INNER WORKINGS
All eyes on Proxima Centauri b
Adam Mann,
Science Writer
An Earth-sized planet next door: that was the startling
announcement last August. Astronomers had found
an exoplanet orbiting the sun
’s closest stellar neigh-
bor, a cool red dwarf star called Proxima Centauri (1).
Even better, the nearby world orbited within its parent
star
’s habitable zone, meaning liquid water could exist
on the planet
’s surface, which raised the prospects for
its harboring life.
But aside from its mass
—at least 1.3 times that of
Earth
’s—and the length of its year—a zippy 11 days—
little was known about the new exoplanet, called Prox-
ima Centauri b. Almost immediately, a rush of papers
appeared, presenting ways to estimate Proxima b
’s
temperature, atmospheric composition and thickness,
and even whether a worldwide ocean spans its sur-
face. The proposed methods are extraordinarily tricky,
pushing the boundary of what
’s possible.
“This planet is so good, so optimum, and so close
to us, that using state-of-the-art technology we [can]
demonstrate that it
’s not science fiction to do these
observations,
” says astronomer Christophe Lovis of the
University of Geneva in Switzerland. With two recently
discovered systems garnering ample headlines
—the
potentially habitable planets near the star TRAPPIST-1
and a super-Earth orbiting the red dwarf LHS 1140
—
Proxima b offers a test case for how astronomers might
take the first steps toward closer inspection of planets
that seem to be prime candidates for life (2, 3).
In the future, enormous ground-based telescopes
and specially designed space-based ones will allow
astronomers to image exoplanets like Proxima b di-
rectly, providing unparalleled information about them.
And with the world a mere 4.25 light-years from Earth, a
few dreamers are starting to think bigger. A project
called Breakthrough Starshot hopes to figure out if it
is possible to accelerate a flotilla of extremely small
satellites to one-fifth the speed of light and reach the
Proxima Centauri system in only a couple of decades.
“You see the stars at night and wonder if we can
visit them,
” says theoretical physicist Abraham Loeb of the
Proxima b, shown here in an artist
’s conception orbiting the red dwarf star Proxima Centauri, may prove to be a test
case for how astronomers conduct initial inspections of potentially habitable planets. Original image courtesy of ESO/M.
Kornmesser; retouched version courtesy of Wikimedia Commons/Karl 432.
6646
–6648 | PNAS | June 27, 2017 | vol. 114 | no. 26
www.pnas.org/cgi/doi/10.1073/pnas.1706680114
INNE
R
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KI
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Harvard
–Smithsonian Center for Astrophysics in
Cambridge, Massachusetts.
“This could be the next ma-
jor leap forward; not just visiting a place like Mars or the
moon, but doing something that is quite extraordinary.
”
In Our Sights
Part of a triple star system
—along with the larger,
yellow, sun-like twins Alpha Centauri A and B
—Proxima
and its kin have been subject to intensive exoplanet
searches ever since astronomers have had the capa-
bility to do so. But Proxima Centauri is particularly
finicky for these kinds of investigations. As a red dwarf,
it is much more active than stars like our sun, throwing
off energetic stellar flares that can interfere with as-
tronomers
’ readings. “You’re trying to look for little
blips around the star but it
’s like someone’s got ahold
of the dimmer switch and they
’re playing around with
it,
” says astronomer David Kipping of Columbia Uni-
versity in New York.
A team led by astronomer Guillem Anglada-
Escud ´e, of the Queen Mary University of London in
England, was able to spot Proxima b by running an
algorithm on several years
’ worth of archival data from
the Very Large Telescope (VLT) and the High Accuracy
Radial velocity Planet Searcher (HARPS) instrument at
the La Silla Observatory, both of which are overseen
by the European Southern Observatory in Chile. The
analysis yielded compelling clues for a planet, which
the researchers then followed up on with a rigorous
months-long observational campaign in early 2016.
