Exoplanets - Copyright ESO L Calcada The system Gliese 667 (Artist’s impression). © ESO/L. Calçada

By Royal Society University Research Fellow Martin Dominik
School of Physics & Astronomy, University of St Andrews

What are exoplanets?

The term "exoplanet" refers to planets orbiting stars other than the Sun or their remnants. The Sun is an ordinary star like around 100 billion others in the Milky Way, which itself is a galaxy like at least 100 billion others in the Universe - the planets in the solar system are just one sample from a vast and diverse range.

Are exoplanets like Earth?

While most of the more than 450 known exoplanets are massive gas giants like Jupiter and Saturn, about 20 "Super-Earths" of between 2 and 10 Earth masses have been revealed. COROT-7b was the first of these shown to have a composition similar to Earth's. However, only Gliese 581 d appears to have the right temperature to sustain liquid water on its surface and thereby being considered suitable to harbour life.

Will we ever be able to inhabit an exoplanet?

The prospects look extremely bleak. The current favourite Gliese 581 d orbits one of the 100 closest neighbours to the Sun. "Close" however still means 20 light-years away from us. Due to the energy released on impact with interstellar dust, space travel has to obey a speed limit of less than a tenth of the speed of light. Therefore, any spacecraft would have to be a floating self-sustainable mini-world itself.

Do you expect us to find alien life on an exoplanet?

Whether alien life actually exists is still an unanswered question. If it does, living generations could well witness some of its signatures spotted. These would however fall short of unambiguous evidence that would arise from carrying out biochemistry lab experiments on another celestial body.