When children learn that we live on the surface of a planet, that there are other planets orbiting around the Sun and, finally, that our Sun is like the stars that we see at night, one of their first thoughts is that those stars may have planets as well. And they are right; there are planets around other stars but they are very difficult to see without sophisticated astronomical instruments. Since 1995, when the first so-called "exo-planet" was discovered, almost 250 have been detected. Most of them are more massive than Jupiter (itself weighing 318 Earths) because the techniques used are more sensitive to these massive planets.
A paper that appeared in a recent issue of the journal Science reported the discovery, as a result of collaborative work between 69 researchers from 11 countries, of not just one, but two planets orbiting a star.
There are already 29 multi-planet extrasolar planetary systems discovered (including this last one), so what makes this particular system unique? It turns out that these two planets are very similar to our Jupiter and Saturn. Their relative masses and distances are similar to those that Jupiter and Saturn have with respect to the Sun, only scaled down a bit. In other words, the bigger planet has a mass around 70% of Jupiter whereas the smaller being 90% of Saturn. Together, they orbit their star at half the distance that our two gas giants planets do: 2.3 and 4.6 astronomical units compared with 5.2 and 9.5 for Jupiter and Saturn respectively.
This discovery was possible thanks to an effect predicted by Einstein called gravitational lensing, where the gravity of a massive object changes the path of light rays coming from a source further away on their way to the Earth. In order to detect extra-solar planets, a technique called gravitational microlensing detects the "bending" of light, coming from a faraway star, that is produced by a closer star and its surrounding planets. That technique is currently the only way to detect planets with masses as small as Saturn's and it is, in principle, capable of detecting planets larger than Mercury.
The collaborative groups involved in this discovery were: OGLE, ТЕFUN, PLANET, and RoboNet. Scott Gaudi from Ohio State University in Columbus was the lead investigator.