- Mysterious red dots, called LRDs, appear in Webb telescope images, puzzling astronomers.
- Initially thought to be early massive galaxies or black holes, theories remain unconfirmed.
- LRDs might be a "missing link" in understanding supermassive black hole formation.
LONDON — 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 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."
