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
Video
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