We have so far found about 2 000 planets outside our solar system. One of the main reasons for hunting and studying these bodies, known as exoplanets, is to identify those that may be similar to earth in a meaningful way, and thus potential places where some form of life may have evolved.

Recent advances in technology – and the capabilities of observatories – both earth-based and in space, have added to astronomers’ arsenal as they sift through the exoplanets for those with earth-like features.

‘New planets are being discovered practically every week,’ said Professor Gilles Chabrier, a theoretical astrophysicist at the Lyon Centre for Astrophysics Research (CRAL) in France and the University of Exeter in the UK. ‘And we are improving our understanding of the physics of such exoplanets pretty much every day. So it is a really exciting time in this field right now.’

Prof. Chabrier leads the PEPS project, which has been funded by the EU’s European Research Council (ERC) to study the genesis of exoplanets, as well as their nature. His research has also looked at the formation and identification of brown dwarfs – space objects too big to be considered planets, but which do not have enough mass to sustain the nuclear processes that make stars shine.

The gases swirling in the atmosphere of an exoplanet can provide a biosignature – the elements, molecules or phenomena that would indicate underlying living organisms of some kind. So even without seeing the surface of an exoplanet, being able to analyse the atmosphere can help to assess if anything is living there.



Such a discovery could be possible in our lifetimes. Dr Xavier Bonfils, National Centre for Scientific Research (CNRS), France

‘In the not too distant future we should be able to identify the atmospheric conditions underlying biosignatures,’ Prof. Chabrier said.

Oxygen, carbon

Among the criteria for planets to have some potential as earth-like candidates are signatures such as oxygen and carbon in their atmospheres, as well as a solvent for biochemical processes – such as water. That means the planet should not be too close nor too far from its sun, but in the ‘habitable zone’ where water is liquid.

But even with these criteria, and the knowledge that the ideal size of a planet to support life is about one to 1.5 times the radius of earth, there could be billions of planets around billions of stars. Sorting through them is an immense task.

Dr Xavier Bonfils, of the Institute of Planetology and Astrophysics of Grenoble, France, aims to improve systems used to analyse the atmospheres of exoplanets.

His ERC-supported EXTRA project is using technology that can boost the power of a traditional technique known as differential photometry, where the brightness of a star or other object is assessed by comparing it to other stars in the image.

Dr Bonfils’ system should make the corrections necessary to eliminate variations caused by the earth’s atmosphere when observing tiny changes in brightness, such as when an exoplanet transits in front of a far-away star.

By collecting the light from many telescopes, and joining it all up through fibre optics to run through an infrared detector, Dr Bonfils envisages a ten-fold improvement in both efficiency and precision of earth-based observations.

‘This should be the most sensitive survey for earth-size planets transiting bright nearby stars,’ Dr Bonfils said. ‘That means we expect to be able to identify and analyse dozens of exo-earths (earth-like exoplanets), and being able to characterise their atmospheres, we will be able to identify those that are potentially habitable.’

By improving the decoding of biosignatures on exoplanets, Dr Bonfils believes researchers may have a chance within the next few decades to answer some fundamental questions.

‘To find planets similar to earth, in the habitable zone of stars, and then to characterise those planets with transmission spectroscopy (studying something by using the light that passes through it), seems to be a very promising strategy,’ he said. ‘If we find planets habitable enough and if we are lucky – like to find that life were to emerge easily on many planets – it seems that such a discovery could be possible in our lifetimes.’