MIT astronomers find a star slowly stripping a brown dwarf 300 light-years away

The clue was a shape. Some supernovae brighten and fade along a rounded, bell-like light curve ^[13]. Several years ago, Kevin Burdge, an assistant professor of physics at MIT, found a light curve in ZTF data that repeatedly traced something closer to a triangle ^[9][12]. "I remember first looking at this and thinking, stars don't make triangular waveforms like this," Burdge said ^[10].

His group was working on another odd signal at the time. That one turned out to come from a black widow binary, in which a rapidly spinning neutron star strips material from a much smaller companion ^[11]. The feeder in ZTF J0440+2325 is a small star, and what it feeds on is a brown dwarf, an object heavier than a typical planet but too light to sustain hydrogen fusion ^[2][3].

The duration is the least settled part of the result. According to the MIT release, the transfer looks slow enough to continue for hundreds of thousands of years, "potentially even billions" ^[6]. The low and high ends of that range are at least a factor of a thousand apart ^[15]. The release's summary does not give the masses of the two objects or the rate at which material moves between them.

Burdge sets the find against the usual expectation. "When we think of stars interacting with planets or brown dwarfs, the picture is always that the star eventually swallows the other thing," he said. "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." ^[7]

His own example limits how far the result reaches. By his account, the sun will still swallow Earth when it becomes a red giant ^[7]. In our view the finding applies to pairs like this one, a small star feeding on a brown dwarf, and the forecast for Earth is unchanged.