Smallest planet yet found
Using a technique known as gravitational microlensing, an international team of astronomers have detected the smallest known extra-solar planet. The discovery, made by the PLANET collaboration who use a network of telescopes in the Southern hemisphere to monitor stars in the Galactic bulge for microlensing events, was published today in Nature magazine.
Gravitational lensing is where one massive object bends the light from a more distant object along roughly the same line of sight (actually it is space which is bent, light just follows the distortion). The more massive the intervening object, the stronger the effect. Lensing is also strongest for objects which line up perfectly, producing a ring of emission known as in Einstein ring. Often, this effect is studied on huge scales where a galaxy lenses the light of a more distant quasar. In this case, however, the astronomers were looking for events much closer to home, they were watching for lensing by stars in the centre of our own Galaxy.
There are many ways to find planets around other stars: dimming caused by transits and Doppler shifts caused by radial motions as the planet pulls the star slightly in it's orbit are two of the most common, but both of these methods are sensitive only to massive planets which either block large amounts of the star's light, or pull the star by a significant amount. Microlensing, on the other hand, is capable of detecting Earth-mass planets by perturbations in the light curve of a lensing event.
An ordinary microlensing event (see a groovy demo of this by the MOA team) produces a sharp spike in brightness of the background star being lensed. A planet around the lensing star will cause an additional perturbation in the brightness measurements, which may look something like this:
Microlensing event OGLE 2005-BLG-71, evidence for an extra-solar planet
CREDIT: A. Udalski and the PLANET collaboration (astro-ph/0505451)
In 2004, astronomers using this technique discovered a planet orbiting a star at a distance of 17,000 light years, the most distant extra-solar planet discovered at the time. The discovery reported today is a planet around a cool, red star at a distance of almost 25,000 light years, with a mass of merely five times the Earth. That's pretty small! This is, in fact, the smallest extra-solar planet ever detected, and it is especially impressive given the distance. It's rather catchy name is OGLE-2005-BGL-390Lb, surely someone can come up with a nickname for it?! The discovery could not have been made by any of the other techniques, they are just not sensitive enough. Why look towards the Galactic bulge? There are huge numbers of stars there so it is the area with the highest probability for a microlensing event to occur, so with a bunch of small telescopes monitoring the area pretty much constantly (PLANET uses 1-m telescopes spread around the Southern hemisphere so that it is always night somewhere), they are likely to pick up any events.
This really is very, very cool. If you have access to Nature, go read the article. You never know, they may find something even smaller yet....