Earth-mass World Found in Habitable Zone of Alpha Centauri Star System
As we all learned in elementary school, Alpha Centauri is the closest star system to the Solar System. At a distance of 4.37 light-years (25 trillion miles), it consists of three stars -- the pair Alpha Centauri A and Alpha Centauri B and a small and faint red dwarf, Proxima Centauri, which may or may not be gravitationally bound to the other two stars.
Astronomers using ESO telescopes and other facilities have found clear evidence of a planet orbiting this third star, Proxima Centauri. This 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. It may also be the closest possible abode for life outside the Solar System.
Proxima Centauri is in the constellation Centaurus and is too faint to be seen with the unaided eye. It lies near to the much brighter pair of stars known as Alpha Centauri A and B.
During the first half of 2016 Proxima Centauri was regularly observed with the HARPS spectrograph on the ESO 3.6 meter telescope at La Silla in Chile and simultaneously monitored by other telescopes around the world. This project was named the "Pale Red Dot campaign," in which a team of astronomers led by Guillem Anglada-Escude, 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. 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 would bathe its orbiting planet in a pale red glow.
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-Escude 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 kilometers per hour (normal human walking pace) and at times it is 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 kilometers from Proxima Centauri -- only five percent of the Earth-Sun distance.
This detection capability 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.
Guillem Anglada-Escude 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.
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 characterization of the planet's atmosphere. Similar factors apply to the planets recently found around TRAPPIST-1.
Reports discussing the habitability of Proxima b and its climate find that the existence of liquid water on the planet today cannot be ruled out. In such a 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 or possibly in a tropical belt. 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 marks 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 (ELT). 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. Proxima b will be a prime target for the hunt for evidence of life elsewhere in the Universe. Indeed, the Alpha Centauri system will likely be the target of humankind's first attempt to explore another star system, although crossing the huge distance between the Sun and the Alpha Centauri using current spacecraft technologies would take several millennia.
Guillem Anglada-Escude 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."
The European Southern Observatory (ESO) is the foremost intergovernmental astronomy organization in Europe and the world's most productive ground based astronomical observatory by far. It is supported by 16 countries: Austria, Belgium, Brazil, the Czech Republic, Denmark, France, Finland, Germany, Italy, the Netherlands, Poland, Portugal, Spain, Sweden, Switzerland, and the United Kingdom, along with the host state of Chile. ESO carries out an ambitious program focused on the design, construction and operation of powerful ground based observing facilities enabling astronomers to make important scientific discoveries. ESO operates three unique world class observing sites in Chile: La Silla, Paranal, and Chajnantor. At Paranal, ESO operates the Very Large Telescope (VLT), the world's most advanced visible light astronomical observatory and two survey telescopes. VISTA works in the infrared and is the world's largest survey telescope and the VLT Survey Telescope is the largest telescope designed to exclusively survey the skies in visible light. ESO is a major partner in ALMA, the largest astronomical project in existence. On Cerro Armazones, close to Paranal, ESO is building the 39 meter European Extremely Large Telescope, which will become the world's biggest eye on the sky.
For more information:
http://www.eso.org/public/news/eso1629/
http://www.astromart.com/news/news.asp?news_id=1300
http://www.astromart.com/news/news.asp?news_id=1320
http://www.astromart.com/news/news.asp?news_id=1115
http://www.astromart.com/news/news.asp?news_id=672
AstroMart News Archive:
http://www.astromart.com/news/search.asp?search=.+
Astronomers using ESO telescopes and other facilities have found clear evidence of a planet orbiting this third star, Proxima Centauri. This 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. It may also be the closest possible abode for life outside the Solar System.
Proxima Centauri is in the constellation Centaurus and is too faint to be seen with the unaided eye. It lies near to the much brighter pair of stars known as Alpha Centauri A and B.
During the first half of 2016 Proxima Centauri was regularly observed with the HARPS spectrograph on the ESO 3.6 meter telescope at La Silla in Chile and simultaneously monitored by other telescopes around the world. This project was named the "Pale Red Dot campaign," in which a team of astronomers led by Guillem Anglada-Escude, 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. 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 would bathe its orbiting planet in a pale red glow.
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-Escude 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 kilometers per hour (normal human walking pace) and at times it is 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 kilometers from Proxima Centauri -- only five percent of the Earth-Sun distance.
This detection capability 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.
Guillem Anglada-Escude 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.
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 characterization of the planet's atmosphere. Similar factors apply to the planets recently found around TRAPPIST-1.
Reports discussing the habitability of Proxima b and its climate find that the existence of liquid water on the planet today cannot be ruled out. In such a 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 or possibly in a tropical belt. 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 marks 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 (ELT). 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. Proxima b will be a prime target for the hunt for evidence of life elsewhere in the Universe. Indeed, the Alpha Centauri system will likely be the target of humankind's first attempt to explore another star system, although crossing the huge distance between the Sun and the Alpha Centauri using current spacecraft technologies would take several millennia.
Guillem Anglada-Escude 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."
The European Southern Observatory (ESO) is the foremost intergovernmental astronomy organization in Europe and the world's most productive ground based astronomical observatory by far. It is supported by 16 countries: Austria, Belgium, Brazil, the Czech Republic, Denmark, France, Finland, Germany, Italy, the Netherlands, Poland, Portugal, Spain, Sweden, Switzerland, and the United Kingdom, along with the host state of Chile. ESO carries out an ambitious program focused on the design, construction and operation of powerful ground based observing facilities enabling astronomers to make important scientific discoveries. ESO operates three unique world class observing sites in Chile: La Silla, Paranal, and Chajnantor. At Paranal, ESO operates the Very Large Telescope (VLT), the world's most advanced visible light astronomical observatory and two survey telescopes. VISTA works in the infrared and is the world's largest survey telescope and the VLT Survey Telescope is the largest telescope designed to exclusively survey the skies in visible light. ESO is a major partner in ALMA, the largest astronomical project in existence. On Cerro Armazones, close to Paranal, ESO is building the 39 meter European Extremely Large Telescope, which will become the world's biggest eye on the sky.
For more information:
http://www.eso.org/public/news/eso1629/
http://www.astromart.com/news/news.asp?news_id=1300
http://www.astromart.com/news/news.asp?news_id=1320
http://www.astromart.com/news/news.asp?news_id=1115
http://www.astromart.com/news/news.asp?news_id=672
AstroMart News Archive:
http://www.astromart.com/news/search.asp?search=.+
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