A Milky Way full of planets

Artist's illustration showing how common planets are in our galaxy. Credit: ESO/M. Kornmesser

The discovery of exoplanets, planets that orbit other stars, has continued to grow exponentially over the last few years. Twenty years ago, when the first ones were found, it was unknown how many may be out there. The first exoplanets were found orbiting a pulsar, the rapidly spinning remains of a dead star, which made them easier to spot than if they orbited a normal star. Were they a fluke or did they indicate that perhaps many stars might have planetary companions?

As it turns out, it is the latter; not only do many, if not most stars apparently have planetary systems, but by some calculations those planets may even outnumber the stars themselves in our galaxy…

new six-year study by an international team of astronomers has concluded that planets are the rule, rather than the exception, in our galaxy. The study was published in the January 12, 2012 issue of Nature.

The group has used a technique called gravitational microlensing, which can detect planets of various masses and farther distances from their star than some other methods. When looking at a star for possible planets, the researchers look at its gravitational field – if there are planets, that gravitational field, combined with the gravitational field of the planets, acts like a lens which magnifies the light of any background stars.

Microlensing, however, requires the rare chance alignment of a background and a lensing star. Finding the actual planet also requires an alignment of the planet’s orbit.

In the six-year survey, three exoplanets were found. That may not sound like very many, but it’s important. Because of the rare chance alignments needed, it means that the astronomers were either very lucky, like finding needles in a haystack, or planets are so abundant that finding them was almost guaranteed.

By comparing the results with previous planetary detections, it was determined that one in six of the stars examined has a planet similar in mass to Jupiter, about half have Neptune-mass planets and about two-thirds have super-Earths (rocky worlds that are larger than Earth but smaller than Neptune). It also suggests that the average number of planets around a star is greater than one.

According to Arnaud Cassan, lead author of the Nature paper, “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.”

Considering these findings plus the incredible results so far from the Kepler space telescope mission and other ground-based telescopes, it would appear that indeed, the Milky Way is full of planets. Extrapolate that then to the billions of other galaxies and, well, you get the idea…

The paper is available here.

This article was first published on Examiner.com.