Exoplanets: tantalising search for life beyond the solar system

This year’s Nobel Prize for Physics honoured Michel Mayor and Didier Queloz, Swiss astronomers who proved the existence of a planet orbiting a star far beyond the Earth’s solar system.

Their find set off a series of so-called exoplanet discoveries, often with features that are nothing like the nine planets circling our sun. Here is a backgrounder on the search for far-away planets that could support some form of life -- though not necessarily as we now understand it.

Any planet outside our solar system is considered an exoplanet. Although their existence had long been theorised in both research labs and in popular culture -- think "Star Wars" -- until 1995 no one had been able to prove one existed.

By setting their telescopic sights on the Sun-like star 51 Pegasi, part of the Pegasus constellation, Mayor and Queloz found it was wobbling -- the light it emitted was blue as it moved toward them and red as it moved away. That proved something was circling the star, even though they couldn’t see it directly, being some 50 light years from Earth.

Nonetheless they could confirm the planet was gaseous and as big as Jupiter, yet very close to its star -- it had a four-day orbit -- and very hot, confounding theories on what types of celestial bodies would revolve where. "We thought other systems would be similar to our own," Ulf Danielsson of the Nobel Committee for Physics said in presenting the award on Tuesday.

"We were wrong." Today there are 4,057 confirmed exoplanets, according to the Nasa Exoplanet Archive, and at least as many likely candidates. The vast majority are far bigger than those of our solar system: there are more than a thousand so-called ice giants, around 1,000 gas giants, and "super Earths" with masses many times higher than the rock we call home.

There are only around 350 smaller terrestrial planets with Earth-like mass, and of those only a handful in a "temperate" zone that would allow for the presence of liquid water -- the key ingredient for life as we know it.

But these are just the planets scientists have detected: several studies have estimated that there could be a trillion exoplanets in our galaxy alone.

Ultimately, there could be as many exoplanets in the universe as there are stars. There are several ways to find planets that cannot be directly observed, often because the light from their suns is so bright, relatively speaking, that it blocks out smaller objects nearby.