By Jacopo Prisco, CNN
(CNN) — Like tiny photobombers, cosmic anomalies resembling small, bright red points show up in almost every snapshot taken by the most powerful space telescope ever made. Astronomers now call them little red dots, or LRDs, but there is no agreement yet on what exactly they are.
Since NASA’s James Webb Space Telescope started peering into the universe four years ago, hundreds of the puzzling objects have appeared in its images. Their unknown origins effectively launched a scientific case that hundreds of studies have attempted to crack.
“This is the first time in my career that I have studied an object where we truly do not 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 pointing you did with James Webb, you were finding a few,” said Greene, referring to the action of focusing the telescope on the same patch of sky for an extended time to collect extremely faint light.
Initially, some astronomers suggested the dots could be massive galaxies from the early universe, or black holes surrounded by dust. However, these initial assumptions were later upended by further observations, paving the way for several new hypotheses, many of them still involving black holes.
“I certainly think they’re powered by growing black holes, but there are other, more exotic suggestions, like some kind of very massive star dying,” Greene said. An expert in supermassive black holes and galaxy evolution, she explained that she believes a black hole as the main component of LRDs fits the largest number of the observations made of the objects so far.
However, she added, someone could make an entirely new observation that overturns every assumption about what LRDs are. “So far, that’s what’s happened. We’ve had an expectation, it’s been wrong. We’ve had another expectation, it’s been wrong. So I would leave that possibility open still.”
Whether these curious dots ultimately confirm older theories or represent a novel discovery, scientists are set to gain a new understanding of the universe.
A ‘missing link’
The name little red dots first appeared in a 2024 study, almost two years after scientists had begun studying the objects. The moniker was coined by Jorryt Matthee, head of the research group on the astrophysics of galaxies at the Institute of Science and Technology Austria, who chose it because it was simpler and catchier than the more scientifically accurate term: “broad-line H-alpha emitters.”
The reason astronomers only spotted LRDs after Webb came online is that other telescopes in operation at the time, like Hubble, didn’t have enough resolution or lacked the sensitivity in the longer infrared wavelengths, beyond the threshold of visible light, to see them. But the Webb telescope, with its 21.6-foot-wide (6.5-meter-wide) primary mirror, has revealed objects that were previously hidden.
The dots appear red because they are so far away, and as the universe expands, light from extremely distant objects gets stretched 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 is one of the trickiest 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 one or two years, but now the consensus has actually changed a bit. We still think they are growing black holes, but we now think they are not red because there’s dust, but because there’s hydrogen gas.”
Much of the uncertainty around the objects stems from their distance. Even though astronomers have detected about 1,000 of them, Matthee noted they are almost all incredibly remote.
“LRDs are widespread in the early universe — primarily the first billion years of cosmic time, with the current age being 13.8 billion years — but they are extremely rare in the more nearby, or later, universe,” he explained, referring to the fact that looking at 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.
“In terms of how LRDs could change our understanding of black holes, I think they might turn out to be some kind of missing link,” Matthee said. “We know that galaxies, like our own Milky Way, have supermassive black holes in their center, and while this is very common, it’s basically a mystery how these supermassive black holes formed. The LRDs may actually be the birth phase, or the baby phase, of this formation, and we might be observing that for the first time.”
‘Black hole stars’
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, started 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 red and bright objects.
“It was really the first program to go after these red sources systematically, observing all sorts of strange objects — not just little red dots — but among them, also 40 or so LRDs,” de Graaff said.
The biggest surprise, de Graaff added, is an object she calls “The Cliff,” the features of which appear to disprove early hypotheses for what LRDs could be. “This source is really the first one where we could say unambiguously, this is neither a normal galaxy nor a dust-shrouded black hole — it has to be something else,” she said. “It was a bit of a breakthrough moment.”
The Cliff got its name because its light spectrum has a very steep transition — from weak ultraviolet to intense 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 think is a black hole. And that is something that has never been observed before,” de Graaff said, underscoring the fact that The Cliff suggests the existence of a new type of cosmic object.
In some papers, de Graaff refers to such objects as “black hole stars,” a name she describes as slightly clickbaity, but not entirely wrong.
“We do think that there is a black hole there that’s powering it, and the light from this black hole is illuminating the gas around it, in a way that is a little bit similar to what we see in stars,” she said. Black holes themselves do not emit light, but the superheated material that falls into them intensely glows, so growing black holes are among the brightest objects in the universe.
‘Truly unknown’
The Cliff also shares similarities with theoretical objects called quasi-stars, which were predicted in 2006 — well before little red dots were discovered — by Mitch Begelman, a professor in the department of astrophysical and planetary sciences at the University of Colorado Boulder, with colleagues Marta Volonteri and Martin Rees.
They described a quasi-star as a star that’s powered not by nuclear fusion but by a black hole, which is surrounded by a massive cloud of gas that makes it shine like a star. Unlike de Graaff’s black hole star, a looser term for a star powered by a black hole of unknown origin, a quasi-star 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 enormous envelopes of matter. I don’t think we necessarily have the 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 picture,” Begelman said.
A strange hybrid between a star and a black hole would be a new type of cosmic object, so there is understandably some caution from researchers on declaring quasi-stars the winners of the little red dot debate.
“It could well be that LRDs are quasi-stars, but in my view we have not yet fully ruled out other scenarios,” Matthee said. “I would definitely love this to be true, as it would imply we discovered a new type of astrophysical phenomena that bridges stars and supermassive black holes, but it’s too early to tell, in my view.”
For de Graaff, the main issue with quasi-stars is that they are a specific type of object, and we simply don’t know enough about LRDs just yet. “It’s very hard to prove that there is a black hole in LRDs, the evidence is nonexistent at the moment,” she said. “The only reason we think that there are black holes in them is because they are so luminous and because there are so many of them. That’s our scientific gut feeling, but actually proving that is difficult.”
It’s hard to pinpoint at what stage of the little red dot debate the scientific community might be right now, but most researchers think they are not even close to a resolution. However, that’s what makes the objects so interesting.
“I think they are the biggest surprise from James Webb, and it’s the sort of surprise that you’d hope for,” de Graaff said.
“James Webb is a $10 billion space mission, and you hope to find things that are truly unknown,” she added. “I think it has delivered. It’s really given us a new puzzle, something that looks a bit like a galaxy, a bit like a black hole and a bit like a star — experts from all these communities are now trying to chip in and put forward their pet theory or their insights. And I think that’s really unique.”
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