If you’ve been counting discoveries of extrasolar planets, now might be a good time to find a new hobby.
According to an article appearing in today’s issue of the British science journal, Nature, exoplanets are so common that our own galaxy contains at least one for each of its 100 billion stars. Furthermore, planetary mass calculations indicate that formation of smaller planets are favored over large ones — meaning that there should be more than 10 billion Earth-sized planets in the Milky Way.
The findings come from a 6-year observational effort by an international collaboration known as PLANET (Probing Lensing Anomalies NETwork), using a technique called microlensing that can detect planets too faint to be seen otherwise.
When one star passes in front of another, gravity from the foreground star act like a lens, bending and amplifying light from the background star. A planet circling the foreground star reveals itself by adding to the amplification, and the more massive the planet, the more the background star is brightened.
Microlensing from a star generally lasts about a month, whereas microlensing from a planet lasts a few hours or days. Unlike other methods used to find exoplanets, microlensing can reveal stars as small as Mercury, in orbits as close as Mercury’s or as far away as Saturn’s.
While the microlensing technique can reveal a planet’s mass, it says nothing about its composition. A planet that is Earth-sized may not be Earth-like, and vice versa. Still, for those contemplating the question as to whether we are alone in the universe, the discovery of so many extrasolar candidates would seem to have altered the odds considerably.
Image: Artist’s rendering of exoplanets in the Milky Way. Credit: NASA, ESA, and M. Kornmesser (ESO)