Star Trek calls them ‘Class M planets’. Earth-like worlds orbiting distant stars and supporting their own flourishing life forms have long been a staple of science fiction.
But they could become science fact within the next 20 years, following the announcement of the first-ever space mission dedicated to searching for planets capable of supporting life.
The PLATO space probe, announced by the European Space Agency in February, will blast off in 2024 – and scientists from the OU’s space research centre, CEPSAR, are already working on the project.
Light years away
It’s no easy task. Even the nearest stars are so distant that it would take thousands of years for any spacecraft we can currently build to reach them. So astronomers will be relying on the information that comes from PLATO’s telescopes.
PLATO (which stands for PLAnetary Transits and Oscillations of stars) will orbit the Earth and sweep the sky with an array of 34 telescopes, looking for the tell-tale clues that indicate the presence of ‘exoplanets’. These are distant planets going round stars other than our Sun.
PLATO will be looking specifically for exoplanets located in the ‘habitable zone’ – the right distance from a star to create the conditions necessary to support life as we know it.
We know there are plenty of exoplanets out there – astronomers using ground-based telescopes have been discovering them since 1995, when the first one was found orbiting the star 51 Pegasi.
But are they capable of supporting life? That’s what the astronomers are hoping PLATO will tell us.
How common are planets like Earth?
“Over the last two decades, more than a thousand exoplanets have been discovered, including several in multi-planet systems,” says Dr Carole Haswell, Senior Lecturer in Astronomy at the OU.
“However, virtually all of these are very different to our Solar System, as they include many giant planets and many in very close orbits around their stars. This is not surprising as they are the easiest systems to find.
“PLATO will tell us whether systems like our own Solar System, and planets like the Earth, are common in the Galaxy.”
Because they only reflect light from the star they orbit, no exoplanets in the habitable zones of stars can be imaged directly, even by instruments with the power of PLATO.
Instead, astronomers look for tell-tale clues to their presence – minute variations in the light emitted by the stars around which they orbit.
Clues in the starlight
The OU’s Professor of Astrophysics Education, Andrew Norton, is working with Dr Haswell on co-ordinating the education and public outreach from the mission. He explains:
“Many people assume that astronomers ‘see’ exoplanets directly through their telescopes, but in most cases all that is seen is a single, stationary pin-point of light – the star itself.
“By measuring the brightness of that speck of light, astronomers may see a tiny repeated dimming, as though the star is winking at us – indicating the transit of an exoplanet in front of the star, blocking a fraction of the light.”
As well as detecting the tell-tale ‘wink’ which allows the planet’s size to be measured, PLATO will be able to gather a great deal more information about the star too.
Combining this with follow-up measurements from other telescopes, it will be possible to paint quite a detailed picture of the unseen planet: its size and mass; its density, surface gravity and temperature; what it is made of; how far it is from its star; and even what its atmosphere is composed of.
Existing telescopes do not have the ability to cover large areas of sky over a long period of time. So the candidate planets they have tended to find are around fainter stars, which are not amenable to follow-up observations that would allow the planets to be properly characterised.
To boldly go
PLATO, by contrast, will be able to search around one million stars, spread over half the sky, during its six-year mission.
From all this information astronomers are confident they will be able to identify any ‘Class M’ planets, and study them further. It is possible they will even be able to detect the actual presence of life.
“Many astronomers are confident that the detection of biomarkers in the atmosphere of an Earth-like exoplanet will happen within the next 20 years, if life actually exists elsewhere in the Milky Way galaxy,” says Professor Norton.
“It may only be a green slime that could be scraped off a rock with a finger nail, but it would transform humanity’s view of its place in the Universe.”
To find out more about the science behind PLATO’s measurements, read Professor Andrew Norton’s explanation in The Conversation.