More planets than stars?

Artists’s cartoon view gives an impression of how common planets are around the stars in the Milky Way Image copyright Other

Just how many planets are there out there?

Going on the available evidence from our own solar system - and there's no obvious reason to assume it is unusual - you might plump for a ratio of 1:8. That would give you a ballpark figure of some 800 billion planets in the Milky Way, and something like 100 billion times that for the entire Universe.

But - as any good statistician will tell you - a sample size of one is hardly scientific.

To get a better idea astronomers have used a variety of methods including the Dopler effect (subtle wobbles in a star's position caused by the gravitational pull of an orbiting planet), and transits (periodic dips in luminosity as a planet crosses in front of its parent star) to infer the existence of extra solar planets.

Planet spotting took a huge leap forward with the launch of the Kepler Space Observatory in 2009. Using the transit method it has taken the total number of candidate planets to over 700 as it repeatedly photographs the stars in a narrow patch of space between the constellations Cygnus and Lyra.

Kepler's planet count remains a work in progress, but research published in the journal Nature indicates that even its final results may represent a massive underestimate.

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Media captionProf Kevin Horne: We believe there are more planets up there than stars

The team, lead by Arnaud Cassan at the Institute of Astrophysics in Paris, used a novel technique - gravitational microlensing - to estimate how common planets may be in the Milky Way.

After a six-year search that surveyed millions of stars they conclude that planets may be commonplace.

"Remarkably," Professor Cassan says, "the data shows that planets are more common than stars in our galaxy. We also found that lighter planets, such as super-Earths or cool Neptunes, must be more common than heavier ones."

Using gravitational microlensing to detect exoplanets relies on the way that the gravitational field of a star acts like a lens, magnifying the light from a background star.

If the star has a planet, or planets, orbiting around it the effect will be amplified - with each planet making a detectable contribution to the brightening effect on the background star.

Microlensing is a very powerful tool, with the potential to detect exoplanets that could never be seen in any other way. But it's also astonishingly rare - depending on the chance alignment of a background and lensing star at the precise moment that a planet transits in front of that.

The team turned up just three direct exoplanet detections in six years worth of microlensing data, but by combining that with seven sightings from previous data, and crunching the probabilities from the huge number of non-detections, they estimate that one in six of the stars hosts a planet the size of Jupiter, half have Neptune-like planets and two thirds have super-Earths.

The results, they claim, suggest that the average number of planets around a star is greater than one. Making planets the rule rather than the exception.

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