22 AUSTRALIAN SKY & TELESCOPE February | March 2019
THE NEAREST STARSSTHE SMALLEST STAR?Plottingacollectionofnearbyredand
brown dwarfs by their radii and surface temperatures reveals a turn:
Suddenly, the stars stop shrinking. The RECONS team thinks the
smallest star in their sample, informally called J0523, might mark the
limit of roughly how small a star can be. Objects to the right of it are
young brown dwarfs, still warm and puffy from their formation.“This is possible, but the theorists would really be wrong by
a large stretch,” says Dieterich. Brown dwarfs should not
be able to cool fast enough to create that huge, invisible
reservoir. However, he admits, “there is very tentative
evidence that there may exist more very cool brown dwarfs
than what the WISE results indicate.”The dividing line
Just beyond the limit of the 10 pc survey is a seemingly
unassuming star, named 2MASS J0523−1403 — let’s call it
J0523 for short — that could turn out to be one of the most
important red dwarfs yet discovered.
Lying 40 light-years away, J0523 is the smallest and dimmest
of the nearby stars, with a surface temperature of about 2100K
and a radius just 8.6% solar. This star marks a turning point
in a diagram comparing the radii and temperatures of red and
brown dwarfs. As you look at smaller and smaller stars on the
plot, the temperatures go down, until you reach J0523.
“There is then a sudden jump to higher radii as the objects
become cooler and less luminous,” says Dieterich. Brown
dwarfs glow with the heat of their contraction, with smaller
browndwarfswarmerthanlargerones.
Furthermore, to be this warm the brown
dwarfs must still be fairly young and
thereforepuffy,makingthemlargerthan
thesmalleststars.Theresultisaclear
disconnect on the diagram. “I believe that
2MASSJ0523−1403isrepresentativeofthe
smallest possible stars, and that everything
cooler than it must be a brown dwarf.”
IfDieterichisright,thenRECONShas
identifiedtheboundarybetweenfull-fledgedstarsandbrown
dwarfs. Not everyone agrees; his results have produced “a
chorusofcomplaints”fromtheorists,hesays,becausesome
stellar models predict that there should be even cooler stars
before we enter the realm of the brown dwarfs. He’s working
on figuring out where the theories might be going wrong.Planet popularity
Insomeways,justasSETImotivatedToddHenrytostart
looking at the nearest stars, the search for extraterrestrial
life is also encouraging researchers to look at the stars that
areclosesttous.“Becausetheexoplanetgamehasgottenso
popular, the nearby stars are back in vogue, in particular the
small red dwarfs,” says Henry.
NASA’s Kepler Space Telescope has taught us that, on
average, there is at least one planet for every star. But only 28
oftheclosest317knownstarsystems(includingtheSun’s)
areknowntohaveplanets—that’slessthan9%.Recently
discovered planetary systems, such as those belonging to the
Mdwarfs Ross 128, Trappist-1 and LHS 1140 (the latter two at
12 pc), could be just the tip of the iceberg, says Henry. “There
arelikelyhundredsmoresolarsystemstofindamongour
nearestneighbouringsystems.”SXTRAPPIST-1The
Trappist-1 system contains
sevenplanetsorbitingared
dwarf. Several of the planets
mightbehabitable.This
graphic shows the Trappist-1
exoplanets, their star, and the
Solar System’s major planets
withsizesscaledtotheSun’s.3 of every 4
stars are
red dwarfs2000 1500J05233000 2500
Surface temperature (kelvin)Radius (solar radii)0.080.100.120.140.16GRAPH: LEAH TISCIONE / S&T, SOURCE SERGIO DIETERICH; TRAPPIST-1: AMANDA SMITH / INSTITUTE OF ASTRONOMY, UNIV. OF CAMBRIDGETrappist-1 System Solar System
SUNMercuryVenusEarthMarsJupiterSaturnUranusNeptuneSTARb c
d e
f
h g