This artist’s impression shows how common planets are around the stars in the Milky Way. The planets, their orbits and their host stars are all vastly magnified compared to their real separations. A six-year search that surveyed millions of stars using the microlensing technique concluded that planets around stars are the rule rather than the exception. The average number of planets per star is greater than one.

Credit: ESO/M. Kornmesser

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SHROPSHIRE: Every star in our galaxy is orbited by, on average, one or more planets: that's the mind-boggling conclusion of a new study by an international team of astronomers.

Published in Nature today, the study reveals that billions of habitable planets may exist within the Milky Way, and that Earth-like planets are more common than larger bodies such as Jupiter and Saturn.

"There are probably more stars than planets in the galaxy," said John Greenhill, a co-author of the study from the University of Tasmania. "Low-mass planets are much more common than gas giants. Much of this was already implied from other experiments, but ours is the first to show that it is true for planets with orbits similar to those in the Solar System."

Detecting habitable planets

Over the past couple of decades, astronomers have detected more than 700 planets outside our Solar System, as well as hundreds more unconfirmed 'candidate exoplanets'.

The most successful exoplanet detection technique is the 'radial velocity method', which looks for variations in a star's motion caused by an orbiting planet. Another common technique is the 'transit method', which analyses the reduction in a star's brightness as a planet passes in front of it.

However, these methods are better at detecting massive planets which orbit relatively closely to their parent star. An alternative technique, known as 'gravitational microlensing', can be used to identify planets which are more like those in our own Solar System.

This method works on the principle that the gravitational field of a star can magnify the light of a distant background star, acting like a cosmic magnifying glass. If the foreground star is orbited by a planet, this lensing effect is altered in such a way that can be detected by astronomers.

Combing the skies

These microlensing events are exceedingly rare because they rely on a chance alignment of two stars relative to Earth. Therefore, in order to detect as many exoplanets as possible, the search strategy is split into two stages.

Firstly, robotic, ground-based telescopes comb the skies for microlensing events, covering millions of stars in a night. Then, once an event is found, a follow-up team monitors the selected candidate to look for the telltale signs of an orbiting exoplanet.

Greenhill and colleagues revisited data collected by one of these follow-up collaborations: the Probing Lensing Anomalies Network (PLANET), which monitored over 400 microlensing events in a six year period between 2002 and 2007. The researchers combined data from ten planet detections, as well as hundreds of non-detections (these are just as important for the statistical analysis), in order to estimate the abundance of exoplanets.

Super-Earths galore

It was found that approximately one in six Milky Way stars hosts a planet with a mass similar to Jupiter, whilst around half of the stars host a Neptune-like planet. These planets orbit their stars at a distance of 0.5 to 10 astronomical units (an astronomical unit, AU, is the average Sun-Earth distance).

However, super-Earths were found to be the most abundant type of exoplanet - around two-thirds of stars are orbited by a planet of 5 to 10 Earth masses. Combining these findings, the astronomers estimate that each star in the Milky Way has at least one companion planet.

"From these results, we would expect any Milky Way star picked at random to have one or two planets with a mass larger than five Earth masses within the zone going from 0.5 to 10 AU," said Michaël Gillon, an exoplanet expert at the University of Liège in Belgium. "For the Solar System this expectation is well respected, with Jupiter at 5.2 AU and Saturn at 9.5 AU. Of course, this is a statistical expectation - some stars should have more, or less, planets."

Is anybody out there?

This study also reveals that an extraordinary number of planets in our galaxy are habitable, orbiting their host star at a distance where water could, in principle, exist in liquid form.

"I have no qualifications in biology," said Greenhill, "but it now seems highly likely that there are millions - probably billions - of planets in our galaxy that have an environment which could support life as we know it. The open question remains - how often does life develop given the right conditions?"