The tantalising prospect of finding a planet like our own which supports life is, arguably, the holy grail of scientific research. In recent years, thousands of exoplanets have been identified by astronomers but only a tiny handful have been deemed potentially habitable, while the vast majority are hundreds, if not thousands, of light years away – making them very difficult to study.
However, in a historic discovery, an international team of scientists has found clear evidence of a planet in orbit within the habitable zone of Proxima Centauri –the closest star to our Sun. The planet, named Proxima b, is the closest exoplanet to Earth ever discovered and is now the prime target in the search for life outside the solar system. The findings have been published in the journal Nature.
The team of astronomers, led by researchers at Queen Mary University London (QMUL), identified the planet using European Southern Observatory (ESO) Telescopes as well as other facilities. Crucially, the exoplanet lies in the so-called Goldilocks or habitable zone, a range of orbits where the presence of liquid water on the surface is possible, given sufficient atmospheric pressure.
Proxima Centauri is located just over four light years away from Earth, a relatively paltry distance in galactic terms. It is so close in fact that, it has been mooted as a possibility for future interstellar travel. The star is a red dwarf meaning it is far fainter than the Sun, so even though Proxima b lies very close to it – closer even than distance between Mercury and the Sun – it still lies within the habitable zone.
A team – headed by Guillem Anglada-Escudé at QMUL – had been observing Proxima Centauri for the first half of this year as part of the Pale Red Dot Campaign – set up to look for signals that could suggest the presence of a planet.
The first hints of a possible planet were spotted back in 2013, but the detection was not convincing, Anglada-Escudé said. 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.
Scientists combined the data from the Pale Red Dot project with previous observations made at ESO facilities and elsewhere, resulting in the extraordinary finding. Their data showed that sometimes Proxima Centuari is moving towards Earth at 5km/h, while at other times it is moving away at the same speed, with the direction of movement switching every 11.2 days. These movements - known as Doppler wobbles – can indicate the presence of an orbiting planet exerting a gravitational pull on the star, which, along with further analysis, allowed the team to confirm its presence.
Very little is currently known about Proxima b, but scientists have been able to infer what kind of planet it is, given the available information. Their data suggests that Proxima b is a rocky world about 1.3 times the size of Earth, orbiting its star every 11 days at a distance of 7 million kilometres. Despite being very close to Proxima Centauri, the planet receives only about 65% of the energy that the Earth receives from the sun. Scientists think surface temperatures could be amenable to supporting liquid water – crucial for life as we know it to flourish.
Because of gravitational forces, it is very likely that the planet is tidally locked, meaning the same side will always be facing the star, leaving the other in perpetual darkness. As a result, there is a strong possibility that it does not have seasons. In addition, if water is found on the surface it will most likely be in the sunny regions or the tropics. Stars like Proxima are very active, meaning conditions on the surface of the planet could be strongly affected by ultraviolet light and intense X-ray flares, which, combined with the other factors, mean the climate is probably quite different to Earths.
These factors, along with traditional views regarding similar planets which lie close to red dwarf stars suggest that life may not be sustainable on Proxima b, however, scientists argue that this does not exclude the possibility of finding it – a question that wont be resolved until direct observations can be made. Whether it is habitable or not, they say, will depend on the planets formation and history with new planetary computer models showing that even tidally locked planets close to red dwarf stars could support life, given the right conditions – climate, atmosphere etc.
Despite the question marks over its habitabilty, Proxima b is an immensely exciting discovery. Because it is the nearest exoplanet ever discovered, it offers an unprecedented opportunity, according to Anglada-Escudé, for extensive further examination using both current technology and next-generation telescopes set to be rolled out over the next few years. These include the European Extremely Large Telescope (E-ELT), the Transiting Exoplanet Survey Satellite and the James Webb Telescope. The latter will have the ability to analyse the atmospheres of exoplanets, which could help researchers determine the habitability of Proxima b.
Perhaps the most fascinating aspect of the Proxima b discovery is the implications it will have for future interstellar travel projects. The Alpha Centauri star system, of which Proxima Centauri is part of, was already the target of an ambitious project funded by Russian billionaire Yuri Milner – with backing from Stephen Hawking and Mark Zuckerberg – to develop viable interstellar spacecraft.
The concept behind Starshot is to launch thousands of nanocrafts – complete with lightsails – into orbit via a mothership. These tiny spacecraft – measuring a few centimetres across and weighing just a few grams – would be propelled by powerful Earth-based laser arrays which focus on the crafts lightsails, accelerating them to speeds of up to 20% the speed of light, meaning they could reach Alpha Centauri in just 20-30 years. By comparison, traditional spacecraft would take tens of thousands of years to cover the same distance.
Now, with news of the Proxima b discovery, the focus will shift to the new exoplanet – which is even closer to Earth than Alpha Centauri – according to Pete Worden, leader of the Starshot project, speaking at an ESO press conference to announce the finding. While Starshot may seem like a fanciful prospect, much of the technology already exists in some form. If sufficient advances are made, the project could be ready for launch in 20 years, meaning a craft could reach Proxima b by around 2060.
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, said Anglada-Escudé. Many peoples 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...
