Last week astronomers using the Anglo-Australian and Keck telescopes announced eight new planets they have found around other stars. These bring the grand total of known exoplanets to nearly 80.
Astronomers found these and the other planets by closely measuring stellar radial velocities. When a planet revolves around a star, its gravity tugs on that star. Over the course of an orbit, the tugging causes a tiny periodic "wobble" as the star swings toward and then away from Earth. Short-period wobbles correspond to short-period orbits; longer-period orbits yield longer-period wobbles.
Planet hunters, such as the Keck team lead by Geoff Marcy (University of California, Berkeley) and R. Paul Butler (Carnegie Institution of Washington) have been monitoring stars for several years now. As a result, they are starting to confirm longer-period wobbles. Out of the eight new detections, seven have Earth-size orbits or larger.
Interestingly, planets with short-period orbits tend to have highly eccentric paths. The exceptions are those so very close to their stars that the orbits have been tidally circularized. However, this is less the case with longer-period extrasolar planets, when generally have more circular paths. This trend raises hopes that solar systems like ours — where the giant planets have circular orbits, thus allowing the existence of the Earth — are common rather than rare.