Like tiny interlopers in photos, cosmic anomalies that look like tiny bright red dots appear in nearly every snapshot taken by the most powerful space telescope ever built. Astronomers now call them little red dots, or LRDs, but there is still no consensus on what exactly they are.
Since NASA’s James Webb Space Telescope began observing the universe four years ago, hundreds of these puzzling objects have appeared in its images. Its unknown origins, in practice, gave rise to a scientific case that hundreds of studies have attempted to solve.
“It’s the first time in my career that I’ve studied an object where we don’t really understand why it looks the way it does,” said Jenny Greene, a professor of astrophysical sciences at Princeton University. “I think it’s fair to call them a mystery.”
One thing was clear from the beginning: these strange objects were common. “Every deep look you did with James Webb, you found some,” Greene said, referring to the action of focusing the telescope on the same portion of the sky for an extended period to collect extremely faint light.
At first, some astronomers suggested that the dots could be massive galaxies from the early universe, or black holes surrounded by dust. However, those initial assumptions were later disproven by subsequent observations, opening the way to several new hypotheses, many of them still related to black holes.
“I certainly think they are powered by growing black holes, but there are other, more exotic suggestions, like some kind of very massive star that is dying,” Greene said. An expert in supermassive black holes and galaxy evolution, she explained that she believes that a black hole as the main component of LRDs fits with the largest number of observations made so far on these objects.
However, he added, someone could make a completely new observation that overturns every assumption about what LRDs are. “So far, that’s what’s happened. We’ve had one expectation and it’s been wrong. We’ve had another expectation and it’s been wrong. So I would still leave that possibility open.”
Whether these curious dots end up confirming older theories or represent a novel discovery, scientists are on the verge of gaining a new understanding of the universe.
The name little red dots first appeared in a 2024 study, almost two years after scientists had begun studying the objects.
The nickname was coined by Jorryt Matthee, head of the galaxy astrophysics research group at the Austrian Institute of Science and Technology, who chose it because it was simpler and more catchy than the more scientifically precise term: “broadline H-alpha emitters.”
The reason astronomers only detected LRDs after Webb came online is that other telescopes operating at the time, such as Hubble, did not have enough resolution or lacked sensitivity at the longer infrared wavelengths, beyond the threshold of visible light, to see them. But the Webb telescope, with its 6.5-meter-wide primary mirror, has revealed previously hidden objects.
The dots look red because they are so far away, and as the universe expands, light from extremely distant objects stretches into the infrared as it travels to reach Earth, a phenomenon astronomers call “redshift.”
But the dots are also inherently red, although the exact reason why they are is one of the most difficult parts of the puzzle.
“The main interpretation in our 2024 study was that these are growing black holes, and that they are red because they are surrounded by dust particles,” Matthee said. “I would say that was the consensus after our paper for at least a year or two, but now the consensus has actually changed a little bit. We still think they are growing black holes, but now we think they are red not because there is dust, but because there is hydrogen gas.”
Much of the uncertainty around objects comes from their distance. Although astronomers have detected about 1,000 of them, Matthee noted that almost all of them are incredibly far away.
“LRDs are widespread in the early universe—mainly in the first 1 billion years of cosmic time, with the current age being 13.8 billion years—but are extremely rare in the closer, or later, universe,” he explained, referring to the fact that observing a distant object in space essentially means looking back in time. That’s because the farther away something is, the longer it takes for its light to reach us.
Last year, a team of researchers found three LRDs much closer to Earth for the first time, and studies are underway to analyze them. But, based on that finding, Matthee said, local LRDs could be 100,000 times rarer than those found farther away in the early universe.
However, if more local LRDs are found, they could reveal more of their secrets, because it is easier to study an object that is closer.
“As for how LRDs could change our understanding of black holes, I think they could turn out to be some kind of missing link,” Matthee said. “We know that galaxies, like our own Milky Way, have supermassive black holes at their centers, and while this is very common, it’s basically a mystery how these supermassive black holes formed. LRDs could actually be the birth phase, or baby phase, of this formation, and we could be observing that for the first time.”
