Most stars in the Milky Way move in regular orbits around the galactic center, but not all do so. Every now and then, a person is caught breaking ranks, advancing at a speed that will eventually catapult them into intergalactic space.

These “hypervelocity” stars are extremely rare, but we just spotted a particularly special example. The star, called CWISE J124909+362116.0 (abbreviated as J1249+36), not only exceeds its Galactic exit velocity by about 600 kilometers (373 miles) per second, but is also a very rare type of small, old main sequence star called L. -subdwarf is also one of the oldest in the Milky Way.

First detected by lay scientists analyzing telescope data in search of a mysterious ninth planet, J1249+36 is one of several hypervelocity stars discovered in the Milky Way, and while it’s far from the fastest star we’ve ever seen, it’s something of a challenge to astronomers; So how did it happen so surprisingly fast?

The discovery was announced at the 244th meeting of the American Astronomical Society, and its paper was recently presented to the US. Astrophysics Journal Letters.

There are several possible explanations for the star’s speed. Researchers examined three of them.

The first is the expulsion from the binary system, which includes a white dwarf star (the core remnant left behind when a Sun-like star runs out of hydrogen, expels most of its outer material, dies, and enters its afterlife). Superdense white dwarfs glow hotly from residual heat, not from fusion, and can be somewhat unstable if they have a binary partner.

If two stars are in close orbit, the white dwarf can steal material from the companion star. The problem is that the white dwarf has an upper mass limit. Even if it gains very little mass, it can survive through repeated explosions called novae. However, if it gains too much mass, it will turn into a Type Ia supernova and completely destroy the white dwarf.

“In this type of supernova, the white dwarf is completely destroyed, so its companion is freed and blown away by the orbital speed at which it was traveling, plus a small shock from the supernova explosion,” says astrophysicist Adam Burgasser. University of California, San Diego.

“Our calculations show that this scenario works. However, the white dwarf no longer exists and the remnants of the explosion that probably occurred several million years ago have long since dispersed, so we have no definitive evidence of its origin.”

The second possibility is that the multibody interaction becomes unstable and ejects one of the objects into the galaxy. The Milky Way has environments that make such an interaction more likely, namely globular clusters, dense spheres that can contain millions of stars. The centers of globular clusters are thought to contain swarms of black holes, but the concentration of black hole binaries is higher than normal.

“When a star collides with a binary black hole system, the complex dynamics of the three-body interaction can blow that star out of the globular cluster,” says Caltech astrophysicist Kyle Kremer, who will soon join UC San Diego.

This is also plausible; but tracing the star’s orbit backwards has not yet allowed researchers to determine the origin of a particular globular cluster.

A third option is that J1249+36 is not from the Milky Way but one of many satellite dwarf galaxies orbiting it. A 2017 study examining the origins of hypervelocity stars found a likely extragalactic origin. And the researchers’ calculations showed that this was also possible for J1249+36.

All three options remain on the table. The best way to find out is to examine the star’s chemical composition in more detail. If J1249+36 had been a white dwarf companion, the supernova might have left traces of elements polluting the L subdwarf’s atmosphere. In contrast, globular clusters contain stars with similar compositional properties, thus connecting the star to the host population.

Failing that, we may need to look at the Milky Way’s moons to determine whether that tiny, dull star is a winking alien from beyond the galaxy.