Astronomers have discovered an unusual star system about 300 light-years from Earth where a small star is gradually consuming a nearby brown dwarf. Instead of swallowing its companion in one dramatic event, the star is steadily pulling material from it, creating a cosmic feeding process that could continue for billions of years.
Most planets, including Earth, travel around their stars in relatively stable orbits. But these peaceful arrangements can sometimes end violently. If a planet ventures too close to its host star, the star’s gravity can drag it inward and eventually engulf it.
Scientists have observed many planetary systems with stable orbits, along with a small number of stars caught rapidly swallowing their planetary companions. Now, researchers have identified something that falls between those two extremes.
In a study published October 5 in Nature Astronomy, an international team led by scientists at MIT describes a star that is slowly feeding on a brown dwarf, an object more massive than a typical planet but not massive enough to sustain the hydrogen fusion that powers ordinary stars.
The system, designated ZTF J0440+2325, lies within our Milky Way galaxy approximately 300 light-years away. It is the first observed example of a low-mass star steadily drawing material from another low-mass object.
Even more remarkably, the transfer of material appears to be happening so slowly that the star could continue feeding on its companion for hundreds of thousands of years, potentially even billions.
“When we think of stars interacting with planets or brown dwarfs, the picture is always that the star eventually swallows the other thing,” says Kevin Burdge, assistant professor of physics at MIT. “This is what will happen to the Earth when the sun becomes a red giant. But here, we’ve found an alternative: Instead of swallowing the thing up, the star can gradually eat it, for billions of years.”
The MIT research team also includes Aaron Householder, Kaitlyn Shin, Saul Rappaport, Joheen Chakraborty, and Emma Chickles. Additional collaborators contributed from Caltech, the University of Hawaii, the Instituto de Astrofísica de Canarias and Universidad de La Laguna in Spain, and the Harvard-Smithsonian Center for Astrophysics.
A Mysterious Signal That Puzzled Astronomers for Years
The first clue to this unusual stellar relationship came from the Zwicky Transient Facility (ZTF), an astronomical survey that searches for objects whose brightness changes over time.
Operating with a telescope at Palomar Observatory in California, ZTF repeatedly photographs large sections of the night sky. Its sensitive camera can detect sudden or unusual changes in light that may indicate powerful cosmic events, including exploding stars called supernovae, gamma-ray bursts, and collisions between neutron stars.
Several years ago, while examining observations collected by ZTF, Burdge noticed a particularly strange pattern.
Astronomers study changes in an object’s brightness using a graph called a light curve. For some supernovae, these graphs form a rounded, bell-like shape as the exploding star grows brighter and then gradually fades.
The signal Burdge encountered looked completely different. Instead of a smooth rise and fall, the brightness repeatedly traced something resembling a triangle.
“I remember first looking at this and thinking, stars don’t make triangular waveforms like this,” he recalls.
At the time, Burdge and his colleagues were investigating another unusual signal that turned out to originate from a system known as a “black widow binary.”
These systems contain a neutron star, the extraordinarily dense remnant of a massive star, that spins rapidly while stripping material from a much smaller companion. They are named after black widow spiders because of the way the neutron star gradually destroys its partner.
Burdge initially considered whether the repeating triangular signal might come from another black widow system. However, the observations did not match what astronomers normally expect.
In a typical black widow binary, the strong contrast between the masses of the two objects produces a distinctive motion. The smaller companion moves rapidly around the much heavier neutron star, creating detectable changes in the light reaching Earth.
But the mysterious triangular signal showed no evidence of the same pronounced motion.
“We weren’t seeing that whipping back and forth here,” Burdge says. “It didn’t make any sense. We couldn’t explain what this was.”
That discrepancy led the researchers to consider another possibility. Perhaps the system did not contain one extremely massive object paired with a much lighter companion. Instead, both objects might have relatively small masses, allowing them to orbit each other with much less dramatic motion.
“If you have less mass in the system overall, things can gently orbit each other without whipping back and forth,” Burdge says. “That was the idea. But we never had any proof. And this weird triangle just sat for years.”
A Brown Dwarf That Orbits Its Star Every 87 Minutes
Eventually, Burdge and Aaron Householder, a graduate student in MIT’s Department of Earth, Atmospheric, and Planetary Sciences, returned to the unexplained signal.
Using the original ZTF observations, they identified its source as an object roughly 300 light-years away in the Milky Way. They then directed several telescopes toward the system, ZTF J0440+2325, to investigate its properties in greater detail.
A key measurement involved determining how much the system’s objects move in response to their mutual gravitational attraction. Although the researchers detected some wobbling, it was far smaller than the motion typically associated with black widow binaries.
