Alpha Centauri. ‘Alpha’: The first. The first letter in the Greek alphabet and a byword for ‘firstness’ is an apt prefix in the name of the closest stellar system to our own…and one which astronomers, futurists and sci-fi aficionados have long considered an ideal first port of call for our first exploratory voyage to another star. Located a mere 4.3 light years from the Sun – an estimated 25.8 trillion miles, a mere flea-hop in astronomical terms – Alpha Centauri has earned legendary status. A ‘triple-star’, it comprises two binaries, dubbed ‘A’ and ‘B’, orbited by an intriguing red dwarf called Proxima Centauri. Earlier this week, astronomers based at the European Southern Observatory’s La Silla Observatory in Chile dramatically announced the discovery of an Earth-sized planet, circling not Proxima…but Alpha Centauri B. Unfortunately for potential life-seekers, the new world’s close proximity to its parent star renders its surface too excessive for even bodies of liquid water to exist.
Officially, the new planet is known as ‘Alpha Centauri Bb’ and once more places the most famous star system beyond our own back into popular consciousness. For years, astronomers considered Proxima to be a potential parent of a planet, although no perturbations in its orbital progress yielded the expected tell-tale signs. Even observations by the Hubble Space Telescope in 1998 failed to find clear evidence of a planet. Moreover, Proxima’s unpredictable nature as a ‘flare star’ – characterised by random dramatic increases in luminosity, caused by magnetic activity – were expected to erode the atmosphere of any planet inside its habitable zone. Nevertheless, debate continues to resonate over whether red dwarfs such as Proxima are capable of sustaining life-bearing planets.
Whatever the truth, the relative closeness of the Alpha Centauri system to Earth marked it out for a possible first visit by one of our unmanned emissaries. In the late 1950s a nuclear-pulse-driven vehicle, known as ‘Project Orion’, was conceived, but the potential risk of radioactive fallout from its volatile propulsion system and the passing of the Partial Test Ban Treaty spelled its demise. More recently, in the late 1980s the nuclear-propelled ‘Project Longshot’ was offered by NASA and the US Naval Academy, targeted to enter orbit around Alpha Centauri B. Barrelling away from Earth at a rate of 8,300 miles per second – a little under five percent of the speed of light – it would take a century to reach its destination and its signals would then take almost four and a half years to get back to Earth.
The new planet found circling Alpha Centauri B, and reported on 17 October in the journal Nature, exerts a minute gravitational tug on its parent. In fact, the European team at La Silla only found it by picking up tiny ‘wobbles’ in the star’s progress, on the range of less than 51 centimetres per second – about the speed of a baby’s crawl – which represents the highest precision ever obtained. This remarkable success was achieved using the High-Accuracy Radial Velocity Planet Searcher (HARPS) instrument on La Silla’s 11-foot-diameter telescope. The planet is of very low mass, only 1.13 times that of Earth, but lies extremely close to its parent star, less than four million miles away. These statistics make it the ‘lightest’ exoplanet ever found around a Sun-like star. “Our observations extended over more than four years using the HARPS instrument and have revealed a tiny, but real, signal from a planet orbiting Alpha Centauri B every 3.2 days,” said Xavier Dumusque of Switzerland’s Geneva Observatory and Portugal’s Centre de Astrofisica da Universidade do Porto. “It’s an extraordinary discovery and it has pushed our technique to the limit.”
That capability is astonishing in terms of its accuracy and sharpness. HARPS measures a star’s radial velocity – its speed ‘toward’ and ‘away’ from Earth – with great precision. Due to the effect of Doppler shifting, this radial velocity change induces a shift of the star’s spectrum towards the ‘red’ end as it moves away from us (‘redshift’) or towards the ‘blue’ end (‘blueshift’) as it approaches us. By measuring these tiny spectral shifts, it was possible for the La Silla astronomers to infer the presence of the planet around Alpha Centauri B. Using the method, they were able to estimate only the planet’s minimum mass, since this is also dependent upon the tilt of its orbital plane, relative to the line of sight, which is unknown. However, it was noted that, statistically, this minimum mass is often very close to the real mass of the planet.
The discovery comes two years after NASA cancelled its Space Interferometry Mission (SIM) Lite, whose mandate was to hunt for Earth-sized planets occupying the habitable zones of distant stars. Had SIM Lite been realised, it was expected to identify far-off planets with an accuracy of a millionth of an arc-second and an expectation that it may have been capable of pinpointing Neptune-sized worlds around up to 2,000 stars and smaller, Earth-like worlds around 60 stars. This would have enabled it to detect planets smaller than three Earth-masses around Alpha Centauri. However, NASA’s Kepler spacecraft, launched in March 2009, has contributed enormously to the science of exoplanet detection, a fact highlighted by former astronaut John Grunsfeld, the Associate Administrator of the agency’s Science Mission Directorate.
“[Kepler] was specifically designed to survey a specific region of our Milky Way galaxy to detect Earth-size and smaller planets in or near the habitable zone — that region around a star where it is theoretically possible for a planet to maintain liquid water on its surface — and determine the fraction of the hundreds of billions of stars in our galaxy that might have such planets,” he said. “Kepler works very differently from HARPS. Rather than detecting the wobble in the host star, Kepler detects the slight dimming of a star when a planet passes in front of it.” Grunsfeld added that the Hubble and Spitzer Space Telescopes also pursue this research, as will the forthcoming James Webb Space Telescope. “NASA is also studying two medium-class exoplanet missions in our Explorer program,” he continued, “and in the spring of 2013 will select one of them to enter development for flight later in the decade.”
The discovery is a triumph for ground-based observatories and is already causing astronomers to refocus their attention on other nearby star systems. “The result represents a major step towards the detection of a twin Earth in the immediate vicinity of the Sun,” exulted Dumusque. Yet the new planet, Alpha Centauri Bb, is not expected to yield evidence of life. Its predicted surface temperature of around 1,200 degrees Celsius is more than three times hotter than that of Venus and above the melting temperatures of many silicate magmas. Still, the similarities between the Alpha Centauri system to our Solar System are too many for it to be overlooked. Alpha Centauri B is about 90 percent the mass of our Sun and some 14 percent smaller in radius; additionally both it and Alpha Centauri A are estimated to be only 250 million years older than the Sun. Further, their respective ‘metallicities’ – their proportions of elements other than hydrogen and helium – are believed to be similar. The system thus remains a tantalising location for further observation.
“We live in exciting times,” Dumusque concluded.