Just over four light-years from the solar system lies a red dwarf star that has been named Proxima Centauri (https://en.wikipedia.org/ wiki/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 (https://en.wikipedia.org/ wiki/Alpha_Centauri).

During the first half of 2016 Proxima Centauri was regularly observed with the HARPS spectrograph (https:////www.eso.org/ instruments/harps) on the ESO 3.6-metre telescope (https:////www.eso.org/public/ teles-instr/lasilla/36) at La Silla in Chile and simultaneously monitored by other telescopes around the world [1].

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 (https://palereddot.org) 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.

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 (https:////www.spaceobs.com) in Chile and the Las Cumbres Observatory (https://lcogt.net) 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 (https://en.wikipedia.org/ wiki/Circumstellar_habitable_ zone) around the star and has an estimated surface temperature that would allow the presence of liquid water.

Two separate papers (https:////www.proximacentauri. info) 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, https://www.eso.org/public/ videos/eso1629g) or in a tropical belt (3:2 resonance rotation, https://www.eso.org/public/ videos/eso1629h).

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, https:////www.eso.org/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 (https:// breakthroughinitiatives.org/ Initiative/3) 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

[0] We are aware that there have been rumours regarding this discovery. These rumours have never been confirmed and have not contained any research content. Whilst the rumours are in the public domain and can be reported, the information in this release, the paper itself and the associated visuals have been provided on an embargoed basis and therefore remain strictly under embargo until 19:00 CEST on 24 August 2016. We would be grateful if any questions or concerns are addressed to us before any action is taken. We thank you for your consideration in this matter.

[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 (https://en.wikipedia.org/ wiki/Carl_Sagan) famous reference to the Earth as a pale blue dot (https://en.wikipedia.org/ wiki/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.

[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.