[Paper Review] Planet Detection via Microlensing

Penny D. Sackett|ArXiv.org|Sep 26, 1997

Stellar, planetary, and galactic studies3 citations

TL;DR

This paper proposes gravitational microlensing as a powerful method for detecting extrasolar planets, particularly those in the 1-10 AU range around distant stars, by monitoring brightness amplification during stellar alignments. It demonstrates that microlensing is sensitive to low-mass planets and provides a statistical approach to measuring planetary frequency across the Milky Way, complementing radial velocity and astrometric methods.

ABSTRACT

Microlensing is the most promising method to study the statistical frequency of extra-solar planets orbiting typical (random) stars in the Milky Way, even those several kiloparsecs from Earth. The lensing zone corresponds to orbital separations of a few times the Earth-Sun distance (AU) -- a good match to many planets in our own Solar System -- and the probability of detection is a rather weak function of planetary mass. Microlensing is thus a perfect complement to radial velocity and astrometric techniques that allow the detailed study of nearby planets with larger masses and smaller orbital separations. This report forms Appendix C of the Final Report of the European Southern Observatory (ESO) Working Group on the Detection of Extrasolar Planets, which contains recommendations to ESO for designing a competitive strategy in the field of extrasolar planets. The full report is available from ESO as document SPG-VLTI-97/002.

Motivation & Objective

• To evaluate microlensing as a viable method for detecting extrasolar planets in the Milky Way, especially those at larger orbital separations.
• To address the limitations of radial velocity and astrometric techniques, which are biased toward nearby, massive planets with small orbital radii.
• To provide a scientific foundation for ESO's strategic investment in microlensing-based planet detection campaigns.
• To quantify the detectability of planets via microlensing as a function of mass and orbital separation.
• To support the design of future observational strategies for detecting planetary systems statistically across the galactic disk.

Proposed method

• Utilizes the gravitational lensing effect where a foreground star's gravity bends light from a background source, causing temporary brightening.
• Models light curves of microlensing events to detect deviations caused by planetary companions in the lensing system.
• Analyzes the sensitivity of microlensing to planetary masses and orbital separations, particularly in the 1-10 AU range.
• Applies statistical analysis to estimate detection probabilities as a function of planetary mass and orbital radius.
• Compares microlensing sensitivity to other detection methods, emphasizing its complementarity with radial velocity and astrometry.
• Uses simulations and theoretical modeling to predict the frequency of detectable planetary systems via microlensing.

Experimental results

Research questions

• RQ1 Can microlensing detect low-mass planets at orbital separations typical of the outer Solar System?
• RQ2 How does the detection probability of a planet via microlensing vary with planetary mass and orbital distance?
• RQ3 To what extent can microlensing provide statistical insights into the frequency of planetary systems in the Milky Way?
• RQ4 How does microlensing compare in sensitivity and coverage to radial velocity and astrometric methods for planet detection?
• RQ5 What observational strategies should be adopted to maximize the detection of extrasolar planets using microlensing?

Key findings

• Microlensing is most sensitive to planets at orbital separations of a few times the Earth-Sun distance (1-10 AU), matching the region of many planetary systems.
• The detection probability of a planet via microlensing depends only weakly on planetary mass, making it effective for detecting low-mass planets.
• Microlensing provides a statistical method to determine the frequency of planetary systems across the Milky Way, independent of distance to the host star.
• The method is particularly suited for detecting planets around distant, randomly distributed stars, including those several kiloparsecs away.
• Microlensing complements radial velocity and astrometric techniques by probing a different region of parameter space—low-mass planets at larger orbital separations.
• Theoretical modeling confirms that microlensing can detect planets with masses as low as Earth-mass at separations of several AU, given sufficient monitoring cadence.