Exoplanets
Gravitational Microlensing
Detecting exoplanets and stellar mass through brief gravitational lensing events
Gravitational microlensing detects compact objects (stars, planets, dark matter) by their gravitational lensing of distant background stars. As foreground object passes near line of sight to background star, gravity bends light → background star appears brighter. Brief brightening (~days to months). Sensitive to wide-orbit planets and free-floating planets — complementary to transit. Discovered ~150 microlensing planets. Future: Roman Space Telescope (Nancy Grace Roman) will revolutionize. Also: probes dark matter (MACHOs), stellar populations.
- First detection1994 (gravitational lens; not yet exoplanet)
- First microlensing planetOGLE-2003-BLG-235 (2003)
- Lensing durationDays to months (compact lens) to years (massive)
- Background star magnification10× to 1000× (typical)
- Sensitive toWide-orbit planets, free-floating planets
- FutureRoman Space Telescope (2027)
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Why microlensing matters
- Wide-orbit planets. Unique detection method.
- Free-floating planets. Only direct sample.
- Mass-distance pairs. Unique constraints.
- Roman Space Telescope. Major future driver.
- Dark matter searches. Constrain MACHO candidates.
- Stellar population. Lensing reveals all compact objects.
- Galactic structure. Bulge versus disk populations.
Common misconceptions
- Microlensing only detects MACHOs. Detects all compact objects.
- Events repeat. One-shot — must catch in time.
- Microlensing competes with transit. Complementary.
- Microlensing is rare. Common in dense star fields.
- Microlensing gives radius. Gives mass; radius needs other methods.
- Microlensing doesn't help cosmology. Probes dark matter and structure.
Frequently asked questions
How does microlensing work?
Foreground compact object (star + planet) passes near line of sight to distant background star. Gravity bends light → background star appears brighter (magnification). For specific geometries: planet near lens star creates additional brief enhancement. Light curve fitted reveals mass ratio. Distinguishes planet from no-planet lens. Magnification: huge (1000×) to brief (factor 2-10).
What does microlensing reveal about planets?
(1) Mass ratio (planet/star). (2) Approximate mass scale via timing. (3) Distance to lens. (4) Orbital separation in physical units. (5) Detection of wide orbits or distant planets — unique among methods. (6) Free-floating planets — those without parent star.
What's a free-floating planet?
Planet not gravitationally bound to a star. Either ejected from solar system or formed in isolation. Microlensing is unique detection method. Estimated: 1-2 free-floating planets per main sequence star. Mostly Jupiter-mass; some Earth-mass. Not really "rogue" — just unbound.
How are events detected?
Survey of millions of stars. Most stars don't change brightness most of the time. Microlensing event: star brightens for hours-months, then returns. Surveys: OGLE (Optical Gravitational Lensing Experiment), MOA, KMTNet. Modern: real-time alerts; ground-based follow-up; high-cadence observations.
Are there limits?
Each event is unique; cannot repeat. Mass and distance derived from event characteristics; some degeneracy. Detection probability depends on geometry. Better with simultaneous observations from spaced platforms (e.g., Roman + ground). Statistics improve with each event.
How does this relate to dark matter?
Original purpose. MACHOs (Massive Compact Halo Objects) — proposed dark matter candidates: white dwarfs, brown dwarfs, primordial BHs in galactic halo. Surveys looking for microlensing in Milky Way's halo. Result: most halo dark matter is NOT MACHOs. Particles preferred. Surveys still continue.
What's Roman's role?
Nancy Grace Roman Space Telescope — NASA mission, 2027 launch. Wide-field IR observations. Microlensing survey toward galactic bulge. Expected: ~1000-3000 exoplanets. Expected discovery of free-floating planets and Earth-mass planets in wider orbits. Will revolutionize microlensing exoplanet science.