Thoughts on Microlensing

by Paul Gilster on January 26, 2006

Some further thoughts on OGLE-2005-BLG-390Lb, the planet whose discovery portends numerous microlensing breakthroughs to come. Note the distance between the Sun and the red dwarf in question, which is variously reported as between 20,000 and 28,000 light years. The red dwarf is in the constellation Sagittarius and lined up between us and the center of the galaxy, a location that is a natural consequence of the microlensing method used to find the new world.

For microlensing relies upon a closer star passing in front of one farther away, thus causing the gravitational effects used to pinpoint OGLE-2005-BLG-390Lb. To maximize the chances of seeing such an event, you need to point toward the largest possible field of stars, which is exactly what the OGLE project does. OGLE stands for Optical Gravitational Lensing Experiment, a study whose telescopes scan the central Milky Way each night. The scans are fruitful, for more than 500 microlensing events are discovered every year.

Bear in mind that the original OGLE data had to be passed along to other projects, like PLANET (Probing Lensing Anomalies NETwork), a global collaboration that can take a single OGLE observation and study it in greater depth. Another key player was RoboNet, a UK operated network of robotic telescopes. And the good news from this coordinated effort, at least to this point, is that microlensing seems to imply the existence of a huge number of small, rocky planets.

“The new discovery provides a strong hint that low-mass planets may be much more common than Jupiters,” said co-author and PLANET researcher David Bennett of the University of Notre Dame. Until recently, most extrasolar planets researchers have found have been Jupiter-like gas giants. “Microlensing should have discovered dozens of Jupiters by now if they were as common as these five-Earth-mass planets. This illustrates the primary strength of the gravitational microlensing method: its ability to find planets of low-mass.”

Indeed, the possibility exists of finding planets even smaller than Earth using these methods. That makes microlensing the chief candidate for finding terrestrial worlds around other stars using today’s technology, although because of the nature of the work, they will be planets thousands of light years away. Our early imaging efforts will, obviously, be devoted to much closer targets, but in terms of establishing the frequency of rocky worlds, microlensing allows us to survey a vast population of stars. A new telescope used by the Japanese/New Zealand Microlensing Observations in Astrophysics project can, in fact, observe 100 million stars per night .

Another thought: 70 percent of the stars in the galaxy are red dwarfs, not unlike the parent star of OGLE-2005-BLG-390Lb. If we have already found a rocky world orbiting such a star, the chances of there being terrestrial-type worlds around other M-class stars seem magnified. We may indeed have to revise our thinking about red dwarfs, which could emerge as an even more likely place to look for life-bearing worlds than the far less numerous G and K-type stars.