Discovering Extrasolar Planets: Methods and Significance

Extrasolar Planets

An extrasolar planet is a planet orbiting a star other than our Sun. There are two general ways of learning about a distant object: directly, which means by obtaining images or spectra of the object, and indirectly, which means by inferring the object’s existence or properties without actually seeing it. There are two major indirect approaches to finding and studying extrasolar planets:

1. Observing the motion of a star to detect the subtle gravitational tugs
2. Observing changes to a star’s brightness that occur when one of its planets passes in front of the star as viewed from Earth.

Planets exert gravitational tugs on their star, causing the star to orbit around the system center of mass. Gravitational tugs: we can detect a planet by observing the small orbital motion of its star as both the star and its planet orbit their mutual center of mass. The star’s orbital period is the same as that of its planet, and the star’s orbital speed depends on the planet’s distance and mass. Any additional planets around the star will produce additional features in the star’s orbital motion.

The Doppler Method

As a star moves alternatively toward and away from us around the center of mass, we can detect its motion by observing alternating Doppler shifts in the star’s spectrum: a blueshift as the star approaches and a redshift as it recedes.

The Astrometric Method

A star’s orbit around the center of mass leads to tiny changes in the star’s position in the sky. The GAIA mission is expected to discover many new planets with this method.

The Transit Method

If a planet’s orbital plane happens to lie along our line of sight, the planet will transit in front of its star once each orbit, causing a dip in the star’s visible-light brightness. An eclipse may occur half an orbit later, during which the system’s infrared brightness will decline because the planet’s contribution is blocked by the star.

Direct Detection

In principle, the best way to learn about an extrasolar planet is to observe directly either the visible starlight it reflects or the infrared light it emits. The astrometric and Doppler methods tell us about planetary mass, while the transit method tells us planetary size.

A transit is when a planet crosses in front of a star. This reduces the star's apparent brightness and tells us the planet's radius. Sometimes an eclipse – the planet passing behind the star – can also be detected.