Proxima Centauri is the closest star to the Sun. It is a faint red dwarf lying just four light-years away in the southern constellation of Centaurus (The Centaur). It is orbited by the Earth-sized temperate world Proxima b, discovered in 2016 and the closest planet to the Solar System. But there is more to this system than just a single planet. The new ALMA observations reveal emission from clouds of cold cosmic dust surrounding the star.
The lead author of the new study, Guillem Anglada , from the Instituto de Astrofísica de Andalucía (CSIC), Granada, Spain, explains the significance of this find: "The dust around Proxima is important because, following the discovery of the terrestrial planet Proxima b, it's the first indication of the presence of an elaborate planetary system, and not just a single planet, around the star closest to our Sun."
Dust belts are the remains of material that did not form into larger bodies such as planets. The particles of rock and ice in these belts vary in size from the tiniest dust grain, smaller than a millimetre across, up to asteroid-like bodies many kilometres in diameter .
Dust appears to lie in a belt that extends a few hundred million kilometres from Proxima Centauri and has a total mass of about one hundredth of the Earth's mass. This belt is estimated to have a temperature of about -230 degrees Celsius, as cold as that of the Kuiper Belt in the outer Solar System.
There are also hints in the ALMA data of another belt of even colder dust about ten times further out. If confirmed, the nature of an outer belt is intriguing, given its very cold environment far from a star that is cooler and fainter than the Sun. Both belts are much further from Proxima Centauri than the planet Proxima b, which orbits at just four million kilometres from its parent star .
Guillem Anglada explains the implications of the discovery: "This result suggests that Proxima Centauri may have a multiple planet system with a rich history of interactions that resulted in the formation of a dust belt. Further study may also provide information that might point to the locations of as yet unidentified additional planets."
Proxima Centauri's planetary system is also particularly interesting because there are plans -- the Starshot project -- for future direct exploration of the system with microprobes attached to laser-driven sails. A knowledge of the dust environment around the star is essential for planning such a mission.
Co-author Pedro Amado, also from the Instituto de Astrofísica de Andalucía, explains that this observation is just the start: "These first results show that ALMA can detect dust structures orbiting around Proxima. Further observations will give us a more detailed picture of Proxima's planetary system. In combination with the study of protoplanetary discs around young stars, many of the details of the processes that led to the formation of the Earth and the Solar System about 4600 million years ago will be unveiled. What we are seeing now is just the appetiser compared to what is coming!"
 In a cosmic coincidence, the lead author of the study, Guillem Anglada shares his name with the astronomer who led the team that discovered Proxima Centauri b, Guillem Anglada-Escudé, himself a co-author of the paper in which this research is published, although the two are not related.
 Proxima Centauri is quite an old star, of similar age to the Solar System. The dusty belts around it are probably similar to the residual dust in the Kuiper Belt and the asteroid belt in the Solar System and the dust that creates the Zodiacal Light. The spectacular discs that ALMA has imaged around much younger stars, such as HL Tauri, contain much more material that is in the process of forming planets.
 The apparent shape of the very faint outer belt, if confirmed, would give astronomers a way to estimate the inclination of the Proxima Centauri planetary system. It would appear elliptical due to the tilt of what is assumed to be in reality a circular ring. This would in turn allow a better determination of the mass of the Proxima b planet, which is currently known only as a lower limit.
This research was presented in a paper entitled "ALMA Discovery of Dust Belts Around Proxima Centauri", by Guillem Anglada et al., to appear in Astrophysical Journal Letters.
The team is composed of Guillem Anglada (Instituto de Astrofísica de Andalucía (CSIC), Granada, Spain [IAA-CSIC]), Pedro J. Amado (IAA-CSIC), Jose L. Ortiz (IAA-CSIC), José F. Gómez (IAA-CSIC), Enrique Macías (Boston University, Massachusetts, USA), Antxon Alberdi (IAA-CSIC), Mayra Osorio (IAA-CSIC), José L. Gómez (IAA-CSIC), Itziar de Gregorio-Monsalvo (ESO, Santiago, Chile; Joint ALMA Observatory, Santiago, Chile), Miguel A. Pérez-Torres (IAA-CSIC; Universidad de Zaragoza, Zaragoza, Spain), Guillem Anglada-Escudé (Queen Mary University of London, London, United Kingdom), Zaira M. Berdiñas (Universidad de Chile, Santiago, Chile; IAA-CSIC), James S. Jenkins (Universidad de Chile, Santiago, Chile), Izaskun Jimenez-Serra (Queen Mary University of London, London, United Kingdom), Luisa M. Lara (IAA-CSIC), Maria J. López-González (IAA-CSIC), Manuel López-Puertas (IAA-CSIC), Nicolas Morales (IAA-CSIC), Ignasi Ribas (Institut de Ciències de l'Espai (IEEC-CSIC), Bellaterra, Spain), Anita M. S. Richards (JBCA, University of Manchester, Manchester, United Kingdom), Cristina Rodríguez-López (IAA-CSIC) and Eloy Rodríguez (IAA-CSIC).
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* Photos of ALMA - http://eso.org/public/images/archive/category/alma/
* Press release about Proxima b discovery - http://www.eso.org/public/news/eso1629/
Instituto de Astrofísica de Andalucía (CSIC)
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The Astrophysical Journal Letters