Detecting Exoplanets by Gravitational Microlensing
Survey Gaps & Rogues
David Bennett
Senior Research Scientist
NASA Goddard Space Flight Center
Sponsored by PSW Science Member Will Blair
About the Lecture
Our current knowledge of exoplanets is dominated by planets that are very different from those in our own Solar System, primarily because the most prolific exoplanet detection methods are most sensitive to planets with short orbital periods or large masses.
The gravitational microlensing method will fill this gap with sensitivity down to Mars-mass planets, with maximum sensitivity just beyond the snow line, where planet formation is thought to be most efficient. For this reason, NASA has selected an exoplanet microlensing survey as a core survey for the Nancy Grace Roman Space Telescope. The Roman Telescope will complete the statistical census of exoplanets started with the Kepler mission’s Galactic Bulge Time Domain Survey, which will be able to detect planets like those in our own Solar System (except Mercury). The Roman Telescope’s sensitivity will extend far beyond the orbital separation of the planets in our own Solar System’s and will be able to detect unbound (“rogue”) planets as well as orbitally bound ones.
Ground-based microlensing surveys have already provided hints of what the Roman Telescope may find. The microlensing data thus far seems to contradict predictions of a sub-Saturn mass desert in the wide orbit exoplanet distribution. Even more surprising than this result is the recent discovery by the MOA microlensing survey that low-mass “rogue” planets appear to be more common than all known types of bound planets, although some of these “rogue” planets could be in very wide orbits around stars. This is a somewhat unexpected clue to aspects of the planet formation process as it is currently understood.
This lecture will review the exoplanet microlensing discoveries to date, and explore how the Nancy Grace Roman Telescope’s exoplanet microlensing survey will add to the statistical census of exoplanets data on wide orbit and rogue planets that is needed to develop our understanding of planet formation.
Selected Reading & Media References
• Gravitational Microlensing Reveals the Lightest Exoplanet Yet Found, Bertram Schwarzschild, Physics Today, vol. 59, issue 4, p. 22, April 2006
• Detection of Extrasolar Planets by Gravitational Microlensing, David Bennett, arXiv:0902.1761, 2008
• Microlensing Surveys for Exoplanets, B. Scott Gaudi, Annual Review of Astronomy and Astrophysics, vol. 50, p.411-453, September 2012
• Telescopes Team Up to Find Distant Uranus-Sized Planet Through Microlensing, Hubble Space Telescope Press Release 2015-27
• Hubble Finds Planet Orbiting Pair of Stars, Hubble Space Telescope Press Release 2016-32
• New Study Reveals NASA’s Roman Could Find 400 Earth-Mass Rogue Planets, Ashley Balzer, NASA Goddard Press Release, Jul 19, 2023
• Our Galaxy Is Home to Trillions of Worlds Gone Rogue, Katrina Miller, The New York Times, Aug. 6-7, 2023
About the Speaker
David Bennett is Senior Research Scientist at the NASA Goddard Space Flight Center and at the University of Maryland. He currently serves as the Science Principal Investigator (PI) for the Nancy Grace Roman Telescope Galactic Exoplanet Survey Project Infrastructure Team and as the US PI for the Microlensing Observations in Astrophysics (MOA) collaboration. Previously, David served as the PI for the Microlensing Planet Finder mission concept, which was combined with two similar mission concepts to create the Nancy Grace Roman Space Telescope (previouly called WFIRST). He served on the first NASA Exoplanet Analysis Group Executive Committee, and on every WFIRST and Roman Space Telescope science team since 2011. Before this work, David had postdoctoral positions in theoretical cosmology at Fermilab and at Princeton.
David’s research has been primarily on microlensing and its use to study dark matter and, more recently, exoplanets. In graduate school he became intrigued by Bohdan Paczyński’s paper showing that a gravitational microlensing survey could confirm or deny the possibility that the Milky Way’s dark matter consists of objects ranging from planetary to stellar masses. He suggested this idea to Charles Alcock at Lawrence Livermore National Laboratory (LLNL) (in 1989!). The next year he joined Alcock to work on the first gravitational microlensing survey, which they called “MACHO” (for Massive Compact Halo Objects). MACHO proved to be quite successful: discovering the first microlensing event (in 1993!) Sun Hong Rhie* in examining the result, had the remarkable insight that a short duration feature in the light curve of this event could be due to a planet. This led her to work out important aspects of gravitational microlensing theory, leading to a seminal 1996 paper (with David Bennett) showing that microlensing was sensitive to very low-mass planets. Rhie and Bennett realized that a space-based microlensing survey would be needed to realize the full potential of exoplanet microlensing, leading them to propose the Galactic Exoplanet Survey Telescope (GEST) to NASA in 2000, and to a revised proposal for GEST, including the cosmology programs that are now a major focus of the Nancy Grace Roman Space Telescope. Much of David’s subsequent research has focused on further development of the space-based exoplanet microlensing survey methods, with recent emphasis on methods to determine the masses and distances of exoplanetary microlensing systems.
David is an author of more than 400 scientific publications. Among other honors and awards he is a Fellow of the American Physical Society, and the recipient of LLNL Physics and Space Technology Distinguished Achievement Awards and NASA Group Achievement Awards.
David earned a BS in Mathematics at Case Western Reserve University and a PhD in Physics at Stanford University.
* Sun Hong Rhie and David met in graduate school, were married and worked together until her untimely death in 2013.
