Astronomers are getting closer to finding a rocky, water-rich planet like earth after proving new technology that can spot minute changes to starlight.
Using an ultra-sensitive light-splitting spectrograph, EU-funded researchers have managed for the first time to identify a rocky, iron-rich planet billions of kilometres away – a critical step towards discovering a far-away earth.
The new device, called HARPS-N, was made with nanoscale precision and operates in a vacuum at temperatures stable within a thousandth of a degree. Researchers at the EU-funded ETAEARTH project, coordinated by Dr Alessandro Sozzetti at the Italian National Institute for Astrophysics (INAF), trained it on Kepler-78, a star 400 light years away in the Cygnus constellation, in order to pick out the tiny fluctuations caused by the planet Kepler-78b.
Kepler-78b was first spotted in 2013 by the Kepler spacecraft, which scans the night sky in search of planetary transits that periodically dim the luminosity of distant stars. The chances of a planet crossing its line of sight are two hundred to one, but Kepler has evened out the odds by tracking some 150 000 stars.
Kepler-78b, a planet just 20 % larger than earth, orbits at the breakneck speed of three revolutions a day. It is this tight orbit that means it pulls its star just enough to allow researchers at the ETAEARTH project to identify its mass and density – elusive properties for extra-solar earth-sized planets and key clues as to their composition.
Astronomers determine the mass of distant planets by investigating how their gravitational pull makes their parent star wobble.
‘It may take another ten years to spot a planet with conditions like earth, and maybe centuries to reach it.’
Dr Emilio Molinari, the director of the TNG telescope at INAF
‘When a star wobbles away from observers on earth, its colour reddens a little. When it wobbles towards us, it shifts towards the blue,’ said Dr Lars Buchhave, an astronomer at the Harvard-Smithsonian Center for Astrophysics in Massachusetts, US, who helped calculate the mass of Kepler-78b. ‘This is the same phenomenon which alters the pitch of a passing police siren.’
This phenomenon, called the Doppler effect, is used to measure things like the speed of an object with radar and as a tool in medical imaging. However, it has also has revealed the existence of hundreds of Jupiter-like gas giants beyond the confines of our solar system since the discovery of the first planet outside our solar system in 1995.
‘Still, the wobble produced by Kepler-78b remains ten times smaller than that of Jupiter on the sun,’ said Dr Emilio Molinari, the director of the TNG telescope at INAF in the Canary Islands.
The HARPS-N spectrograph was mounted on the 3.6 metre wide TNG telescope and pointed at Kepler-78 for 80 nights. ‘It was very challenging to achieve such high accuracy, but in the end we got it. We now know that Kepler-78b is just 60 % more massive than earth.’
The Doppler wobble of Kepler-78b’s parent star reveals that the planet’s density is the same as that of earth, suggesting that it is also composed of rock and iron. However, whether we would want to visit this new world is another matter. The proximity of Kepler-78b to its parent star leads to temperatures so high as to melt the rock on its surface and strip away its atmosphere.
The results obtained are of philosophical as well as scientific interest. Theorists are still trying to understand how Kepler-78b landed in such a tight orbit. But the fact that the planet exists at all indicates that, from an astronomical perspective, the earth is not unique.
The ultra-sensitive HARPS-N spectrograph. Copyright: © UNIGEFor a planet to harbour life, scientists believe it must sit in the habitable zone – a precise distance from a star where the temperature is just right for water to be liquid. That means its Doppler wobble will be much fainter than that caused by the tight orbit of Kepler-78b.
The biggest problem that planet-hunters face in detecting this wobble is stripping out effects of activity on the surface of the star. ‘If we can crack this problem, we will be in a much better position to detect earth-mass planets orbiting in the habitable zones of solar-type stars,’ said Professor Andrew Cameron, who heads the School of Physics and Astronomy at the University of Saint Andrews in the UK.
They reckon that the best chance of spotting an earth-like planet is to focus on small stars called red dwarfs, where the habitable zone is closer to the star, increasing the Doppler wobble. ‘It may take another ten years to spot a planet with conditions like earth, and maybe centuries to reach it,’ said Dr Molinari at INAF. ‘But even if we just see the first steps of this great journey, someone has to start it.’
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