Microlensing

Chapter 6 of 15

When a foreground star (the lens) passes almost in front of a background star (the source), its gravity bends and magnifies the source's light for days to weeks. A planet around the lens adds a short extra spike. The lens itself is often too faint to see.

Illustration

Speed

A perspective grid of space, dented under a foreground lens star and its planet, with a distant source star behind and a telescope in front. The lens drifts across the line of sight; below, the source's brightness rises and falls in a symmetric magnification curve with a short planetary spike. The bending is exaggerated a million-fold.

Point-lens magnification: A(u) = (u² + 2)/(u√(u² + 4)), with u the lens–source separation in Einstein radii; A(1) = 1.342. The Einstein angle is θ_E = √(κ M π_rel), κ ≈ 8.14 mas/M☉ (Gaudi 2012). A planetary spike lasts about √q · t_E, where q is the planet-to-star mass ratio. An event never repeats, so the planet cannot be re-observed this way.

Quantity	Value	From
A(u = 1)	1.342	[calc]
θ_E for 0.3 M☉ and π_rel = 125 µas	0.55 mas	Gaudi 2010 example [calc]
Typical Einstein crossing time	≈ 19 days	Gaudi 2010
First microlensing planet	2004	Bond et al. 2004

356 of the 8,049 planets with a stored distance in exoplanet.eu were found by microlensing (the first method listed). Show them on the Galaxy map ›

peak 60 (2022)

19882026

Planets found by microlensing per year, 1988–2026: 356 in all; the most in one year was 60 in 2022. Source: exoplanet.eu, fetched 2026-10-18.

Planets in this chapterOGLE-2005-BLG-390L b ›

Sources