The challenge

There are hundreds of billions of galaxies in the observable Universe, with each galaxy such as our own containing some hundred billion stars. Surrounded by this seemingly limitless ocean of stars, mankind has long speculated about the existence of planetary systems other than our own, and the possibility of life existing elsewhere in the Universe.

Only recently has evidence become available to begin to distinguish the extremes of thinking that has pervaded for more than 2000 years, with opinions ranging from ‘There are infinite worlds both like and unlike this world of ours’ (Epicurus, 341–270 BCE) to ‘There cannot be more worlds than one’ (Aristotle, 384–322 BCE).

Shining by reflected starlight, exoplanets comparable to solar system planets will be billions of times fainter than their host stars and, depending on their distance, at angular separations from their accompanying star of, at most, a few seconds of arc. This combination makes direct detection extraordinarily demanding, particularly at optical wavelengths where the star/planet intensity ratio is large, and especially from the ground given the perturbing effect of the Earth's atmosphere.

Alternative detection methods, based on the dynamical perturbation of the star by the orbiting planet, delivered the first tangible results in the early 1990s. Radio pulsar timing achieved the first convincing detection of planetary mass bodies beyond the solar system in 1992.