An international team of astronomers has discovered the smallest ever star to date – a tiny red dwarf only slightly larger than the planet Saturn. Known as EBLM J0555-57Ab, it sits roughly 600 light years from Earth, and is so small that it was discovered using a technique ordinarily used for hunting exoplanets.

Low-mass bodies such as red dwarfs are the most prevalent star types in the galaxy. Their diminutive size and luminosity in comparison to a stellar body such as our Sun, however, makes them relatively hard to detect. Despite their seemingly unimpressive nature, red dwarfs represent some of the most exciting stars in the Milky Way, as they are thought to be the stellar bodies with the greatest chance of harboring Earth-sized exoplanets, themselves capable of hosting liquid water on their surfaces.


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One such red dwarf, known as TRAPPIST-1, generated a massive amount of excitement in February, when scientists sensationally announced that it played host to seven Earth-sized exoplanets, three of which orbited in the star's habitable zone.

EBLM J0555-57Ab first came to the attention of the team in October 2008, when the exoplanet-hunting experiment, Wide Angle Search for Planets (WASP), spotted a dip in the output of light from the Sun-like star EBLM J0555-57, as the red dwarf made a transit across its disk.

Follow-up observations from the CORALIE spectrograph mounted on the 1.2-m (3.9-ft) Euler telescope, and the smaller TRAPPIST telescope, both located at the La Silla observatory in Chile, confirmed that the two stars belonged to an eclipsing stellar binary system.

The red dwarf transits across the disk of the primary Sun-like star EBLM J0555-57 once every 7.75 days.

Artist's impression of a brown dwarf (Credit: NASA/JPL-Caltech)

"This star is smaller, and likely colder than many of the gas giant exoplanets that have so far been identified," said lead author of the study Alexander 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."

The newly-discovered red dwarf boasts a mass similar to that of TRAPPIST-1, despite being only a little larger than the planet Saturn, which has a radius of 72,000 miles (115,872 km), or nearly 10 times the size of Earth.

It is also important to remember that small is contextual. Though only the size of Saturn, if, by some miracle, you found yourself standing on the "surface" of the red dwarf, its gravity would exert a force 300 times that of Earth's. So, you know, don't go there.

That said, the star is so small that it sits only slightly above the mass threshold at which a stellar body is capable of turning hydrogen nuclei into helium through nuclear fusion. Were EBLM J0555-57Ab to be any smaller, there would not be enough pressure at the heart of the star to maintain the fusion process, and it would transform into a failed stellar body, known as a brown dwarf.

A paper on the findings has been published in the journal Astronomy and Astrophysics.

Source: University of Cambridge

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