Astronomers using ESO telescopes and other
facilities have found clear evidence of a planet orbiting the closest
star to Earth, Proxima Centauri. The long-sought world, designated
Proxima b, orbits its cool red parent star every 11 days and has a
temperature suitable for liquid water to exist on its surface. This
rocky world is a little more massive than the Earth and is the closest
exoplanet to us -- and it may also be the closest possible abode for
life outside the Solar System. A paper describing this milestone finding
will be published in the journal Nature on 25 August 2016.
Just over four light-years from the Solar System lies a red dwarf
star that has been named Proxima Centauri as it is the closest star to
Earth apart from the Sun. This cool star in the constellation of
Centaurus is too faint to be seen with the unaided eye and lies near to
the much brighter pair of stars known as Alpha Centauri AB.
During the first half of 2016 Proxima Centauri was regularly observed
with the HARPS spectrograph on the ESO 3.6-metre telescope at La Silla
in Chile and simultaneously monitored by other telescopes around the
world [1]. This was the Pale Red Dot campaign, in which a team
of astronomers led by Guillem Anglada-Escudé, from Queen Mary University
of London, was looking for the tiny back and forth wobble of the star
that would be caused by the gravitational pull of a possible orbiting
planet [2].
As this was a topic with very wide public interest, the progress of
the campaign between mid-January and April 2016 was shared publicly as
it happened on the Pale Red Dot website and via social media.
The reports were accompanied by numerous outreach articles written by
specialists around the world.
Guillem Anglada-Escudé explains the background to this unique search:
"The first hints of a possible planet were spotted back in 2013, but
the detection was not convincing. Since then we have worked hard to get
further observations off the ground with help from ESO and others. The
recent Pale Red Dot campaign has been about two years in the planning."
The Pale Red Dot data, when combined with earlier
observations made at ESO observatories and elsewhere, revealed the clear
signal of a truly exciting result. At times Proxima Centauri is
approaching Earth at about 5 kilometres per hour -- normal human walking
pace -- and at times receding at the same speed. This regular pattern
of changing radial velocities repeats with a period of 11.2 days.
Careful analysis of the resulting tiny Doppler shifts showed that they
indicated the presence of a planet with a mass at least 1.3 times that
of the Earth, orbiting about 7 million kilometres from Proxima Centauri
-- only 5% of the Earth-Sun distance [3].
Guillem Anglada-Escudé comments on the excitement of the last few
months: "I kept checking the consistency of the signal every single day
during the 60 nights of the Pale Red Dot campaign. The first 10 were
promising, the first 20 were consistent with expectations, and at 30
days the result was pretty much definitive, so we started drafting the
paper!"
Red dwarfs like Proxima Centauri are active stars and can vary in
ways that would mimic the presence of a planet. To exclude this
possibility the team also monitored the changing brightness of the star
very carefully during the campaign using the ASH2 telescope at the San
Pedro de Atacama Celestial Explorations Observatory in Chile and the Las
Cumbres Observatory telescope network. Radial velocity data taken when
the star was flaring were excluded from the final analysis.
Although Proxima b orbits much closer to its star than Mercury does
to the Sun in the Solar System, the star itself is far fainter than the
Sun. As a result Proxima b lies well within the habitable zone around
the star and has an estimated surface temperature that would allow the
presence of liquid water. Despite the temperate orbit of Proxima b, the
conditions on the surface may be strongly affected by the ultraviolet
and X-ray flares from the star -- far more intense than the Earth
experiences from the Sun [4].
Two separate papers discuss the habitability of Proxima b and its
climate. They find that the existence of liquid water on the planet
today cannot be ruled out and, in such case, it may be present over the
surface of the planet only in the sunniest regions, either in an area in
the hemisphere of the planet facing the star (synchronous rotation) or
in a tropical belt (3:2 resonance rotation). Proxima b's rotation, the
strong radiation from its star and the formation history of the planet
makes its climate quite different from that of the Earth, and it is
unlikely that Proxima b has seasons.
This discovery will be the beginning of extensive further
observations, both with current instruments [5] and with the next
generation of giant telescopes such as the European Extremely Large
Telescope (E-ELT). Proxima b will be a prime target for the hunt for
evidence of life elsewhere in the Universe. Indeed, the Alpha Centauri
system is also the target of humankind's first attempt to travel to
another star system, the StarShot project.
Guillem Anglada-Escudé concludes: "Many exoplanets have been found
and many more will be found, but searching for the closest potential
Earth-analogue and succeeding has been the experience of a lifetime for
all of us. Many people's stories and efforts have converged on this
discovery. The result is also a tribute to all of them. The search for
life on Proxima b comes next..."
Notes
[1] Besides data from the recent Pale Red Dot campaign, the
paper incorporates contributions from scientists who have been observing
Proxima Centauri for many years. These include members of the original
UVES/ESO M-dwarf programme (Martin Kürster and Michael Endl), and
exoplanet search pioneers such as R. Paul Butler. Public observations
from the HARPS/Geneva team obtained over many years were also included.
[2] The name Pale Red Dot reflects Carl Sagan's famous
reference to the Earth as a pale blue dot. As Proxima Centauri is a red
dwarf star it will bathe its orbiting planet in a pale red glow.
[3] The detection reported today has been technically possible for
the last 10 years. In fact, signals with smaller amplitudes have been
detected previously. However, stars are not smooth balls of gas and
Proxima Centauri is an active star. The robust detection of Proxima b
has only been possible after reaching a detailed understanding of how
the star changes on timescales from minutes to a decade, and monitoring
its brightness with photometric telescopes.
[4] The actual suitability of this kind of planet to support water
and Earth-like life is a matter of intense but mostly theoretical
debate. Major concerns that count against the presence of life are
related to the closeness of the star. For example gravitational forces
probably lock the same side of the planet in perpetual daylight, while
the other side is in perpetual night. The planet's atmosphere might also
slowly be evaporating or have more complex chemistry than Earth's due
to stronger ultraviolet and X-ray radiation, especially during the first
billion years of the star's life. However, none of the arguments has
been proven conclusively and they are unlikely to be settled without
direct observational evidence and characterisation of the planet's
atmosphere. Similar factors apply to the planets recently found around
TRAPPIST-1.
[5] Some methods to study a planet's atmosphere depend on it passing
in front of its star and the starlight passing through the atmosphere on
its way to Earth. Currently there is no evidence that Proxima b
transits across the disc of its parent star, and the chances of this
happening seem small, but further observations to check this possibility
are in progress.
More information
This research is presented in a paper entitled "A terrestrial planet
candidate in a temperate orbit around Proxima Centauri," by G.
Anglada-Escudé et al., to appear in the journal Nature on 25 August 2016.
