From counting to characterising exoplanets

An artist's impression of an exoplanet orbiting close to a star

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We've come a long way since 1995 when Michael Mayor and Didier Queloz claimed the first official detection of an exoplanet orbiting a distant star - the somewhat prosaically named 51 Pegasi b, orbiting a sun-like star some 51 light-years from earth in the constellation Pegasus.

According to Nasa's Jet Propulsion Laboratory that figure now stands at 932, with a further 2,717 planet candidates waiting to be confirmed.

Much of the heavy lifting when it comes to spotting these new worlds has been done by the Kepler Space Telescope. Trailing in the earth's wake as it orbits the sun, Kepler has been staring, unblinking, at a narrow patch of the Milky Way between Cygnus and Lyra for the last three-and-a-half years - waiting for the telltale dimming of a distant star's light as an orbiting planet passes in from of it.

Kepler's share of the planet-spotting booty, according to the latest tranche of data released at the American Astronomical Society in January, is 2,740 including 114 confirmed planets.

It's an astonishing achievement by any standards, but Kepler's enduring legacy may be much more profound that its contribution to a simple head count of exoplanets. According to David Latham at the Harvard Smithsonian Centre for Astrophysics, a better measure of the project's success would be to acknowledge that it has shifted the goalposts of scientific debate. The question now is not are there planets orbiting distant stars, or even how many, but what do these planets look like and, crucially, could any support life?

"There are planets out there, but that's not the same as saying there's life on them. How often the Universe has planets truly like the earth - true earth twins, planets the size of the earth, rocky planets with a surface that supports liquid water - well, we don't have that number yet, but I'm optimistic we'll figure it out."

Some measure of how far the scientific debate has shifted can be gauged from the extensive list of sub-clauses in the title of a two-day Royal Society discussion meeting on exoplanets earlier this week. It reads "Characterising Exoplanets: detection, formation, interiors, atmospheres and habitability."

One of the key presentations at the conference was given by Dr Giovanna Tinetti who's leading the Exoplanet Characterisation Observatory, or EChO, mission recently selected for further development by the European Space Agency. EChO will use spectroscopy to study the atmospheres of exoplanets, hoping to spot the tell-tale chemical signature of life.

"EChO won't search for exoplanets directly." Dr Tinetti says "Instead we will observe planets we already know exist. We will study their composition to see if they're really similar to earth, and we'll look at the light of the star filtered through the atmosphere of the planet to tell us about the molecules present in those atmospheres and habitability".

And the EChO project is not alone. One of five European Space Agency missions being considered for launch in the early 2020's, Nasa also has a series of exoplanet projects at various stages of development, and both the James Webb Space Telescope and ground based observatories are getting in on the act.

And because what we're finding - planets that are bigger than Jupiter or smaller than Mercury, denser than iron or lighter than styrofoam - Bill Borucki, the Kepler Mission's principal investigator, believes the next ten or 15 years is going to be even more exciting than the last.

"Everything we're finding is different from what was predicted by the theorists. We didn't expect to find planets bigger than Jupiter. We certainly didn't expect to find small planets which are almost entirely gas. So lots of surprises in every way."