The closest thing to a census of the little red dots came in 2023, after a team of researchers led by Anna de Graaff, a Clay Fellow at the Harvard–Smithsonian Center for Astrophysics, began a program called RUBIES, or Red Unknowns: Bright Infrared Extragalactic Survey. The program spent a significant amount of Webb telescope time—60 hours—analyzing thousands of bright, red objects.
“It was really the first program to go after these red sources systematically, looking at all kinds of strange objects — not just little red dots — but among them, also about 40 LRDs,” de Graaff said.
The biggest surprise, de Graaff added, is an object he calls “The Cliff,” whose characteristics seem to refute early hypotheses about what LRDs could be. “This source is really the first one where we were able to say unambiguously: this is neither a normal galaxy nor a black hole wrapped in dust; it has to be something else,” he said. “It was a breakthrough moment.”
The Cliff got its name because its light spectrum has a very pronounced transition—from a weak ultraviolet to a deep red. “A feature that can only be caused by very dense hydrogen gas that is somewhat warm in temperature,” de Graaff said. “This is surprising, because it means that LRDs are not red because they have old stars or because they have dust, but they are red because the light is being absorbed by a very dense gas surrounding a central engine, which we believe is a black hole. And that is something that has never been observed before,” explained de Graaff, underlining the fact that The Cliff suggests the existence of a new type of cosmic object.
In some articles, de Graaff refers to such objects as “black hole stars,” a name he describes as a bit sensational, but not entirely wrong.
“We do believe that there is a black hole there that is feeding it, and the light from this black hole illuminates the gas around it, in a way that is a little similar to what we see in stars,” he said. Black holes themselves do not emit light, but the superheated material that falls into them shines brightly, which is why growing black holes are among the brightest objects in the universe.
The Cliff also shares similarities with theoretical objects called quasistars, which were predicted in 2006—long before the little red dots were discovered—by Mitch Begelman, a professor in the department of astrophysical and planetary sciences at the University of Colorado Boulder, along with colleagues Marta Volonteri and Martin Rees.
They described a quasistar as a star that is powered not by nuclear fusion, but by a black hole, which is surrounded by a huge cloud of gas that makes it shine like a star. Unlike the black hole star described by de Graaff, a looser term for a star powered by a black hole of unknown origin, a quasistar is a defined theoretical model, in which the black hole is the result of the collapse of a massive protostar.
“I realized that we had predicted the existence of black holes with huge envelopes of matter. I don’t think we necessarily have smoking gun that this is the explanation for LRDs, but so far I haven’t seen any evidence that poses an insurmountable problem for that scenario,” Begelman said.
A rare hybrid between a star and a black hole would be a new type of cosmic object, so there is understandably some caution among researchers about declaring quasi-stars the winners of the little red dot debate.
“It could well be that LRDs are quasi-stars, but in my opinion we haven’t completely ruled out other scenarios yet,” Matthee said. “I would certainly love this to be true, as it would imply that we discovered a new type of astrophysical phenomenon that connects stars and supermassive black holes, but it is too early to tell, in my opinion.”
For de Graaff, the main problem with quasistars is that they are a specific type of object, and we simply don’t know enough about LRDs yet.
“It is very difficult to prove that there is a black hole in the LRD; the evidence is non-existent at the moment,” he said. “The only reason we believe there are black holes in them is because they are so luminous and because there are so many of them.
“That is our scientific intuition, but truly proving it is difficult.”
It’s hard to pinpoint what stage of the debate over the little red dots the scientific community might be at right now, but most researchers believe they’re not even close to a resolution. However, that’s what makes the objects so interesting.
“I think they’re James Webb’s biggest surprise, and it’s the kind of surprise you’d expect,” de Graaff said.
“James Webb is a $10 billion space mission, and you hope to find things that are really unknown,” he added. “I think it’s delivered. It’s really given us a new puzzle, something that looks a little bit like a galaxy, a little bit like a black hole, and a little bit like a star — experts from all of these communities are now trying to contribute and propose their favorite theory or ideas. And I think that’s really unique.”
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