In the constellation of Centaurus, Proxima Centauri is a red dwarf star that is located just over four light-years from our solar system. This star is cool and therefore too faint to be seen with the naked eye. Alpha Centauri A and B are close by and, since they are hotter, they are much brighter.
Carnegie’s Paul Butler, together with an international team of astronomers, has found clear indications that there is a planet orbiting Proxima Centauri. This planet has been called Proxima b and it orbits its star every 11 days. If liquid water is present, the temperature of the new world is suitable for water to exist on its surface. Proxima b is the closest exoplanet to Earth and is slightly more massive than our home planet. It is also conceivable that this is the closest possible habitat suitable for life beyond our own Sun.
Amateur and professional telescopes around the world regularly look at Proxima Centauri. A team of astronomers led by Carnegie alum Guillem Anglada-Escudé of Queen Mary, University of London, used the HARPS spectrograph on the European Southern Observatory‘s 3.6-meter telescope at La Silla during the first half of 2016. The team (known as the Pale Red Dot campaign) was looking for a small back-and-forth vibration in the star. If detected, this would likely be caused by the gravitational pull of an orbiting planet.
A research paper was released incorporating contributions from scientists who have been watching Proxima Centauri for years, as well as the data gathered by the Pale Red Dot campaign. The topic has wide public interest, and the campaign’s progress was shared publicly while it occurred between mid-January and April 2016. Various social media and the Pale Red Dot website was used. The reports on data collection was accompanied by many outreach articles from international specialists.
The paper containing the combined data revealed a fascinating result. Proxima Centauri approaches Earth at about 5 kilometers per hour at regular intervals and moves away again at the same speed. This happens every 11.2 days. Analyzing the small Doppler shifts indicate that this is caused by a planet with a mass of at least 1.3 times that of the Earth. The planet orbits Proxima Centauri at about 7 million kilometers.
Although this is only 5 percent of the distance between the Earth and the Sun and closer than what Mercury orbits our Sun, Proxima Centauri is much cooler than the Sun. Scientists estimate that if water were present on Proxima b, the lower temperature would allow it to exist in a liquid state on the planet’s surface. This would put Proxima b in the “habitable zone” around the star.
Red dwarfs like Proxima Centauri are active stars, which means their natural brightness variations could mimic the presence of a planet without there actually being one. The team monitored the changing brightness of the star very closely during the campaign in order to eliminate this possibility. To do this, they used the Las Cumbres Observatory Global Telescope Network and the ASH2 telescope at the San Pedro de Atacama Celestial Explorations Observatory in Chile.
The surface conditions on Proxima b may however be strongly affected by the x-ray and ultraviolet flares from Proxima Centauri. Due to its closeness to the star, these would be far more intense than the Earth experiences from the Sun.
Butler is ecstatic about finding a potentially habitable planet around Proxima Centauri and notes it is the culmination of 30 years of work. This work has resulted in the discovery of hundreds of planets around the nearest stars, and has finally culminated in the finding of a potentially habitable planet around the nearest star in the sky.
Precision velocity and Kepler satellite studies showing that potentially habitable planets are common have been validated. It also shows how such planets can be directly observed with giant space- and ground based telescopes in the future.