The Star That Is Slowly Eating a Brown Dwarf

When a star meets a companion too small to be a star itself, you might expect a fast and messy ending: the bigger object swallows the smaller one. A new study describes something slower. In a system called ZTF J0440+2325, about 300 light-years away, a small M dwarf star is pulling material off a brown dwarf a little at a time. If the team's reading is right, it could keep doing that for a very long time.

"Instead of swallowing the thing up, the star can gradually eat it, for billions of years," MIT astronomer Kevin Burdge said in a statement released by MIT News on Oct. 5.

The work appears in Nature Astronomy, where the authors describe it as the direct observation of stable mass transfer from a substellar object onto a main-sequence star. According to the paper's abstract, such accretion had not been observed before.

A very tight orbit

The two objects in ZTF J0440+2325 complete an orbit in 86.65 minutes, Universe Today reported (the paper rounds this to 87 minutes). That is an extremely close pairing: MIT News says the orbit would fit within the diameter of the Sun. At that separation the brown dwarf overflows onto its companion, and material flows from the smaller body onto the star.

Here the direction of the flow is what's unusual. MIT News describes a brown dwarf as a planet-like object that is more massive than a planet yet not quite as big as a star. In this system the substellar object is the donor and the star is the one feeding.

Universe Today, citing the research, puts the transfer rate at roughly one hundred-thousandth of an Earth mass per year. That is a slow trickle by astronomical standards. It is also why the arrangement can last: MIT News notes this is a very small fraction of the brown dwarf, so it is worn down gradually instead of being destroyed at once. MIT News says the feeding could continue for anywhere from hundreds of thousands to billions of years.

Found in old data

The team didn't find this system by pointing a telescope at a promising target. According to Universe Today, Burdge was studying archival data from the Zwicky Transient Facility (ZTF) sky survey, hunting for the light curves of black widow binaries, when he noticed a strange pattern. The system takes its name from the survey. What gave it away was its light curve: the brightness traced a triangular pattern as the system went through its orbit. "Stars don't make triangular waveforms like this," Burdge recalled thinking. The triangle sat unexplained for years, until Burdge and graduate student Aaron Householder, the paper's first author, revisited it.

Follow-up observations used several large instruments: the LRIS spectrograph on Keck I, the ESI spectrograph on Keck II, and the HiPERCAM high-speed camera on the Gran Telescopio Canarias (GTC). Universe Today reports that the researchers concluded the hot spot lies on either the day side of the M dwarf or the night side of the brown dwarf, and that there is strong evidence the brown dwarf is transferring mass onto the M dwarf. A hot spot is the kind of feature you would expect where a stream of incoming material hits the star's surface. Burdge compares it to a continuous fireball whose hot spot comes in and out of view as the pair orbit, with the peak of the triangle arriving when we look right at it.

A second candidate, with a twist

ZTF J0440+2325 may not be alone. The paper also names ZTF J1444+4820 as a strong candidate for the same process. Its orbital period is even shorter, 67 minutes, and the abstract says it sits in a hierarchical triple, meaning a third body is gravitationally tied to the tight pair.

The two systems appeared earlier in a preprint, "Stars stably accreting from substellar objects," which was posted to arXiv on March 17, 2026 and submitted to Nature Astronomy. Authors listed include Householder, Burdge, Kareem El-Badry, and Shrinivas Kulkarni.

One caveat: the full Nature Astronomy paper is paywalled, and this article relies on its abstract, the arXiv listing, and the MIT and Universe Today write-ups. Detailed figures such as the error bars on the transfer rate are not covered here.

Why It Matters

Stars swallowing planets is the picture most people carry. The paper's abstract notes that substellar objects are generally expected to stay detached from their host stars unless orbital decay or stellar expansion brings them into contact, leading to rapid engulfment and destruction. Theory also allows for stable, long-lived mass transfer, but until now it had not been directly observed. ZTF J0440+2325 gives researchers an actual system to test those ideas against.

The timescale matters too. If such systems can stay stable for up to billions of years, as both the abstract and the MIT release suggest, they aren't fleeting accidents. The authors conclude that the fate of some substellar objects is gradual consumption rather than rapid destruction. That raises a population question: how many are out there?

The discovery method points toward an answer. ZTF J0440+2325 turned up because of a distinctive light curve in survey data that already existed. Universe Today notes that the Vera C. Rubin Observatory's Legacy Survey of Space and Time (LSST) is expected to detect multitudes of transients, and some are bound to resemble this one. Householder said in the MIT release that the discovery is inspiring new searches. With a larger sample, astronomers could move from studying one well-observed case and one candidate to working out how often stars slowly eat their substellar neighbors.

For now there is one confirmed example, about 300 light-years away. Its star is taking a very long time to finish its meal.