Planet orbiting Proxima Centauri is likely to be the focus of future interstellar voyages.
Proxima Centauri, the star closest to the Sun, has an Earth-sized
planet orbiting it at the right distance for liquid water to exist. The
discovery, reported today in Nature1, fulfils a longstanding dream of
science-fiction writers — a potentially habitable world that is close
enough for humans to send their first interstellar spacecraft.
“The search for life starts now,” says Guillem Anglada-Escudé, an
astronomer at Queen Mary University of London and leader of the team
that made the discovery.
Humanity’s first chance to explore this
nearby world may come from the recently announced Breakthrough Starshot
initiative, which plans to build fleets of tiny laser-propelled
interstellar probes in the coming decades. Travelling at 20% of the
speed of light, they would take about 20 years to cover the 1.3 parsecs
from Earth to Proxima Centauri.
Proxima’s planet is at least 1.3
times the mass of Earth. The planet orbits its red-dwarf star — much
smaller and dimmer than the Sun — every 11.2 days. “If you tried to pick
the type of planet you’d most want around the type of star you’d most
want, it would be this,” says David Kipping, an astronomer at Columbia
University in New York City. “It’s thrilling.”
Earlier studies
had hinted at the existence of a planet around Proxima. Starting in
2000, a spectrograph at the European Southern Observatory (ESO) in Chile
looked for shifts in starlight caused by the gravitational tug of an
orbiting planet. The resulting measurements suggested that something was
happening to the star every 11.2 days. But astronomers could not rule
out whether the signal was caused by an orbiting planet or another type
of activity, such as stellar flares.
Star and planet align
In January 2016, Anglada-Escudé and his colleagues launched a campaign
to nail down the suspected Proxima planet. ESO granted their request to
observe using a second planet-hunting instrument, on a different
telescope, for 20 minutes almost every night between 19 January and 31
March. “As soon as we had 10 nights it was obvious,” Anglada-Escudé
says.
The team dubbed the work the ‘pale red dot’ campaign, after
the famous 'pale blue dot' photograph taken of Earth by the Voyager 1
spacecraft in 1990. Because Proxima is a red-dwarf star, the planet
would appear reddish or orangeish, perhaps bathed in light similar to
the warm evening tints of Earth.
Although the planet orbits at a
distance that would permit liquid water, other factors might render it
unlivable. It might be tidally locked — meaning that the same hemisphere
always faces the star, which scorches one side of the planet while the
other remains cool. The active star might occasionally zap the planet
with destructive X-ray flares. And it's unclear whether the planet has a
protective, life-friendly atmosphere.
Proxima itself belongs to
the triple-star system Alpha Centauri. In 2012, a Nature paper reported
that an Earth-mass planet orbited another member of that stellar trio,
Alpha Centauri B2. That result has now mostly been dismissed3, 4, but
exoplanet specialists say the Proxima claim is more likely to hold up.
“People call me Mr Sceptical, and I think this result is more robust,”
says Artie Hatzes, an astronomer at the Thuringian State Observatory in
Tautenburg, Germany.
False alarm
This time, the
combination of new observations and older measurements dating back to
2000 increases confidence in the finding, Anglada-Escudé’s team argues.
“It’s stayed there robustly in phase and amplitude over a very long
time,” says team member Michael Endl, an astronomer at the University of
Texas at Austin. “That’s a telltale sign of a planet.” The data even
contain hints that a second planet may exist, orbiting Proxima somewhere
between every 100 and 400 days.
The researchers now hope to
learn whether the Proxima planet's pass across the face of its star can
be seen from Earth. The chances are low, but such a ‘transit’ could
reveal details of the planet, such as whether it has an atmosphere. A
team led by Kipping has been independently looking for transits around
Proxima, and is frantically crunching its data in search of any signal.
The discovery of the Proxima planet comes at a time of growing
scientific interest in small planets around dwarf stars, says Steinn
Sigurdsson, an astrophysicist at Pennsylvania State University in
University Park. NASA’s Kepler space telescope has shown that rocky
planets are common around such stars, which themselves are the most
common type of star in the Galaxy. “This is a total vindication of that
strategy,” he says.
One day, the Proxima planet might be seen as
the birth of a new stage in planetary research. “It gives us the target
and focus to build the next generation of telescopes and one day maybe
even get to visit,” says Kipping. “It's exactly what we need to take
exoplanetary science to the next level.”
http://www.nature.com/…/billionaire-backs-plan-to-send-pint…
http://www.nature.com/…/earth-sized-planet-around-nearby-st…
http://www.nature.com/news/the-exoplanet-files-1.18809
http://www.nature.com/news/the-exoplanet-next-door-1.11605
http://www.centauri-dreams.org/?p=36210
Cecile G. Tamura
