Astronomers have made the exciting discovery of an Earth-mass exoplanet in the habitable zone of our nearest neighbouring star, Proxima Centauri, a red dwarf star 4.25 light years (1.3 parsec) from the Sun. The new-found world, designated Proxima b, is a little more massive than the Earth, implying that it has a rocky surface and is not a gas giant world like Jupiter or Saturn. It follows a tight orbit around its primary, much closer than scorched little Mercury goes around our own Sun, however as Proxima Centauri is a cooler, less luminous star than the Sun, this new planet is in its “Goldilocks zone” . This is the name for the region around a star where planets could have surface temperatures allowing liquid water to exist. Water is a necessity for life on Earth (and so too throughout the Universe?)It is no wonder that astronomers are thrilled; we have found that our nearest stellar neighbour is home to a possibly habitable world.
Discovered in 1915, Proxima Centauri lies in the southern constellation of Centaurus (the Centaur) which is too far south to be seen from Ireland. Proxima Centauri takes hundreds of thousands of years to orbit around a much brighter pair of more Sun-like stars, Alpha Centauri A and B. In 2012 a suspected planet was reported to be orbiting around Alpha Centauri B, but this turned out to be an error. At the moment Proxima Centauri’s enormous orbit around its two larger partners makes Proxima the closest star to the Sun, but eventually it will slowly swing away from us. About 25 000 years from now it will be so far away that Alpha Centauri A and B will be our closest stars! Proxima Centauri is a faint little star, too dim to be seen by unaided eyes, and any planet orbiting it would be fainter still, so how was this discovery made?
This is was no chance discovery made by a lucky astronomer who just happened to be looking in the right place at the right time. Rather it was the result of the Pale Red Dot campaign, a concerted programme of careful observations made in 2016 using the High Accuracy Radial velocity Planet Searcher (HARPS) spectrograph on the European Southern Observatory’s 3.6m telescope at La Silla in Chile supported by other telescopes across the globe. The campaign’s aim was to scrutiniseProxima Centauri for minute “wobbles” in the star’s position caused by the gravitational tug of orbiting planets. These wobbles appear as small movements in the spectrum of the star’s light (called Doppler shifts) caused by the star’s light being stretched and squeezed as its position with respect to the Earth swings backwards and forwards. This is a proven planet-finding method called the radial velocity technique.
The results obtained (which are supported by earlier observations) show that Proxima Centauri does indeed wobble just the astronomers hoped. It oscillates back and forth in a regular pattern repeating every 11.2 days. This is the tell-tale sign of a planet at least 1.3 times heavier than the Earth orbiting about 7 million kilometres (about 4.3 million miles) from Proxima Centauri. This is only 5% of the Earth to Sun distance.
So far Proxima b seems to circle its sun alone, previous observations seem to rule out the existence of super giant planets (larger than Jupiter) orbiting the star, but I would expect there are other planets in the system to be found.
If astronomers on Proxima b looked back towards the Sun, what would they see? To unaided (human) eyes our own star would appear as a 0.4 magnitude star in the constellation Cassiopeia. Astrophotographers on Proxima b would see the Sun as hanging in front of two large nebulae. These are the Heart ( IC 1805) and Soul nebulae which lie about 7500 light years (2300 parsec) beyond the Sun.
What can we surmise about the conditions on Proxima b itself? A planet so close to its star is almost certainly tidally-locked to it. This means its rotation has, over time, become synchronised with its orbital period, so that one face of the planet experiences an unending day, while the other endures an eternal night. On the dayside, the planet’s mountains, plains and valleys will be illuminated by the orange-red tone light from its star, Proxima Centauri. The two companion stars Alpha Centauri A and B are so distant from Proxima B that they are merely the brightest stars in its sky, contributing no heat to warm the planet’s surface. Proxima Centauri is much smaller and fainter than the Sun so even though Proxima b is very close to it, the planet still has an estimated surface temperature that would allow the presence of liquid water. However this does not mean unprotected humans could survive on its surface. In our own Solar System, hellish Venus and frigid, airless Mars are arguably in the Sun’s Goldilocks Zone yet have environments which would kill unprepared humans in seconds. We have no information on the the atmosphere of Proxima b or indeed if it has an atmosphere at all. The behaviour of the planet’s star may also present a lethal difficulty to life…
Red dwarf stars like Proxima Centauri are “flare stars”, prone to sudden and violent outbursts of ultraviolet and X-rays. At Proxima Centauri these eruptions of radiation occur about eight times a year and are far more intense than anything we see from our Sun so would be highly dangerous. Apart from the immediate hazard from these stellar flares, it is possible that over time frequent large flares could strip away a planet’s atmosphere. The prospect of life arising on planets around red dwarf stars (the most common and longest-lived stars in the Universe) is a currently a subject for theoretical speculation. This important discovery of a terrestrial planet so close opens up the possibility of direct observations to add facts to the debate. For example, if Proxima b passes in front of its star (“transits” the star in astronomical terminology) an atmosphere around the planet might be just detectable by the large telescopes planned for the near future (although the frequently occurring flares would confuse such a search). Looking further still into the future, in the decades and centuries to come we now have a target for speculative interstellar voyages such as Projects Starshot and Icarus.
Proxima b is a wonderful discovery and will inspire new research in the years to come. Expect to hear much more about our neighbouring planetary system. Expect a revolution in our knowledge of planets across the Universe.
(Article by Colin Johnston, Science Education Director, who wishes to thank colleagues ApostolosChristou, Gerry Doyle, David Asher and Stefano Bagnulo for their suggestions of additional topics to include in this article.)