Stars

The Tiniest Known Star Shows Just How Small Stars Can Get

Astronomers love labels: this is a star, that is a planet, this is a brown dwarf, that is a black hole. But the universe doesn't always package itself up in such neat categories. Take the object recently discovered by Cambridge researchers: it's smaller than Jupiter and it orbits a star like a planet does, but it fuses hydrogen into helium so it counts as a star itself. The fact that we even found this tiny star means we could find more — and that means finding more Earth-like exoplanets.

The sizes of Saturn and EBLM J0555-57Ab are compared to one another. In the shadows, Jupiter and TRAPPIST-1 are represented to scale.

Barely Made It

The teeny tiny star, described in a 2017 paper where it's poetically dubbed EBLM J0555-57Ab, is roughly the size of Saturn. Lead author Alexander Boetticher says that it's on the knife's edge of how small a star can be. "Had this star formed with only a slightly lower mass, the fusion reaction of hydrogen in its core could not be sustained, and the star would instead have transformed into a brown dwarf."

It's got more similarities to a planet than just its size. Because it orbits a larger companion star as part of a binary star system, the researchers discovered it in the same way we discover exoplanets: they spotted the telltale dimming of its companion star as the small star transited, or passed in front of it. That transit method let the astronomers measure the size and mass of the small star, which ended up being about 30 percent smaller than the famed TRAPPIST-1, a star system with seven Earth-sized planets, but of roughly equal mass.

Who You Calling Tiny?

In a universe full of huge, powerful objects like blazars and supermassive black holes, who cares about a dim, tiny star? Exoplanet hunters, that's who. Small stars are our best bet for finding Earth-sized planets, and Earth-sized planets are our best bet for finding liquid water and, potentially, extraterrestrial life. "Nature just seems to make more little planets around these stars," astronomer Sarah Ballard told NASA. "Chances are that where you find a small, Earth-size planet, it orbits a small star."

Luckily, small, dim stars like those are the most common ones in the universe. But until now, we've had a heck of a time finding them, since they're, well, small and dim. The EBLM project, which identified this tiny star, is working to change that by first finding, and then understanding these small stellar objects. With that many small stars out there, who knows what kind of planets could exist?

Astronomers Say This Is The Smallest Star Ever Discovered

Written By
Ashley Hamer
July 28, 2017