The astronomers spotted a tiny periodic wobble in the
light from Proxima Centauri, indicating that a planet
was tugging gravitationally on the star. Though some
astronomers were initially skeptical, the data were
robust enough to win over critics.
“I heard about this potentially habitable planet
through the press and said
‘Yeah, yeah, sure,’” says
astrophysicist Laura Kreidberg of Harvard University in
Cambridge, Massachusetts.
“And then I saw the paper
and said,
“Oh wow, this is actually legit.”
Soon thereafter, Kreidberg published a paper with
Loeb on how to answer one of the first important
follow-up questions for Proxima b: whether it has an
atmosphere (4). During the first billion or so years of
their lives, red dwarf stars shine far brighter in UV and X-
ray radiation than they do later on, blasting any orbiting
planets. Furthermore, a planet like Proxima b is so close
to its parent star that it is likely to be tidally locked,
meaning one side is always turned to the burning sun
while the other faces the darkness of space. Combined
with the excessive flaring problem, this increases the
chances that atmospheres of these sorts of rocky worlds
can burn off long before life can develop.
Astronomical Answers
The upcoming James Webb Space Telescope (JWST),
scheduled to launch next year, could test Proxima b
’s
atmospheric retention abilities. As the exoplanet or-
bits, viewers on Earth will see different parts of it illu-
minated, much like the moon changing phases in the
night sky. JWST
’s infrared capabilities will be able to
capture the combined thermal emission from both the
star and planet, which should vary sinusoidally as the
little world goes through phases during its 11-day orbit.
An airless body will show wild temperature swings;
the sunlit side of the moon can reach 117 °C, whereas
its dark side remains at
–179 °C. In contrast, an at-
mosphere will efficiently move heat around: day
–night
temperature differences on our planet can be as little
as a few degrees. The signal would be a good proxy
for atmospheric thickness; a tenuous Mars-like atmo-
sphere wouldn
’t redistribute heat as well as a denser
Earth-like one.
Like any world, Proxima b is reflective. As starlight
bounces off its surface, it could pass through a hypo-
thetical atmosphere, imparting information about the
gases present. The trouble is that Proxima Centauri
is about 10 million times brighter than its planetary
companion, so the planet
’s reflected light is com-
pletely washed out in the star
’s glare. Working with
astronomer Ignas Snellen, of the University of Leiden in
the Netherlands, Lovis came up with the idea of using a
two-part system at the VLT to tease out Proxima b
’s
secrets (5).
First, the researchers will use a sophisticated adap-
tive optics instrument called Spectro-Polarimetric High-
contrast Exoplanet Research (SPHERE), which cor-
rects for the turbulence in Earth
’s atmosphere that
blurs telescopic images, creating a much clearer
picture of the star and planet together.
“Instead of
having a planet that
’s 10 million times fainter, we
will have a planet that
’s 1,000 times fainter,” says
Lovis.
“Which is still 1,000 times, but it’s already
much better.
”
The data will then be passed through the Echelle
SPectrograph for Rocky Exoplanet and Stable Spec-
troscopic Observations (ESPRESSO) instrument, which
can disentangle the planet
’s light from the star’s using
an ingenious trick. When a body emitting radiation
moves, the radiation
’s wavelength will grow longer
as it travels away from an observer and shorter as it
comes closer, an effect known as a Doppler shift.
Although the starlight from Proxima Centauri will
remain constant, the reflected light from Proxima b
will get ever-so-slightly Doppler-shifted as it travels
through its orbit, an effect that ESPRESSO can detect
to unravel the two light effects from one another and
hence provide astronomers a peek at the planet
’s
molecular make-up. Biosignature molecules, such as
oxygen and water vapor, would make for exciting
findings.
The technique could also capture small changes in
reflectivity as the planet rotates. An ocean on Proxima
b would act like a mirror, creating a bright glint that
“You’re trying to look for little blips around the star but
it
’s like someone’s got ahold of the dimmer switch and
they
’re playing around with it.”
