This project is focused on the detection of exoplanets from gravitational microlensing events. Towards this end, a series of activities will be undertaken as extensions to Dr. Gould's previous work and in support of ongoing searches for new exoplanets.
Dr. Gould leads the Microlensing Follow Up Network (micro-FUN), a collaboration of amateur and professional astronomers that searches for planets in high-magnification microlensing events. Micro-FUN has detected 5 of the 7 microlens planets, including a 'cold Neptune' and the first Jupiter-Saturn analog system, both from parameter space that is not accessible to other planet-search methods. Dr. Gould will continue this work, both organizing observations and leading the interpretation of the rather complex light curves that bear planetary signatures. The detection rate is expected to increase from the current level of 2 per year because of improved surveys as well as work carried out here. Because microlensing is probing regions of parameter space not covered by other methods, new surprises can be expected.
As part of this project, a semi-automated pipeline for high magnification events will be developed. This will in part allow the collaboration to concentrate resources on high magnification events. It will speed up the extraction of planetary parameters from the light curve data as well. Dr. Gould will also begin a careful analysis of selection effects required to begin to draw statistical conclusions on planet frequencies from microlensing detections. Additional projects include searches of all publicly available data for short-timescale events that otherwise might be missed and a new investigation of parallax effects on high magnification events. Finally, the PI will continue to seek target of opportunity spectroscopy of bulge dwarfs during magnification events.
The projects here will lead to the training of a graduate student who will be an integral part of the program. Also, under Dr. Gould's guidance, microlensing has become one of a handful of areas in which amateurs are making a major contribution to astronomy. Dr. Gould will focus substantial effort on raising the public's scientific awareness of microlensing and exoplanets through the continued participation of amateurs in microlensing studies.
The principal scientific goal was discovery and characterization of extrasolar planets using gravitational microlensing. At present, microlensing discovers far fewer planets than other methods (about 20 versus 2000), but these planets would be difficult or impossible to discover otherwise. This is because all other methods depend on light from the host star or the planet, whereas microlensing finds planets by their gravitational effect on light from a more distant (unrelated) star. Hence microlensing can find planets that orbit very faint stars, including completely dark stars or very distant stars. It can find planets that are very far from their hosts or even completely ejected from their host system and now "floating freely" in space. Based partly on a "White Paper" submitted by the PI, the US is now committed to launching a satellite (WFIRST) for which a major objective will be finding planets with microlensing. Hence, work under this award involved both efforts to find planets and to train young scientists for much bigger future efforts. The single most important outcome of this work was publication of the first measurement of planet frequency using the microlensing technique. Published in 2010, this paper found that gas giant planets (e.g., Jupiter, Saturn) are much more common out past the "snow line" where microlensing is sensitive, than they are closer in where they had previously been measured by other techniques. The "snow line" marks the region that is cold enough to form ice when planets are forming. In the solar system, it is 2.7 astronomical units from the Sun. All the Solar-System giant planets are out past the snow line, and it is generally believed that all giant planets must form beyond the snow line because they require ice cores to get started. Hence, it was initially a major puzzle why other techniques were finding giant planets inside the snow line. Most astronomers now believe that these planets formed outside the snow line, but then migrated inward. It was still a big question whether the Solar System is unusual because its planets did not migrate very far (i.e., stayed outside the snowline where they were born) or whether the systems discovered by other techniques were unusual, and were found simply because those techniques are most sensitive to close-in planets. Our work showed that the Solar-System is more typical: giant planets usually do not migrate very far. It also showed that the Solar System is about three times richer in giant planets than typical stars, and probably about six times richer in multi-giant-planets. However, the latter conclusion is only tentative because the statistical uncertainties are still large and require more data to resolve. Many very interesting science results were obtained as a byproduct of this research. For example, a very old brown dwarf (failed star) was discovered because it gave rise to a spectacular microlensing event that was then intensively monitored to search for planets. No planets were found, but it was noticed that the light curve as seen from Chile, South Africa, and the Canary Islands looked different. This was the first example of the long-predicted "terrestrial parallax effect" in microlensing and was used to measure the lens mass and distance. This is the faintest free-floating brown dwarf that has ever been detected. All previous brown dwarfs had been detected from their light or their effect on stars that they orbit. But as stated above, microlensing can detect objects that emit little of no light. Also microlensing acts as a "natural telescope", sometimes magnifying stars by 100 to 1000 times. We therefore notified stellar astronomers about these events as they were happening so that they could get detailed spectra of these stars, which would otherwise be impossible. Probably the greatest long term benefit of the work will be the training given to graduate students. The PI gives very intensive training to students, who often spend several years working directly with him in his office. The one student that graduated under this award won a prestigious "Sagan Fellowship" to continue his work on finding and characterizing planets. Another student is poised for graduation and her prospects look very good. In addition, the PI trained two postdoctoral researchers. Salaries for these researchers came primarily from Ohio State funds, but the PI's time working with them was primarily funded under this award. These researchers will eventually go back to their home countries (Poland and France) with enhanced skills, thus strengthening international cooperation. Finally, the PI has founded an international network of amateur astronomers to carry out microlensing observations. Many of the planets discovered have relied on data from these amateurs. At the same time, the amateurs are trained in microlensing science and take out this training to other amateurs and the broader public. Thus, the project has high impact well beyond its immediate scientific outcomes.
