# Exoplanetary Microlensing

**Authors:** B. Scott Gaudi  
**arXiv:** [1002.0332](https://arxiv.org/abs/1002.0332)  
**Date:** 2010-02-01  
**Summary page:** https://www.summarizepaper.com/en/paper/1002.0332v1/

## Key points

- Gravitational microlensing occurs when a foreground star passes close to our line of sight to a more distant background star
- The foreground star acts as a lens, causing the light from the background star to split into two images
- If the foreground star has a planet with a projected separation near the paths of these images, the planet can further perturb the images
- Microlensing is sensitive to low-mass planets located just beyond the snow line
- Microlensing can detect multiple-planet systems containing analogs of all solar system planets except Mercury
- It can also identify free-floating planets, providing insights into planet formation theories

## Summary

Gravitational microlensing is a phenomenon that occurs when a foreground star passes close to our line of sight to a more distant background star. This foreground star acts as a lens, causing the light from the background star to split into two images. These images are usually unresolved but are magnified depending on the angular separation between the lens and source. If the foreground star happens to have a planet with a projected separation near the paths of these images, the planet can further perturb the images, resulting in a distinct and short-lived signature. This chapter provides an introduction to the discovery and characterization of exoplanets using gravitational microlensing. The theoretical foundation of this method is reviewed, with a focus on understanding planetary perturbations. The strengths and weaknesses of microlensing as a technique for detecting exoplanets are discussed, emphasizing its sensitivity to low-mass planets located just beyond the snow line. These planets can orbit stars throughout the Galactic disk and foreground bulge. The chapter also presents an overview of how microlensing planet searches are conducted, highlighting some of the challenges associated with detecting and analyzing planetary perturbations. It concludes by reviewing the results obtained so far and discussing both near-term and long-term prospects for microlensing planet surveys. Ultimately, gravitational microlensing has the potential to detect multiple-planet systems containing analogs of all solar system planets except Mercury. It can also identify free-floating planets, providing valuable insights into planet formation theories by determining the demographics of planets across the Galaxy.