watching
Epsilon Indi Ab orbits Epsilon Indi A, a star slightly smaller and cooler than the Sun, and one of the nearer stars to us. That proximity, combined with the planet's brightness in the infrared where JWST is most sensitive, makes it one of the best cold giant planets we can study in detail. Follow-up observations have continued to probe its atmosphere, refining what we know about its temperature, composition, and weather.
The importance goes beyond one planet. By proving that JWST can directly image and characterize a cold, mature giant, Epsilon Indi Ab opened a new category of target. The vast population of old, cold planets, long invisible to direct imaging, is beginning to come within reach. We have spent years photographing the universe's newborn worlds because they were the only ones bright enough. Now, for the first time, we are starting to see the grown-ups.
A single point of infrared light, twelve light-years away, resolved into a frozen giant with clouds of ice in its sky. It is the first portrait of a world grown old.
Frequently Asked Questions
What is Epsilon Indi Ab?
Epsilon Indi Ab is a cold super-Jupiter exoplanet about 12 light-years away, directly imaged by JWST and reported in 2024. At around 275 kelvin, roughly 2 degrees Celsius, it is the coldest exoplanet ever photographed directly.
Why is imaging a cold exoplanet so difficult?
Directly imaging a planet requires separating its faint light from its star's glare. Young planets still glow with birth heat above 1,000 kelvin, making them easier to see. Cold, mature planets are far dimmer, so almost none had ever been directly imaged until Epsilon Indi Ab.
What did JWST find in its atmosphere?
JWST's mid-infrared instrument detected less ammonia than models predicted. The best explanation was a thick, patchy layer of high water-ice clouds, similar to cirrus clouds on Earth, which could hide the ammonia beneath them.
How cold is Epsilon Indi Ab?
Its effective temperature is about 275 kelvin, or roughly 2 degrees Celsius, just above the freezing point of water. That makes it far colder than the young, glowing planets that make up nearly all other directly imaged exoplanets.
Why is this discovery important?
It proved JWST can directly image and study a cold, mature giant planet, not just hot young ones. Since most planets in the universe are old and cold, this opens a vast new population of worlds to direct imaging that was previously out of reach.