Webpage: https://science.gsfc.nasa.gov/sci/bio/david.p.bennett
Minutes
On March 21, 2025, Members of the Society and guests joined the speaker for a reception and dinner at 5:45 PM in the Members’ Dining Room at the Cosmos Club. Thereafter they joined other attendees in the Powell Auditorium for the lecture proceedings. In the Powell Auditorium of the Cosmos Club in Washington, D.C., President Larry Millstein called the lecture portion of the 2,512th meeting of the Society to order at 8:02 p.m. ET. He began by welcoming attendees, thanking sponsors for their support, announcing new members, and inviting guests to join the society. Scott Mathews then read the minutes of the previous meeting which included the lecture by Johanna Drucker, titled “Alphabet Histories: The Origins of Letters from Antiquity to the Present”. The minutes were approved, pending a minor correction.
President Millstein then introduced the speaker for the evening, David Bennett, of the Goddard Space Flight Center. His lecture was titled “Detecting Exoplanets by Gravitational Microlensing: Survey Gaps & Rogues”.
The speaker began by saying that although humans have been discussing the possibility of life on other planets since the time of the Ancient Greeks, the real story of exoplanets began in 1995 with the discovery of the first two planets outside our solar system. He then discussed the “habitable zone”; the range of orbital distances over which a planet is likely to have liquid water. Bennett discussed the physics of planet formation, claiming that the models were too complex for “first-principles” calculations and required a number of approximations and guesses. He said, “Exoplanet theory makes no grand predictions, and progress comes from observation.”
The speaker then discussed the methods for detecting exoplanets. These included: ground-based and space-based imaging, astrometry, Doppler radial velocity, transits, and gravitational microlensing. He showed an animation, depicting “reflex motion”; the change in position and velocity of a host star, due to an orbiting planet. He discussed how Doppler radial velocity and astrometry measurements could reveal this reflex motion. Bennett indicated that the majority of the first 500 exoplanets discovered were found by radial velocity measurements. He described the transit method, wherein the apparent brightness of the host star is reduced as the planet passes in front of the star, noting that the majority of recently discovered exoplanets were found using this method and the Kepler Space Telescope.
Bennett then discussed gravitational lensing: the prediction of Einstein’s theory of general relativity that light propagating in a gravitational field will appear to bend due to the curvature of space-time. He listed several of the seminal papers on the subject, including: Einstein’s 1936 paper on the “lens-like” action of a gravitational field, Paczynski’s 1986 paper on the search for dark matter, Mao & Paczynski’s 1991 paper on the search for planetary systems, Alcock’s 1993 paper on the observation of microlensing in the Large Magellanic Cloud, and the 1996 paper by the speaker and his late wife, Sun Hong Rhie, on the detection of Earth-sized planets by gravitational microlensing.
Bennett described the physics of planetary microlensing. As the lens star (or host star) transits in front of the source star, the gravitational field of the lens star causes the light from the source star to bend in such a way that the source star appears larger. This magnification, which is initially small, increases to a maximum value, and subsequently decreases as the lens star passes in front of the source star. This smooth increase and subsequent decrease in the apparent size of the source star can be interrupted by sharp spikes in the magnification if a planet is present. Additionally, this change in magnification generally causes an increase in the apparent brightness of the source star. It is these spikes in the magnification and brightness as a function of time that constitute the observation of an exoplanet by gravitational lensing.
The speaker indicated that the probability of finding a source star and lens star in the right position, as observed on the Earth, was quite small: on the order of a few parts in 106. He said that the Galactic Bulge, toward the center of the Milky Way, was the best place to look for planetary companions. He then discussed some of the results from recent microlensing surveys, including: MOA-II, which found 30 exoplanets between 2005 and 2012, and KMTNet survey, which found 63 exoplanets during its 6-year operation. He discussed how these results compare to various models of planet formation, or core accretion models. Bennett showed animations and hydrodynamic simulations of gas accretion during the early phases of planet formation.
The speaker ended his talk by describing the types of observations expected to be performed with the Nancy Grace Roman Space Telescope, scheduled to be launched in 2026. The search for exoplanets is one of the primary objectives of this mission. This instrument will have significantly higher resolution and significantly faster survey times than current ground-based observatories.
The lecture was followed by a Question and Answer session.
A member asked if our current observational techniques would allow detection of “non-planetary” features around other stars; particularly asteroids or asteroid belts. Bennett responded that we have not been able to make such measurements with ground-based systems, and that there was some discussion as to whether the Roman Space Telescope would be capable of such observations.
A member noted that some researchers have claimed that the distribution of planets in our solar system is logarithmic, and is related to Boyle’s Law [sic] (actually known as Bode’s Law or more commonly Titius-Bode Law). He asked if such logarithmic distributions have been observed in other systems. Bennett responded that Boyle’s Law [sic] is “partly an accident”, saying that the law does not appear to be true in general, but that there may be planet formation mechanisms that make such a distribution more likely.
A guest on the live stream asked whether you could detect exoplanets using gravitational waves and could you use microlensing with gravity waves, as opposed to light. Bennett responded, “No”, saying that the gravitational fields of planets are quite small, and that current technologies are unable to detect gravity waves of such low amplitude.
After the question and answer period, President Millstein thanked the speaker and presented him with a PSW rosette, a signed copy of the announcement of his talk, and a signed copy of Volume 17 of the PSW Bulletin. He then announced speakers of up-coming lectures and made a number of housekeeping announcements. He adjourned the 2,512th meeting of the society at 9:57 pm ET.
Temperature in Washington, DC: 12.8° Celsius
Weather: Mostly Cloudy
Audience in the Powell auditorium: 44
Viewers on the live stream: 26
For a total of 70 viewers
Views of the video in the first two weeks: 419
Respectfully submitted, Scott Mathews: Recording Secretary