“That was the real clincher for this system,” Householder says. “When we measured that wobble, we found we were not seeing a black widow. This was a low-mass star that’s orbited by a brown dwarf. The wobble was too small in amplitude to be anything else.”
The observations revealed two unusually compact objects orbiting at an extremely close distance.
The central star has approximately 85 times the mass of Jupiter, while the brown dwarf weighs about 25 Jupiter masses. For comparison, the Sun is more than 1,000 times as massive as Jupiter.
The brown dwarf completes a full revolution around its host star in just 87 minutes. Its entire orbit is so small that it would fit within the diameter of the Sun.
That extraordinarily close arrangement suggested that gravity might be doing more than simply keeping the objects in orbit. The researchers suspected that the star could be actively pulling matter away from the brown dwarf.
Scientists Discover a New Form of Stellar Cannibalism
When one astronomical object draws gas or other material from another, astronomers call the process accretion.
Accretion is especially familiar in systems involving black holes and neutron stars. Although these objects can be extremely massive, their physical dimensions are relatively small. Material falling toward a black hole, for example, often forms a swirling disk around it before moving inward.
But ZTF J0440+2325 appears to work differently.
Rather than a black hole or neutron star collecting matter, an ordinary low-mass star is receiving material from a nearby brown dwarf. Because the star has a much larger physical surface than a compact object such as a black hole, the incoming material can strike it directly.
“The difference here is, the thing absorbing matter is not a tiny black hole but a star, which is relatively big in size,” Burdge explains. “So matter just pummels directly onto the surface, at very high speeds, like an asteroid hitting the Moon.”
To investigate whether this was actually happening, the researchers created computer simulations of the system.
They modeled particles originating from the brown dwarf and calculated how those particles would move under the gravitational influence of both objects. The simulations incorporated the measured properties of the star and its companion, along with the physical equations governing orbital motion.
The results supported the team’s suspicion. Material leaving the brown dwarf followed trajectories that carried it directly into the star.
“When we track those test particles, we see they indeed fall right onto the surface of the star,” Householder says. “This is the first time we’ve caught a low-mass star actively accreting from another low-mass object.”
A Cosmic Feast That Could Continue for Billions of Years
The team also estimated how quickly the brown dwarf is losing material.
According to their calculations, the star is consuming approximately one hundred-thousandth of Earth’s mass every year.
That amount may sound insignificant, but it represents an enormous quantity of material. The researchers compare it to approximately 40 million dump trucks’ worth of matter, or about 1.3 trillion one-pound burritos every second.
Despite those staggering figures, the brown dwarf is so massive that this material loss represents only a tiny portion of its total mass.
In astronomical terms, the feeding process is remarkably gradual.
Based on the properties of the system and the estimated rate of mass transfer, researchers believe the star could continue drawing matter from its companion for billions of years.
Instead of a sudden, catastrophic collision that destroys the smaller object, the star appears to be sustaining a relatively steady flow of material from its neighbor.
That realization also helped explain the strange triangular light pattern that first attracted Burdge’s attention.
As matter streams from the brown dwarf toward the star, it strikes the stellar surface at high speeds, generating an intensely heated region.
This bright hotspot acts like a persistent fireball on the star.
As the brown dwarf moves through its 87-minute orbit, the hotspot repeatedly rotates into and out of the observer’s view. Those changes in visibility produce the distinctive pattern of brightening and dimming that the astronomers had struggled to understand.
“It’s like you’ve got this continuous fireball onto one of the objects, and as one orbits the other, that hotspot comes in and out of view, and the peak of the triangle signal is when you’re looking right at the fireball,” Burdge explains.
A Strange Star System Opens a New Window Into Planetary Evolution
With the triangular signal finally explained, the researchers are now interested in identifying other stellar systems that behave in similar ways.
Until now, observations of close interactions between stars and their planetary companions have largely emphasized either stable orbits or relatively rapid engulfment events.
ZTF J0440+2325 demonstrates that another possibility exists. Under the right conditions, a small star can steadily extract matter from a nearby brown dwarf without immediately destroying it.
Finding additional examples could help astronomers determine how common this process is, how long it can persist, and what ultimately happens to objects trapped in such extraordinarily close relationships.
The discovery may also provide new insights into the evolution of planets and brown dwarfs that orbit near their host stars.
“It’s inspiring a lot of new searches on our part,” Householder says. “I think we’re going to learn a lot about a different kind of way that planets and brown dwarfs interact with their host stars.”
This research received partial support from the National Science Foundation.
