Clear evidence of a planet orbiting Proxima Centauri, the closest star to the Solar System, has been found by an international team of scientists led by astronomers at Queen Mary University of London (QMUL).
24 August 2016
Using facilities operated by ESO (the European Southern Observatory) and other telescopes, the research, which is published in the journal Nature, reveals a world with a similar mass to Earth orbiting around Proxima Centauri.
The planet, called Proxima b, orbits its 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 planet outside our Solar System. Planets around other stars are commonly referred to as exoplanets.
Scientists are excited because Proxima b may also be the closest possible home for life outside the Solar System.
Lead author and coordinator of the project, Dr Guillem Anglada-Escudé from QMUL’s School of Physics and Astronomy, said: “Succeeding in the search for the nearest terrestrial planet beyond our Solar System has been an experience of a lifetime, and has drawn on the dedication and passion of a number of international researchers. We hope these findings inspire future generations to keep looking beyond the stars. The search for life on Proxima b comes next."
Proxima Centauri is a red dwarf star that is four-light years from the Solar System and yet 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 studied with the HARPS spectrograph at ESO's La Silla observatory in Chile, and simultaneously monitored by other telescopes around the world.
Planet-hunters use the Doppler Effect, the shift in the star’s light spectrum depending on its velocity, to investigate the properties of exoplanets, such as their masses and periods of orbit.
The results indicate that at times Proxima Centauri is approaching Earth at around normal human walking pace (about 3 mph) and at times receding at the same speed. This regular pattern repeats with a period of 11.2 days. Careful analysis of the resulting tiny Doppler shifts in this case indicate 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 five per cent of the Earth-Sun distance .
The announcement of the new found planet has been a long time in the making.
Professor Hugh Jones from University of Hertfordshire explained: “Initial observations of the planet were made more than 15 years ago in March 2000. We first submitted a scientific paper presenting the planets existence back in February 2013. My colleague Dr Mikko Tuomi also from the University of Hertfordshire had discovered the planet's fingerprints in archived data taken before 2009, but we didn’t have enough evidence to conclusively support such a major discovery.”
The team involves more than 30 researchers from different countries and institutions, and includes three UK universities. The latest observing campaign was called Pale Red Dot and involved looking for the tiny back and forth wobble of the star that would be caused by the gravitational pull of a possible orbiting planet. In addition to acquiring the new data, the campaign was also a public outreach exercise in which the team, led by Dr Anglada-Escudé reported the progress of the observations in real-time via a website, and social media .
Red dwarfs like Proxima Centauri are slightly active 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 telescopes at the San Pedro de Atacama Celestial Explorations Observatory in Chile and the Las Cumbres Observatory global network.
Co-author, Dr John Barnes from The Open University was instrumental in establishing that the observed ‘wobble’ of Proxima Centauri was caused by a nearby planet.
Dr Barnes said: “Once we had established that the wobble wasn’t caused by star spots, we knew that that there must be a planet orbiting within a zone where water could exist, which is really exciting. If further research concludes that the conditions of its atmosphere are suitable to support life, this is arguably one of the most important scientific discoveries we will ever make.”
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 — these would be far more intense than those the Earth experiences from the Sun .
This discovery will be the beginning of extensive further observations, both with current instruments  and with the next generation of giant telescopes such as ESO’s European Extremely Large Telescope (E-ELT). Proxima b will be a prime target for the hunt for evidence of life elsewhere in the Universe.
 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.
 Science and Technology Facilities Council (STFC) funds the UK subscription, giving UK astronomers access to ESO telescopes.
 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 pioneers such as R. Paul Butler. Public observations from the HARPS/Geneva team obtained over many years were also included.
 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 maintain 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.
 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.
For media information, contact:Sarah Birdsall