My first Magazine Sky & Telescope - 01.2019 | Page 40

The Nearest Stars
The Dividing Line
Just beyond the limit of the 10 pc survey is a seemingly unas-
suming star, named 2MASS J0523−1403 — let’s call it J0523
for short — that could turn out to be one of the most impor-
tant red dwarfs yet discovered.
Lying 40 light-years away, J0523 is the smallest and dim-
mest 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
brown dwarfs warmer than larger ones. Furthermore, to be
this warm the brown dwarfs must still be
3 of every 4
fairly young and therefore puffy, making
stars are
them larger than the smallest stars. The
red dwarfs
result is a clear disconnect on the diagram.
“I believe that 2MASS J0523−1403 is repre-
sentative of the smallest possible stars, and
that everything cooler than it must be a
brown dwarf.”
If Dieterich is right, then RECONS has
identified the boundary between full-
fledged stars and brown dwarfs. Not everyone agrees; his
results have produced “a chorus of complaints” from theo-
rists, he says, because some 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 theo-
ries might be going wrong.
0.16
“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 reser-
voir. However, he admits, “there is very tentative evidence
that there may exist more very cool brown dwarfs than what
the WISE results indicate.”
0.14
0.12
0.10
J0523
0.08
3000
2500
2000
1500
Surface temperature (kelvin)
p THE SMALLEST STAR? Plotting a collection of nearby red and 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.
Trappist-1
System
b
d
f
h
Solar
System
c
e
g
Mercury
STAR
Venus
Earth
Mars
Planet Popularity
38
In some ways, just as SETI motivated Todd Henry to start
looking at the nearest stars, the search for extraterrestrial
life is also encouraging researchers to look at the stars that
are closest to us. “Because the exoplanet game has gotten so
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
of the closest 317 known star systems (including the Sun’s)
are known to have planets — that’s less than 9%. Recently
discovered planetary systems, such as those belonging to
the M dwarfs Ross 128, Trappist-1, and LHS 1140 (the latter
two at 12 pc), could be just the tip of the iceberg, says Henry.
“There are likely hundreds more solar systems to find among
our nearest neighboring systems.”
JA N UA RY 2 019 • SK Y & TELESCOPE
SUN
pu TRAPPIST-1 The
Trappist-1 system con-
tains seven planets orbit-
ing a red dwarf. Several
of the planets might be
habitable. This graphic
shows the Trappist-1
exoplanets, their star,
and the solar system’s
major planets with sizes
scaled to the Sun’s.
Jupiter
Saturn
Uranus
Neptune