How small can a star be? The smallest known star ever discovered is only slightly bigger than Saturn

The smallest known star is named EBLM J0555-57Ab and it's the smallest star ever measured, at just a sliver larger than Saturn.

A star can never be smaller than one of its orbiting planets, but just how small can a star be?

This star has a mass just large enough to support nuclear fusion of hydrogen, the process required to power stars, and so represents a lower bound on star size.

Named EBLM J0555-57Ab and located 600 lightyears away, it is part of a binary star system and was detected when it passed in front of its much bigger companion, in what is called an eclipsing stellar binary system.

“Our discovery reveals how small stars can be,” said Alexander von Boetticher at the University of Cambridge, lead author of the study that announced the discovery of EBLM J0555-57Ab, the smallest known star, on 12 July 2017.

“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.”

How the smallest known star was found

EBLM was identified by the Wide Angle Search for Planets (WASP), a collaboration aiming to find exoplanets, using the transit method.

Astronomers noticed that its parent star became dimmer in a periodic manner.

This signifies an orbiting object, normally a planet, is passing in front of it, and the mass and size of the companion can be accurately measured from observation data.

“This star is smaller, and likely colder, than many of the gas giant exoplanets that have so far been identified,” says von Boetticher.

“While a fascinating feature of stellar physics, it is often harder to measure the size of such dim low-mass stars than for many of the larger planets.

"Thankfully, we can find these small stars with planet-hunting equipment, when they orbit a larger host star in a binary system.

"It might sound incredible, but finding a star can at times be harder than finding a planet.”

Scientists believe these very small, dim stars are the best candidates for finding Earth-sized planets with liquid-water on their surface because of examples like TRAPPIST-1, an ultra-cool dwarf surrounded by seven temperate Earth-sized worlds.

“The smallest stars provide optimal conditions for the discovery of Earth-like planets, and for the remote exploration of their atmospheres,” says co-author Amaury Triaud, senior researcher at Cambridge’s Institute of Astronomy.

“However, before we can study planets, we absolutely need to understand their star; this is fundamental.”

Despite the majority of stars in the Universe having masses less than 20% of that of our Sun, they are difficult to detect and not very well understood.

This project hopes to fill the gap in our understanding by studying EBLM and any planets potentially orbiting it.