Although few exoplanets have been detected using this technique, it's finding worlds that challenge planetary formation models.
As we detect more exoplanets, astronomers are realizing standard planetary formation models cannot account for many of them.
The use of microlensing -- where the gravity of a planetary body bends the light of its host star -- can highlight the presence of small worlds.
The importance of microlensing as an exoplanet-hunting tool is slowly being realized.
In exoplanet hunting today, there seems to be one burning question that nearly every new article published asks: Where did these planets come from?
As astronomers discovered the first extrasolar planets, it quickly became obvious that the formation theories we'd built around our own solar system were only part of the story.
For starters, these theories didn't predict the vast number of "hot Jupiters" astronomers were finding throughout our galaxy.
Astronomers went back to the drawing board to put more details into the theory, breaking formation down into quick, single collapses and more gradual accretion of gas disks, and worrying about the effects of orbital migration.
It's likely all these formation effects take place to some extent, but ferreting out just how much is now the big challenge for astronomers.
Hampering their efforts is the biased sample from the gravitational-wobble detection technique that preferentially discovers high mass, tightly orbiting exoplanets.
The addition of the Kepler space telescope to the planet hunter's arsenal has removed some of this bias, finding planets of far lower masses, but still prefers planets in short orbits where they are more likely to transit.
However, the addition of another technique, gravitational microlensing, promises to find worlds down to 10 Earth masses, much further out from their parent stars. Using this technique, a team of astronomers has just announced the detection of a rocky planet in this range.
Gravitational microlensing works by a planetary body -- or any body for that matter with significant mass, such as a brown dwarf or even a black hole -- passing in front of a star. As it does so, its gravity bends the starlight around itself, focusing the light for an observer on Earth. The body is then detected by monitoring transient brightenings of distant stars.
According to the Extrasolar Planet Encyclopaedia, astronomers have discovered 13 planets using gravitational microlensing. The newly announced one, MOA-2009-BLG-266Lb, is estimated to be just over 10 times the mass of Earth and orbits at a distance of 3.2 AU around its parent star with roughly half the mass of the sun.
This new finding is important because it's one of the first exoplanets in this mass range that lies beyond the "snow line" -- the distance during formation of a planetary system beyond which ice can form from water, ammonia, and methane.
The presence of icy grains is expected to assist in the formation of planets since it creates additional, solid material to form the planetary core. Just beyond the snow line, astronomers would expect that planets would form the most quickly since, as you move beyond this line, the density of planet-making material drops.
Models have predicted that planets forming here should quickly reach a mass of 10 Earth masses by accumulating most of the solid material in the vicinity. The forming planet can then slowly accrete gaseous envelopes. If it accumulates this material quickly enough, the gaseous atmosphere may become too massive and collapse, beginning a rapid gas accretion phase forming a gas giant.
The timing of these three phases, as well as their distance dependency, makes testable predictions that can be contrasted with the observations as astronomers discover more planets in this vicinity.
In particular, it has suggested that we should see few gas giants around low mass stars because the gas disk is expected to dissipate before the atmosphere collapse leading to a rapid accretion phase. This expectation has been generally supported by the findings of the 500+ confirmed extrasolar planets, as well as the 1,200+ candidates from Kepler, lending credence to this core collapse and slow accretion model.
Additionally, Kepler has also reported a large population of relatively low mass planets, inside the snow line. This too supports this hypothesis since the greater difficulty in forming cores without the presence of ice would hamper the formation of large planets. However, other predictions, such as not expecting massive planets in tight orbits, is still largely contradictory to the hypothesis and greater testing with additional discoveries will be needed.
Assisting with this, several new observing programs will be coming on line in the near future. The Optical Gravitational Lensing Experiment IV (OGLE-IV) has just entered operation and a new program at Wise Observatory in Tel Aviv will begin operation following up on microlensing events next year.
Also expected in the near future is the Korean Microlensing Network (KMT-Net) that will operate telescopes in South Africa, Chile, and Australia using 1.6 meter telescopes covering 4 square degrees of the galactic bulge.