Sky and Telescope - December 2016 - 10

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News Notes

EXOPLANETS I World Found Around Proxima Centauri
Astronomers have discovered a planet
orbiting the closest star to our solar
system, Proxima Centauri. Proxima is a
small, red M dwarf only about half as hot
as the Sun and 14% as wide. It's member C
of the Alpha Centauri triple-star system,
which lies just over 4 light-years away.
Guillem Anglada-Escudé (Queen Mary
University of London) and colleagues
report in the August 25th Nature that the
newly discovered world, dubbed Proxima
Centauri b, has a mass at least 1.3 times
that of Earth, meaning it's potentially
rocky. It orbits the star every 11.2 days at
a distance of only 0.05 astronomical unit,
or roughly 10% Mercury's orbital distance
from the Sun. That places it right in the
star's putative habitable zone.
We've had our hopes raised for alien
worlds in the Alpha Centauri system
before. For example, in 2012 observers
reported that they'd found what looked
like an Earth-mass exoplanet hugging the
orange Sun-like star Alpha Centauri B.
But follow-up work by other researchers

raised a red ﬂag (S&T: Sept. 2013, p. 14).
The alleged planet has since proved to be
a subtle "ghost" in the data, created by
how the original team sliced up the observations for analysis.
But Proxima Cen b stands on more
solid ground. Astronomers suspected for
some time that the star has a planet: data
from two European Southern Observatory (ESO) spectrographs had found a
shift in the star's light that seemed to
repeat every 11.2 days. This shift might
have come from a tiny wobble in the star's
position toward and away from us, due to
the gravitational tug of an orbiting planet.
Yet being an M dwarf, Proxima is a
fairly active star. Astronomers didn't have
enough data to tell if the planet signal
was real or instead just muck in the starlight from ﬂares and other stellar hubbub.
So Anglada-Escudé's team started the
Pale Red Dot campaign to ﬁnd out. The
campaign took a two-pronged approach.
First, the team used the renowned
HARPS spectrograph on La Silla's

Habitable zone
S&T: LEAH TISCIONE, SOURCE: ESO / M. KORNMESSER / G. COLEMAN

Proxima
Centauri b orbit
Mercury's
orbit
(average)

Sun
Proxima
Centauri

This ﬁgure compares the solar system (left) with the Proxima Centauri system. Proxima is far
smaller and cooler than the Sun, so even though its newfound planet orbits much closer than
Mercury does to the Sun, the world lies well within the hypothetical habitable zone.

December 2016 sky & telescope

3.6-meter telescope in Chile to observe
Proxima Centauri for 20 minutes every
clear night from January 19th to March
31st this year. Second, a network of other
telescopes simultaneously performed photometry (watched for changes in the star's
brightness) to pinpoint the star's rotation
period and to record if any ﬂares went oﬀ.
The team found that the new, 2016
HARPS data contain a periodic signal,
just like the one in the archival data. And
when the researchers combined both sets
of data into one and analyzed the whole,
16-year-spanning shebang, the statistical
signiﬁcance (read: "how believable the
signal is") went "sky high," AngladaEscudé said in a press conference.
But the real kicker is the photometry.
Proxima itself didn't do anything during
the observations that could explain the
periodic signal. Instead, the best solution
is a planet.
"I'm very excited about this planet,
but not terribly surprised," says David
Charbonneau (Harvard-Smithsonian
Center for Astrophysics), who heads up
the MEarth Project to look for exoplanets
around M dwarfs. A few years ago, he and
Courtney Dressing (now at Caltech) used
Kepler data to estimate that one out of
every four M dwarfs has a planet that's at
most 1.5 times as wide as Earth. Expand
the size to twice that of Earth, and the
rate goes up to nearly one out of every
two. "With those odds, it isn't at all that
surprising that the closest M dwarf has
one," he explains.
But we don't know how big Proxima
Cen b is. MEarth and other projects have
failed to detect a transit in front of the
star, which would reveal the exoplanet's
diameter. The "failure" is expected, Charbonneau says: geometrically speaking,
there's only a 1.5% chance that the world
passes across the face of Proxima.
The best way to follow up may be taking actual pictures. The star is so close
to Earth that upcoming adaptive-optics
instruments will be able to directly image
the planet - assuming they can properly

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Sky and Telescope - December 2016

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