The telescope that will be able to detect life outside our solar system in 2020

The radial velocity technique, which measures the induced Doppler shift of features in the spectrum of the parent star, can only find certain kinds of planets. With the current generation of telescopes, this technique is limited both by the precision and the stability of the velocity measurements: current measurements have pushed the limit down to an already impressive ~1 m/s precision retained over several years.

Unfortunately, though, a planet like the Earth, orbiting a star like the Sun, will only induce a radial velocity of about a tenth this size, which lies at the limit of what can be achieved with even the next generation of instruments on current telescopes. In contrast, ultra-stable spectrographs profiting from the large collecting power of the E-ELT will achieve measurement precisions of ~1 cm/s over periods ranging from minutes to years. For the detection of rocky planets in habitable zones, this precision is needed in order to overcome measurement contamination by oscillations, seismology, granulation and magnetic activity of the parent star.

More than 400 exoplanets have been found so far. With the E-ELT, the sensitivity of the radial velocity method will be improved by a factor of one hundred.

Thus, the E-ELT is essential for finding Earth twins in habitable zones, for determining how common they are and for understanding the properties of their parent stars. This will allow a complete census of rocky Earth- to Neptune-mass planets around nearby stars for the first time and will provide an understanding of the architecture of planetary systems with low-mass planets. These studies will lead to an understanding of the formation of Solar System twins and will provide an answer to an important part of the fundamental question: just how unique are we?

By 2020, ground- and space-based facilities will have discovered thousands of massive (Neptune- and Jupiter-mass) exoplanets. The E-ELT will start detecting Earth-twin targets in habitable zones using the radial velocity technique described above. By then, the statistical understanding of the properties of the parent stars and the distributions of the masses and orbits of exoplanets will have matured. The next step in exoplanet research will be the physical characterisation of the then known planets.

An artist’s view of the exoplanet CoRoT-7b, the closest known to its host star. The role of the E-ELT is to characterise similar rocky planets, but in habitable zones.

In order to achieve this, direct light from the planet must be detected and separated from the glare of its parent star. Overcoming this difference in brightness (usually referred to as the contrast) is the main challenge for this type of observation, and requires extremely sharp imaging. This capability will be a huge strength of round-based telescopes. Planet-finder instruments on 8-metre-class telescopes will achieve similar contrasts to the James Webb Space Telescope: around 10–5 to 10–6 at sub-arcsecond distances from the parent stars.

The detection of an Earth-twin requires a contrast of 10–9 or better within less than 0.1 arcseconds from the star. The unprecedented light-gathering power of a 40-metre-class telescope, and the implementation of extreme adaptive optics in the E-ELT are absolutely crucial to reaching this limit.

With the E-ELT, the detailed study of the atmospheres of young, massive exoplanets becomes feasible. Indeed, with its unprecedented sensitivity and spatial resolution at mid-infrared wavelengths, the E-ELT will be able to detect young, self-luminous exoplanets of Jupiter-mass. The contrast ratio between star and planet at these wavelengths becomes so advantageous that, for the nearest stars, hydrogen, helium, methane, water ammonia and other molecules can all be detected in low resolution spectra of the atmospheres of Neptune-like planets in habitable zones.

Using the Hubble Space Telescope astronomers have found water vapour and methane in the atmosphere of the Jupiter-sized planet HD 189733b.

Alternatively, exoplanet atmospheres can be observed during transits. Ground- and space-based facilities (such as the CoRoT and Kepler missions) are accumulating target stars for which an exoplanet, as seen from Earth, transits in front of its parent star. During these events (lasting a few hours every few months or years), spectral features of the exoplanet’s atmosphere, back-lit by their parent star, can be seen in the spectrum of the system. Such measurements are challenging, but lie within reach of the E-ELT. In the case of rocky planets in the habitable zone, the spectra can be examined for the biomarker molecules that are indicative of biological processes, offering perhaps the best opportunity to make the first detection of extraterrestrial life.