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14 CHRIS LINTOTT
16 LEWIS DARTNELL EDGE
The latest astronomy and space
Our experts examine the hottest
news written by Elizabeth Pearson new astronomy research papers

EBLM J0555-57Ab is smaller
than Saturn, but many times
denser, which is ultimately why
it is a star and not a planet

COMMENT
by Chris Lintott

This new star is smaller
than Saturn, but the team
estimate it has the mass of
85 Jupiters. That makes
this one of the densest
objects known – not quite
up there with neutron
JUPITER SATURN EBLM J0555-57Ab TRAPPIST-1 stars and black holes,
Gas giant Gas giant Low-mass star Low-mass star but ahead of almost
everything else.
Smallest star ever this is a star not a planet.
That density is why
At the surface the star is
nearly as hot as the Sun,
DISCOVERED and at the star’s core the
temperature is high enough
for the nuclear fusion of
hydrogen into helium to
Red dwarf found by planet hunters examining another star proceed. That’s about as
close to a sensible
The tiniest star ever observed, with a diameter star not only makes it easier to find planets, dividing line between
star and planet as there
slightly larger than Saturn, has been discovered but also means that features caused by
is. Large planets like
600 lightyears away. EBLM J0555-57Ab, a potential atmospheres will stand out and
Jupiter do produce some
red dwarf in a binary system, was found by can be measured. Last year it was announced
internal heat, but are
researchers on the WASP exoplanet hunting that red dwarf TRAPPIST-1, which is only
nowhere near hot enough
experiment, who were looking for 30 per cent larger than EBLM J0555-57Ab, is
for hydrogen fusion.
planets passing in front of a larger star now surrounded by seven Earth-sized worlds. The importance of
thought to be its stellar companion. Despite their importance, little is known EBLM J0555-57Ab is
“Our discovery reveals how small stars can about red dwarfs. Although stars with that it lies very close
be,” says Alexander von Boetticher, the lead masses less than 20 per cent of the Sun are to what’s supposed
author of the study, from Cambridge’s the most numerous in the Universe, their to be the theoretical
AMANDA SMITH/UNIVERSITY OF CAMBRIDGE a slightly lower mass, the fusion reaction of many of the gas giant exoplanets that have Weigh in at anything
minimum, a mass
weak light output makes them incredibly
Cavendish Laboratory and Institute of
equivalent to 83 Jupiters.
difficult to find.
Astronomy. “Had this star formed with only
“This star is smaller and likely colder than
less than that, and
hydrogen in its core could not be sustained,
your chance of
and the star would instead have transformed
so far been identified,” says von Boetticher.
stardom is gone.
“It might sound incredible, but finding a star
into a brown dwarf.”

CHRIS LINTOTT co-
can at times be harder than finding a planet.”
Red dwarfs are thought to be fertile grounds
for exoplanet hunters, as the dim light of the
presents The Sky at Night
> See Comment, right
skyatnightmagazine.com 2017