Question

What is the gravitational lensing signature of a planet orbiting a star? Draw a picture of the spatial relationship between the star being lensed, the planet, and the star around which the planet is orbiting. Also, draw a typical light curve showing how the planet manifests itself.

Answer 1

Light travels through empty space unaffected, and any observer could track the position of the source by tracing back this light ray. But the universe is not empty, and according to general relativity theory large masses can deflect light in a similar way as regular lenses do. This is way objects of the size of a planet or a star are called gravitational lenses, and they can bend the light stronger the closer they are to the ray. Planets that are very far (thousands of light-years away) can be detected using the microlensing method: when one star passes in front of another it makes it brighter because of the gravitational lensing. If a planet (which emmits no light) orbits the lens making star, it acts as a second lens, making the source star even brighter. A typical light curve of the brighness of the source star has 2 characteristic spikes, due to the lens star's presence and to and the planet's presence.