Brown Dwarfs: NASA's SPHEREx Found Hundreds of Failed Stars Colder Than Boiling Water — and They May Outnumber Every Sun in the Galaxy

Some of them are colder than boiling water. None of them have ever lit up the night sky. And according to NASA's newest space telescope, they might be the most common type of object in the entire galaxy — and we've barely started counting them.

This week, NASA's SPHEREx telescope revealed one of the most comprehensive catalogues of brown dwarfs ever assembled: a menagerie of celestial objects so strange they don't fit neatly into any category. Too massive to be planets. Too small to be stars. Just sitting there, fading in the dark. Invisible to the naked eye for billions of years.

What exactly is a brown dwarf?

A brown dwarf is what happens when a cloud of gas collapses under its own gravity, heats up — and then stalls. It never reaches the critical mass needed to ignite full hydrogen fusion like our Sun. The result is a permanent in-between: not a star, not a planet, just an object ranging from roughly 13 to 80 times the mass of Jupiter, glowing faintly in infrared as it slowly bleeds heat into the void.

They fuse deuterium briefly when young — a different, lighter fuel. Then that runs out. And they cool. And cool. For billions of years. Indefinitely. There is no endpoint, no explosion, no dramatic death. A brown dwarf that formed 10 billion years ago is simply a colder version of its younger self.

That middle number deserves a moment. 250 Kelvin is minus 23 degrees Celsius. Below freezing. The coldest known brown dwarfs — a class called Y dwarfs — are actually colder than a winter morning in Moscow. These are objects more than 13 times the mass of Jupiter, and they are cooler than a freezer. Completely invisible to ordinary telescopes. Just mass, cooling in the dark.

Why is SPHEREx finding so many of them now?

Because we finally have a telescope that sees in the right light. SPHEREx — the Spectro-Photometer for the History of the Universe, Epoch of Reionization, and Ices Explorer, launched in March 2025 — doesn't take dramatic photos of individual nebulae. It scans the entire sky, every six months, in 102 separate infrared wavelength bands simultaneously. Brown dwarfs, which radiate entirely in infrared, light up clearly in SPHEREx data. They were always there. We were just looking in the wrong light.

That estimate — 100 billion — is uncertain but scientifically credible, based on how brown dwarfs form relative to stars. For every sun-like star in our galaxy, there may be a brown dwarf lurking nearby. Cooling quietly. Never visible. The Milky Way may have been hiding half its population in plain sight, in a wavelength we simply weren't watching.

What kinds did SPHEREx find?

The word NASA used was menagerie — and that's deliberate. Brown dwarfs span a temperature range so large they're split into entirely different spectral classes:

• L dwarfs — the warmest, still showing metallic vapours in their atmospheres, ranging from about 1,300 to 2,200 Kelvin. Still hot enough to glow faintly orange-red.
• T dwarfs — cooler, with methane appearing in the atmosphere around 600–1,300 K. Methane is a planetary molecule. Finding it in a near-star is unsettling.
• Y dwarfs — the coldest known class, below ~600 K, some with actual water ice clouds drifting in their upper atmospheres.

That last point is worth pausing on. Y dwarfs have weather. Water ice clouds — cirrus-like wisps of frozen vapour — floating high in the atmosphere of an object that is 30 times the mass of Jupiter. Clouds on a failed star. The line between star and planet does not just blur at this end of the spectrum. It dissolves entirely.

Could a brown dwarf host life?

It's a question scientists are genuinely starting to ask — carefully. Some brown dwarfs have confirmed planets orbiting them, discovered in recent years. At the right orbital distance from a warm brown dwarf, surface temperatures could theoretically permit liquid water. The odds are long. Tidal locking is almost certain at close range, creating a permanent day side and a frozen night side. Radiation environments are unclear. Atmospheric chemistry would be alien.

But if brown dwarfs outnumber stars, and if even a fraction host planets in warm orbits, then the total number of potentially interesting worlds in the galaxy just expanded enormously — toward objects we didn't even know to look for a decade ago. Researchers emphasise they are still at the census stage. Confirming habitability around any of these objects would require a generation of follow-up. For more on what makes a world potentially habitable, the SkyLens learn section breaks down the conditions astronomers look for.

What does SPHEREx actually do — and why does it matter beyond brown dwarfs?

SPHEREx is doing something no prior telescope has done: mapping everything, all the time, in hundreds of colours. Its primary science goal is to trace the large-scale structure of the universe and the history of star formation across cosmic time. But a complete infrared sky survey produces surprises as a side effect. Along with brown dwarfs, early SPHEREx data is flagging organic molecules in interstellar clouds, water ice signatures across the galaxy, and faint structures that don't yet have obvious explanations.

Every sky survey in history has changed astronomy. The Sloan Digital Sky Survey redrew galaxy maps. The 2MASS infrared survey tripled the known star count. SPHEREx is doing both at once, in a wavelength range that the human eye cannot see and that Earth's atmosphere blocks almost entirely from the ground. What it finds in its next twelve months of operation may rewrite multiple textbook sections simultaneously.

Why does any of this matter for the rest of us?

Because it changes the head count. Astronomy has spent centuries refining its census of what exists. Stars: counted. Galaxies: counted. Planets: getting there. Brown dwarfs: until now, essentially a mystery. If the SPHEREx catalogue confirms that brown dwarfs approach stars in total number, then our map of the Milky Way — the thing we've been building since Galileo pointed a tube at the sky — has been missing roughly half the objects in it.

That isn't a minor correction. That is a revision of what the galaxy is. And it came from a telescope that's been operational for less than two years, scanning wavelengths that most of history couldn't see. Follow the SkyLens blog for updates as the full SPHEREx data pipeline continues to publish results — there is significantly more coming. To see what's actively orbiting right now, from the 15,000+ tracked satellites to the objects scientists are just beginning to map, the live tracker is updated in real time.