HOW EXOPLANETS ARE FOUND

DIRECT IMAGING

The most obvious way to find an exoplanet is to see it using a telescope. This isn’t as easy as it sounds, because distant planets are so faint they are usually lost in the bright glare of their host stars. Nevertheless, this approach has been successful in some cases.

TRANSIT

The majority of exoplanets have been found using this method, which involves looking for a slight dimming of the host star as the planet passes in front of it. However, this can only be used if the geometry of the planet’s orbit allows us to see the transit from Earth.

ASTROMETRY

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As a planet orbits a star, it causes the star to wobble slightly. In principle, using a precision measuring technique called astrometry, this wobble can be observed directly. It was the first exoplanet detection method to be tried, but has only been successful in a few cases.

RADIAL VELOCITY

In practice, it’s much easier to detect the tiny wobble caused by a planet by looking at a star’s velocity rather than its position in the sky. This can be done using spectroscopy. The resulting ‘radial velocity’ method has been very successful.

GRAVITATIONAL MICROLENSING

This method requires Einstein’s general theory of relativity. This shows how light from a distant object is bent as a star passes between it and the observer. The pattern of bending is slightly different if there’s a planet in orbit.