Like Earth, most planetary bodies circle their star in stable, detached orbits. These companionable systems can suddenly change when a planet comes too close to its star. In such a close encounter, a star can pull the planet in and swallow it whole. 

Across the galaxy, astronomers have seen plenty of stable, detached planetary systems. They have also observed a handful of stars quickly engulfing their planet. Now, for the first time, scientists have spotted a system that is striking a curious balance between the two extremes. And it’s revealing a new way that stars can interact with planetary companions. 

In a paper appearing today in Nature Astronomy, scientists at MIT and elsewhere have discovered a star leisurely snacking on a closely orbiting brown dwarf — a planet-like object that is more massive than a planet yet not quite as big as a star. 

The new system, named ZTF J0440+2325, is within the Milky Way galaxy, roughly 300 light years from Earth, and represents the first observation of a low-mass object that is slowly and steadily consuming material from another low-mass object. 

The rate at which the star is feeding from the brown dwarf suggests that this slow stellar cannibalism could carry on for hundreds of thousands, or even billions of years.

“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 study’s MIT co-authors include Aaron Householder, Kaitlyn Shin, Saul Rappaport, Joheen Chakraborty, and Emma Chickles, along with collaborators from Caltech, the University of Hawaii, the Instituto de Astrofísica de Canarias and the Universidad de La Laguna in Spain, and the Harvard and Smithsonian Center for Astrophysics.

A “weird triangle”

The new system was spotted initially by the Zwicky Transient Facility. The ZTF uses a camera as part of a telescope at the Palomar Observatory, in California, to scan the sky for rapid changes in brightness, which could signal the presence of a supernova, a gamma-ray burst, or colliding neutron stars. 

Several years ago, Burdge was looking through ZTF data when he noticed a strange light curve, or pattern in brightness. Light curves for supernova resemble a bell curve, signaling the gradual brightening and then fading of a star as it bursts. But what Burdge picked out looked more like a triangle, that didn’t appear once, but again and again.

“I remember first looking at this and thinking: Stars don’t make triangular waveforms like this,” he recalls. 

At the time, he and his colleagues were focused on a different signal, which they identified as a “black widow binary” — a system in which an extremely dense, spinning neutron star is slowly consuming a much smaller companion star, similar to how its arachnid namesake plays with its prey. 

Burdge wondered whether the triangle signal might also be from a black widow. But the light from the signal was puzzling. In black widow binaries, the light appears to wobble, as a result of a very light, low-mass object, such as a small companion star, whipping around a much heavier object, such as a neutron star. 

“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.” 

But they had a hunch: Could the signal be coming not from a wobbly, David-and-Goliath system, but from a more balanced pair of objects, each with a similarly low mass? 

“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 slow and steady fireball

Recently, Burdge and Householder, a graduate student in MIT’s Department of Earth, Atmospheric and Planetary Sciences, decided to revisit the triangle mystery. From the original ZTF signal, they determined the location of its source to be within the Milky Way galaxy, around 300 light years from Earth. They focused multiple telescopes on the source, named ZTF J0440+2325. From these observations, they measured various properties of the source, including its wobble. Compared to black widows and other similar binaries, the wobbling from ZTF J0440+2325 was much smaller — but not insignificant. 

“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.”

They determined that the star and the brown dwarf are extremely close, with the brown dwarf circling the star every 87 minutes, in an orbit that could fit within the diameter of the sun. Both objects are small by stellar standards. The star is around 85 times as massive as Jupiter, while the brown dwarf is around 25 times as massive. 

With two low-mass objects circling at such close range, the scientists wondered if one object could be pulling material from the other. Such a process, known as accretion, is most often seen around objects that are extremely massive, though small in actual size, such as black holes and neutron stars. When a black hole accretes, or draws material from a much smaller nearby object, it pulls the matter around it in a disk.

“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.”

The team carried out simulations of possible accretion in ZTF J0440+2325. Taking into account the properties of the star and the brown dwarf, they simulated particles of matter on the brown dwarf, and how these particles should behave within the system over time, according to the laws of physics and equations of motion. 

“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.”

What’s more, the team calculated that the brown dwarf must be feeding material to its star at a rate of about 1/100,000 of an Earth’s mass each year. That’s about 40 million dump trucks’ worth of material, or roughly 1.3 trillion one-pound burritos every second. While that may seem like a lot of matter to be losing, it is in fact a very small fraction of the brown dwarf. This rate, the researchers estimate, is actually quite slow and steady. Given the size of the system, they say the star could continue leisurely snacking on the brown dwarf, for billions of years. 

This slow accretion, they say, would resemble a steady stream from the brown dwarf, onto the star. The researchers realized that if they were to view the system from afar, the brightness from the system would chart as a triangle, as the brown dwarf and its stream of matter circles its star. 

“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. 

With the mystery of the triangle light curve solved, the team hopes to spot similar slow-feeding systems nearby. 

“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 was supported, in part, by the National Science Foundation.