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.
This plot shows how the motion of Proxima Centauri
towards and away from Earth is changing with time.
Credit: ESO/G. Anglada-Escudé
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 Xray flares from the star — far more
intense than the Earth experiences
from the Sun.
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
This infographic compares the orbit of the planet around Proxima Centauri (Proxima b) with the same region
of the Solar System. Proxima Centauri is smaller and cooler than the Sun and the planet orbits much closer
to its star than Mercury. As a result it lies well within the habitable zone, where liquid water can exist on the
planet’s surface. Credit: ESO/M. Kornmesser/G. Coleman
The relative sizes of a number of objects, including the
three (known) members of Alpha Centauri triple system and some other stars. The Sun and planet Jupiter
are also shown for comparison. Credit: ESO
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 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...”
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