Institute of Astronomy

A star the size of Saturn

Published on 08/07/2017 

The smallest star yet measured has been discovered by a team of astronomers led by the University of Cambridge. With a size just a sliver larger than that of Saturn, the gravitational pull at its stellar surface is about 300 times stronger than what humans feel on Earth. This star is likely about as small as stars can possibly become, meaning that this is one of the most compact instance of a naturally occurring hydrogen-fusion reactor.

Most planets have sizes between Neptune and Earth. However planets do not emit much light, they only radiate the heat they receive from their star. Exotic objects such as neutron stars and white dwarfs are smaller than most planets and do emit light, but those are bright only because they are cooling down from a hot beginning.

What makes this star, called EBLM J0555-57Ab, particular, is that, despite its diminutive size, it is just massive enough to enable the fusion of hydrogen nuclei into helium. This is the same process that powers the Sun’s luminosity, and that scientists are attempting to replicate as a powerful energy source, here on Earth.

Alexander Boetticher, the lead author of the study, and an MPhil student at the University of Cambridge, was delighted by the discovery: “Had EBLM J0555-57Ab 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. Our discovery reveals how small stars can be!”

EBLM J0555-57Ab was identified by a planet-finding experiment called WASP, which is run by the Universities of Keele, Warwick, Leicester and St Andrews. EBLM J0555-57Ab was noticed when it passed in front of another, larger, parent star, forming what is called an eclipsing stellar binary system. The parent star became dimmer in a periodic fashion, the signature of an orbiting object. Thanks to this special configuration, researchers can accurately measure the masses and radii of an orbiting companion, here a small star. This method is also routinely used to study exoplanets, where it is called a transit. The mass of EBLM J0555-57Ab was established via the Doppler, wobble method, using data from the CORALIE spectrograph. CORALIE is an instrument built by the University of Geneva, whose main goal is also the study of exoplanets.

Alexander von Boetticher explains: “This star is smaller, and likely colder than many of the heated gas-giant exoplanet that have been identified. Whilst such low-mass, dim stars are a fascinating feature of stellar physics, measuring their physical properties can be more difficult than for many of the larger exoplanets. Thankfully, we can find these small stars with planet-hunting equipment, when they orbit a larger host star in a binary system.” Sam Gill, a PhD student at Keele University continues: “It might sound incredible, but finding a star can at times be harder than finding a planet!”

As it happens, EBLM J0555-57Ab has a mass comparable to the current estimate for TRAPPIST-1, an ultracool dwarf surrounded by seven temperate Earth-sized worlds, but has a radius that is nearly 30% smaller. “This star was not found serendipitously.“ explains Amaury Triaud, senior researcher at Cambridge’s Institute of Astronomy. “EBLM J0555-57Ab was found as part of a dedicated observing campaign.” The EBLM project aims to measure the mass and size of the smallest stars that exist. Amaury Triaud continues: “The smallest stars provide optimal conditions for the discovery of Earth-like planets, and for the remote exploration of their atmospheres. However, before we can study planets, we absolutely need to understand their star; this is fundamental.”

All the crucial planetary parameters, like radius and mass, are entirely dependent on what radius and mass are assumed for their host stars. Although being the most numerous stars in the Universe, stars with sizes and masses less than 20% that of the Sun are poorly known. The EBLM project aims to plug that lapse in knowledge. Didier Queloz, professor at the Cavendish Laboratory concurs: “Thanks to the EBLM project, we will achieve a far greater understanding of the planets orbiting the most common stars that exist, planets like those orbiting TRAPPIST-1.”

Numerous projects like the pale red dot search, as well as the MEarth, TRAPPIST/SPECULOOS/SAINT-EX, Apache and ExTra experiments, are attempting to discover Earth-like planets orbiting small red dwarf stars.  “The EBLM project is essential in supporting these multiple international efforts,” concludes Stéphane Udry, director at the Department of Astronomy, at the University of Geneva.

The discovery has been accepted for publication as a Letter to the journal Astronomy & Astrophysics. A pdf version can be accessed here:

local IoA contact:  Dr Amaury Triaud ( at the Institute of Astronomy, Cambridge.

Images (click on image to obtain full resolution version)

The sizes of Saturn and EBLM J0555-57Ab are compared to one another. In the shadows, Jupiter and TRAPPIST-1 are represented to scale. Image credit: IoA/Amanda Smith

As above, but now with accompanying text.  Image credit: IoA/Amanda Smith


Page last updated: 12 July 2017 at 07:40