A cosmic census by international astronomers has discovered that the Milky Way galaxy must be packed with planets. The alien worlds, though largely invisible to us, are more common than the stars themselves, they conclude. 

Since the first planet orbiting a star other than our own Sun was detected in 1995, the number of confirmed discoveries has rocketed to more than 700. Most of these are giant gasballs, rather like Jupiter, beause they are easier to find, usually either by the wobble effect they cause on their host star or by the dip in the star’s brightness if they happen to pass in front of it.

This is still a small number compared to the hundreds of billions of stars that must lie in our own Galaxy, which is itself just one of many billions of other galaxies in the Universe. But the techniques used mean that planets which are not massive or orbiting close to the stars get missed.

The new planet estimate is the result of a six-year survey of millions of stars using a different method called gravitational microlensing. This used highly sensitive instruments attached to some of the world’s largest telescopes, in Australia, Chile and South Africa, to find planets by detecting how their tugging effects warped the light from far more distant stars behind them.

The technique is able to detect planets over a wide range of mass and those that lie much further from their stars, the discovery team say in a paper in the journal Nature. The only problem is that you need a rare chance alignment of a star with the planet to detect it.

Three planets were found in the six-years search, one a super-Earth and two gas giants comparable either to Neptune or Jupiter. However, the number found suggests either that the astronomers had been exceptionally lucky in catching microlensing events or that it was inevitable due to the abundance of planets in the Milky Way. But by combining their discoveries with seven planets they found by looking back at earlier microlensing data, they concluded that planets are comonplace and that they outnumber stars.

Lead author Arnaud Cassan, of the Institut dʼAstrophysique de Paris, said: “We have searched for evidence for exoplanets in six years of microlensing observations. Remarkably, these data show 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.”

The international team joined up in a collaboration to which they gave the excellent acronym PLANET – short for Probing Lensing Anomalies NETwork. The group’s leader Jean-Philippe Beaulieu, also of the Institut d’Astrophysique de Paris, said: “The PLANET collaboration was established to follow up promising microlensing events with a round-the-world network of telescopes located in the southern hemisphere, from Australia and South Africa to Chile.”

More than half of the data came from an instrument called OGLE – Optical Gravitational Lensing Experiment – attached to the Danish 1.54-metre telescope at the European Southern Observatory’s La Silla telescope in Chile. The astronomers say their microlensing technique is capable of finding planets in the same range of distances as from Venus to Saturn in our own Solar System.

Co-lead author of the Nature paper, Daniel Kubas, said: “We used to think that the Earth might be unique in our galaxy. But now it seems that there are literally billions of planets with masses similar to Earth orbiting stars in the Milky Way.”

A NASA study using a different approach, by examining Sun-like stars in our own neighbourhood, came to a similar conclusion that the Galaxy must be swarming with planets in 2010.

Astronomers are following up their microlensing success with a new hunt called MiNDSTEp – the Microlensing Network for the Detection of Small Terrestrial ExoPlanets campaign – which pushes the search for new worlds into hitherto uncharted territory down to the mass of the Moon.