Gravitational lensing

The gravitational micro-lensing method is an application of Einstein’s theory of gravity. This predicts that when a body of mass M (deflector) is at a distance projected onto the sky b ("impact parameter") of the line of sight of a remote source, the apparent brightness of this source is amplified by a factor:

$$A=\frac{\sqrt(M)}{b}$$

When the source and the deflector move in relation to each other on the plane of the sky, the impact parameter b varies with time. The same applies to the amplification factor which becomes a function of time.

Lens
A very distant star passes exactly behind another star around which a planet is orbiting. The nearer star focuses the light of the more distant star, which creates an increase in brightness. The planet also focuses the light from the distant star, which creates the two brightness peaks visible before the maximum.
(Observatoire de Paris / UFE)

If the background star aligns in turn with a star and a planet, its light curve will show a significant increase (because of the star) onto which a peak (planet) will be superposed.

The first detection (below) was presented by the team of the Scottish astrophysicist Ian Bond in 2004. This event was detected simultaneously by the dark matter research programmes OGLE (Optical Gravitational Lensing Experiment) and MOA (Microlensing Observations in Astrophysics).

First gravitational micro-lensing event
istorical graph presenting the light curve from a very remote star, light focused by the passage of a star and a planet located at 5,200 parsecs ? . The X-axis shows the days of observation and the Y-axis the brightness of the background star.
(Bond et al. 2004, ApJL)

This method has so far led to the discovery of 15/10/2026,

objects.

Advantages:

  • The campaigns can observe several stars simultaneously.
  • Detection of the planet is instantaneous.

Drawbacks:

  • Detection is a single event: the planet detected cannot be re-observed.
  • Detection gives little information about the star-planet system.