Dr. David Bennett will continue the microlensing planet search program at the University of Notre Dame in collaboration with several research teams around the world. Mirolensing is a gravitational phenomenon which produces a significant amplification of the brightness of a distant star when another star passes almost directly in front of it. The latter star is called the "lens," and the manner in which the distant star's brightness rises and falls during a microlensing event can be used to detect planets around the (generally unseen) lens star. In this project, Dr. Bennett will work to improve the analysis techniques used to detect planets, in particular to increase the number of planets detected in future microlensing events and to model the amplification from binary or multiple-planet lenses.
This project will emphasize international collaboration, since microlensing observations are performed by many different groups worldwide. The PI will transfer his technical expertise and software to the international scientific community to ensure that those who provide the hardware for this project will also share in the scientific discovery, and to enable new investigators to learn the technical details of microlensing analysis. The impact of the project also extends to amateur scientists and the general public.
This research project supported the Notre Dame group's participation in the study of extrasolar planetary systems with the gravitaional microlensing method as a part of the Microlensing Observations in Astrophysics (MOA) and Probing Lensing Anomalies NETwork (PLANET) collaborations. The major results of this research project were statistical in nature. The most prominant is the MOA Collaboration discovery of a previously unknown population of rogue planets that have no evidence that they orbit a host star. It was expected that such a population exists, but the number of planets discovered, about 1.8 Jupiter-mass planets per normal, Hydrogen buring star, was higher than expected. This provides a challenge to theories of planet formation and may indicate that violent gravitational interactions between planets are more common than previously thought. It might also indicate that binary star systems are both efficient at forming planets and also ejecting them into interstellar space. This discovery was deemed newsworthy enough to appear as the cover story for the June 2012 issue of Astronomy Magazine (see the accompanying Figure), and it was also featured in the Weather Channel documentary, "Forecasting the End". Another newsworthy outcome of this research project was the PLANET Collaboration result on the frequency of planets orbiting stars beyond the snow line, in Jupiter-like orbits (Cassan et al. 2012). This paper found that ~20% of stars have gas-giant planets, like Jupiter, ~50% of stars have ice-giant planets, like Neptune, and ~60% of stars have sub-Neptune or super-Earth planets. This implies that planets are more common than stars and that most stars have at least one planet in a Jupiter-like orbit. This result also made the national and international news. This work also resulted in the discovery of 8 new planets orbiting other stars, including the first cold planet with a mass measured to be smaller smaller than Saturn. (This planet, MOA-2009-BLG-266Lb, has a mass of 10 Earth masses.) All three students supportted by this grant are women, who are under-represented in astronomy and physics.
