D10 Exoplanet Detection: Methods

Topic

There are four complementary methods for detecting planets outside the Solar System, each sensitive to a different physical effect that the planet exerts on its surroundings.

Photometric transit detects the periodic dimming caused by a planet passing in front of its star; the depth of this dimming increases with the planet's size relative to the star's, meaning that large planets orbiting small stars produce transits that are much easier to detect than those of small planets orbiting large stars, while the periodicity of the dimming also reveals the planet's full orbit.

Radial velocity, on the other hand, detects the wobble a planet induces in its star; the amplitude of this wobble increases with the planet's mass but decreases as the star's mass increases and as the planet's orbital distance from the star grows.

Astrometry—already described as a separate method—can detect this same planet-induced wobble, but it does so by observing a shift in position in the sky rather than measuring velocity.

Direct imaging suppresses the host star's overwhelming glare to reveal the planet itself; this method is viable only for young, still-hot planets orbiting far from their star.

A system's habitable zone is the orbital ring—determined by the star's luminosity—within which a planet's equilibrium temperature would allow liquid water to exist on its surface. Thus, from the four complementary methods that open this document to the habitable zone that concludes it—encompassing photometric transit, radial velocity, and direct imaging—an extrasolar planet betrays its presence (usually invisible to the naked eye) in four distinct ways to anyone who knows where to look.

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