Australasian Science: Australia's authority on science since 1938

Planets Are the Norm, Not the Exception

By Stephen Luntz

There are probably many more planets in our galaxy than stars.

There are probably many more planets in our galaxy than stars, according to an international study published in Nature.

Over recent years the subtle movements in nearby stars, triggered by the gravitational effects of their companions, have been used to detect hundreds of planets. The Kepler space telescope, which is watching the dimming caused by planets passing across the face of the Sun, has turned the rate of discovery into an avalanche, with more than 2000 prospective planets revealed last year.

However, both techniques are limited, revealing only planets lying close to the parent star.

Microlensing, on the other hand, works best with planets orbiting at about twice the Earth–Sun distance, and can often pick up planets far further out, providing a much better guide to the true nature of other stellar systems.

Microlensing occurs when the gravity of a large object bends light from a more distant star around it, causing a temporary brightening of the object behind. On any given night one in a million stars lying in the direction of the centre of the galaxy will be lensing a more distant object.

“We have a couple of survey scopes watching millions of stars for signs of microlensing events,” says Dr Stefan Dieters of the University of Tasmania. “From these we pick the 50 or 60 most promising each year to watch in more detail.”

Promising events are those where the brightening appears likely to at least double the apparent light of the more distant star and where the relative velocities of the two objects are sufficiently slow that there is reasonable time to observe. An additional blip in the slow lensing curve (which can take up to 130 days to complete) indicates the presence of a planet whose mass provides its own additional lensing.

Ten planets have been drawn from this sample, including three previously reported (AS, March 2006, p.9). Many planets will be missed, for example if they are lying too close to the parent star during the pass or if their effect occurs in a short burst while no observations are being made.

Using estimates of the proportion of planets they would expect to catch, the international team concluded that they must either have been remarkably lucky, or that planets are very common indeed.

“Our analysis shows that Earth-sized and smaller planets are even more common than suspected,” says Dr John Greenhill of the University of Tasmania.

A breakdown of the objects observed by size indicates that planets similar in mass to the Earth or Neptune are more common than those closer in mass to Jupiter. “We think about two-thirds of stars have planets a few times the size of the Earth, while half have objects more like Neptune. Perhaps 20% have objects the size of Jupiter or larger,” Dieters says.

While this makes intuitive sense, early planet-spotting techniques were so skewed towards larger objects that some astronomers concluded that planets larger than Jupiter are particularly common.

Soon-to-be-launched space telescopes, designed to seek out dark energy, are also suited to detecting microlensing events, and Dieters says with luck they will be able to spot a well-placed object as small as Mars, testing the theory that such objects are even more common than the ones we can currently detect.