Aka: Doppler Spectroscopy, Doppler, Wobble
Gravity seems, to most of us, a fairly one-sided relationship. A big object pulls a smaller object towards it; an apple falls to the Earth, not the Earth to an apple (sorry, armchair philosophers). But gravitational pull is simply proportional to an object’s mass, and as such it actually works both ways; small objects also have their effect on larger ones, even if it’s not always easily measured. Such is the case in our solar system.
While the Earth orbits the much larger Sun, the Sun itself has a small orbit around the system’s center of mass, one driven by the pull of the planets orbiting it. The Earth’s gravity is one part of this cumulative displacement; our planet’s pull causes the sun to wobble at a speed of .1 meters per second.
(Cosmic Journeys, The Search for Earthlike Planets)
Tracing the effects of this wobble in order to find the presence of an exoplanet is the principle behind the
Radial Velocity technique, or
Doppler spectroscopy.
As a star rotates toward and away from its observer, the light it emits will shift from the blue to red ends of the spectrum, respectively, due to changing frequencies. Doppler spectroscopy allows us to view these color shifts, which reveal the wobbles and displacements in the speed at which the star is moving toward us—its radial velocity. Current instruments are able to observe variations of radial velocity down to about 1 m/s. This wobble, in turn, betrays the presence of an orbiting exoplanet.
But there’s more. On a similar principle, according to the
Rossiter-McLaughlin Effect, by noting the order and timing of these light shifts, scientists can deduce the direction and speed of the orbit.
(Ohta, The Rossiter-McLaughlin Effect)
Developed in the 70s by Bruce Campbell and Gordon Walker, and first tested atop a Hawaiian volcano in the 80s (as is everything worth testing), the radial velocity method has proven one of the most effective means of exoplanet detection. It is responsible for the first exoplanet discoveries (including Belerophon), and the majority of the several hundred discovered since.
(Erskine, Externally Dispersed Interferometry...)
Part of the method’s success lies in its applicability to stars with a wide range of characteristics, providing certain details which other methods do not; it’s especially useful for telling us about a planet’s mass and its orbit. Radial Velocity and Transit together make up the majority of exoplanet findings (and the most commonly cited in news stories); however, other more exotic methods do exist, and can assist in painting a fuller picture of the alien world.
Next in the Planet Hunting series:
From pulsars to star-quakes, the next part of our planet hunting series will cover some of the more exotic methods of exoplanet detection...
