The universe is full of surprises, and a colossal young star has been hiding a stellar one.

While observing infant star MM 1a, astronomers found that its massive disk was actually forming another star instead of planets. The much smaller companion, dubbed MM 1b, was detected just outside the behemoth star’s dusty disk, and could actually house a planet-forming disk of its own. The discovery of the new star, published on December 14 in the Astrophysical Journal Letters, marks one of the first times astronomers saw a star forming in the fragmented disk of another.

Stellar Siblings

Binary stars are pretty common in the universe, and it’s thought that they form the same way single stars do: from a massive cloud of dust and gas that collapses under its own gravity. If the molecular cloud is large enough, it can birth two similar-sized stars instead of one.

And since binary pairs are easily detectable, astronomers from the University of Leeds were rather surprised when they observed MM 1a. Homing in on the seemingly single star, they found an unexpected, much smaller companion star lurking in the outskirts of its dense disk — the region of dust and gas where planets typically form.

“In this case, the star and disk we have observed is so massive that, rather than witnessing a planet forming in the disk, we are seeing another star being born,” said John Ilee, a researcher at the University of Leeds and head of the study, in a news release.