Most planetary systems have planets on stable, detached orbits around their star. When a planet drifts too close, the star can pull it in and swallow it whole. Astronomers have already seen many rapid‑engulfment events, while stable systems are far more common. A team from MIT and other institutions now reports the first case that lies between these extremes—a low‑mass star is slowly “eating” a tightly bound brown dwarf. The system, named ZTF J0440+2325, lies about 300 light‑years away in our Milky Way.
The object was first flagged by the Zwicky Transient Facility (ZTF) at Palomar Observatory. In the ZTF data the researchers noticed an unusual triangular light curve: brightness rose sharply, fell gradually, then rose again, forming a repeatable triangle shape. Initially it was thought to be a “black‑widow” binary (a neutron star devouring a low‑mass companion), but the amplitude and lack of a clear wobble did not match that model.
Follow‑up observations with several telescopes measured a very small radial‑velocity wobble, confirming that the system consists of a low‑mass star about 85 × Jupiter’s mass and a brown dwarf about 25 × Jupiter’s mass. The two objects orbit each other every 87 minutes, in a separation that would fit inside the Sun’s diameter.
At such a close separation the brown dwarf’s outer layers are tidally stripped and form an accretion stream onto the star. The team ran numerical simulations, injecting test particles to represent the material and evolving them under Newtonian equations of motion. The particles consistently fell directly onto the stellar surface, akin to an asteroid hitting the Moon at high speed. This is the first direct observation of a low‑mass star actively accreting from another low‑mass object.
From the observations and models the accretion rate is about 1/100 000 Earth‑mass per year—roughly the mass of 40 million dump trucks, or about 1.3 trillion one‑pound burritos each second. Although that sounds large, it is a tiny fraction of the brown dwarf’s total mass, implying a slow, steady flow. At this rate the star could continue to snack on the brown dwarf for billions of years.
The researchers note that, when viewed from a distance, the persistent hotspot on the star rotates in and out of view, producing the triangular light curve originally detected. This discovery opens new searches for similar slow‑feeding systems and expands our understanding of how planets and brown dwarfs can interact with their host stars.
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