—David Kipping
Mann
PNAS | June 27, 2017 | vol. 114 | no. 26 | 6647
the VLT might potentially observe. Getting SPHERE
and ESPRESSO up to such challenges will require
upgrades, including a new coronagraph in SPHERE
and an optical fiber link between the instruments, but
Lovis and Snellen think they should be able to provide
the first information about Proxima b
’s composition
and surface features within 3
–5 years.
Pioneering Planet
In many ways, Proxima b will be a trailblazer. The
exoplanet does not appear to eclipse its parent star,
meaning starlight can
’t filter through any potential
atmosphere (6). But the methods described above
don
’t rely on such light, making them applicable to
rocky worlds around other nearby red dwarf stars and
complementary to techniques used for transiting
exoplanets, such as those at TRAPPIST-1 (7). For nearby
stars, the odds of finding a transiting Earth-sized exo-
planet in the habitable zone is low,
“less than 2 per-
cent,
” says astrophysicist Jayne Birkby, also of the
Smithsonian Center for Astrophysics.
“So if we really
want to understand our local neighborhood of exo-
planets, we need all these techniques to work.
”
The best characterization, though, may be several
more years away. In the 2020s, a new generation of
ground-based observatories, such as the Thirty Meter
Telescope, Giant Magellan Telescope, and European
Extremely Large Telescope will begin focusing on
nearby exoplanets with their huge mirrors. The Wide
Field Infrared Survey Telescope, a space-based ob-
servatory scheduled to launch in the mid-2020s, might
similarly use an enormous petal-like screen, known as
a starshade, to selectively filter out Proxima Centauri
’s
light and see the exoplanet. Beyond that, the astro-
nomical community hopes to select either the Habit-
able Exoplanet Imager or the Large UV Optical and
Infrared telescope to fly in the 2030s, either of which
would be direct exoplanet imaging powerhouses.
But perhaps the most audacious scheme devised
for Proxima b is Breakthrough Starshot, announced in
April 2016, even before the exoplanet was discov-
ered. Based on previously published ideas, the project
would shoot centimeter-sized spacecraft with a 50-
gigawatt laser, imparting each with the same thrust as
the Space Shuttle at launch (6). The laser light would
push a solar sail, a highly reflective extremely thin
sheet, attached to each nanocraft, accelerating them
toward the Proxima Centauri system. The initiative
began with $100 million from Russian venture capi-
talist Yuri Milner. Loeb, who chairs the project
’s advi-
sory committee, says they hope to use the next 5 years
to identify whether such a mission is feasible while
also working on developing some of the necessary
technology.
“I think if it works and human ingenuity is up to the
task, it would be a tremendous return,
” says Kipping,
who is not involved in the initiative.
“There’s only so
much you can do from 4 light-years away.
”
1 Anglada-Escud ´e G, et al. (2016) A terrestrial planet candidate in a temperate orbit around Proxima Centauri. Nature 536:437–440.
2 Gillon M, et al. (2017) Seven temperate terrestrial planets around the nearby ultracool dwarf star TRAPPIST-1. Nature 542:456–460.
3 Dittmann JA, et al. (2017) A temperate rocky super-Earth transiting a nearby cool star. Nature 544:333–336.
4 Kreidberg L, Loeb A (2016) Prospects for characterizing the atmosphere of Proxima Centauri b. Astrophys J Lett 832:L12.
5 Lovis C, et al. (2017) Atmospheric characterization of Proxima b by coupling the SPHERE high-contrast imager to the ESPRESSO
spectrograph.
Astron Astrophys 599:A16.
6 Kipping D, et al. (2017) No conclusive evidence for transits of Proxima b in MOST photometry. Astron J 153:93–112.
7 Lubin P (2016) A roadmap to interstellar flight. JBIS J Br Interplanet Soc 69:40–72.
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Mann