Most stars in our galaxy have planets, study suggests
Rocky, roughly earth-like planets orbiting stars seem to be the rule, rather than the exception, an new study reveals.
Alien planets are incredibly common in our Milky Way galaxy, outnumbering stars by a large margin, a new study suggests.Skip to next paragraph
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On average, each of the 100 billion or so stars in our galaxy hosts at least 1.6 planets, according to the study, bringing the number of likely alien worlds to more than 160 billion. And large numbers of these exoplanets are likely to be small and rocky — roughly Earth-like — since low-mass planets appear to be much more abundant than large ones.
"This statistical study tells us that planets around stars are the rule, rather than the exception," said study lead author Arnaud Cassan of the Paris Institute of Astrophysics. "From now on, we should see our galaxy populated not only with billions of bright stars, but imagine them surrounded by as many hidden extrasolar worlds."
Using a cosmic gravity lens
The vast majority of these exoplanet detections have been made using two different techniques: transit photometry and radial velocity. [Gallery: The Strangest Alien Planets]
Kepler employs the transit method, which watches for the tiny, telltale dips in a star's brightness caused when a planet crosses the star's face, blocking some of its light. Radial velocity looks for minuscule wobbles in a star's movement caused by the gravitational tugs of orbiting planets.
While these two methods have been highly productive, they're biased toward finding planets that orbit relatively close to their parent stars. In the new study, Cassan and his colleagues employed a different technique, known as gravitational microlensing, that feels this bias less strongly.
In gravitational microlensing, scientists watch what happens when a massive object passes in front of a star from our perspective on Earth. The nearby object's gravitational field bends and magnifies the light from the distant star, acting like a lens.
This produces a light curve — a brightening and fading of the faraway star's light over time — whose characteristics tell astronomers a lot about the foreground object.
In many cases, this nearby body is a star. If it has any planets, even ones in relatively far-flung orbits, these can generate secondary light curves, alerting researchers to their presence.
Studying millions of stars
In the new study, the researchers looked at data gathered by a variety of Earth-based telescopes, which scanned millions of stars from 2002 to 2007 for microlensing events.
The team closely analyzed about 40 of these events and discovered that three betrayed the presence of an alien planet around a star. One of these planets is a bit more massive than Jupiter, one is comparable to Neptune and the third is a so-called "super-Earth" with a mass about five times that of our home planet. [Gallery: Smallest Alien Planets Ever Seen]
Considering how perfectly aligned multiple bodies must be to yield an explanet detection via microlensing, that's a pretty impressive haul, researchers said.
The astronomers used all of this data, as well as information about seven additional planets detected by other microlensing efforts, to put a number on their planet-detection efficiency — and, by extension, the number of alien worlds that may populate the Milky Way.