Cosmic Rulers
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Radial Velocity Technique
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Radial Velocity Technique

Astronomers have discovered more than 400 possible planets orbiting stars beyond our own solar system. Most of those discoveries were made with the radial velocity technique, in which the gravitational pull of an orbiting planet causes its parent star to “wiggle” a little bit. Astronomers detect this wiggle as a slight back-and-forth motion in the star’s spectrum.

Spectroscopy is one of the most important tools in astronomy. With this technique, astronomers split the light from a star or other astronomical object into its component wavelengths or colors, producing a spectrum.


Astronomers can detect the presence of a planet orbiting a star by measuring a back-and-forth shift in the star's light, known as the Doppler shift. This illustration shows an example. The strip in the middle shows the simulated spectrum of a star that is not moving toward or away from us. The black lines represent the chemical 'fingerprint' of key elements. At top, those lines have been shifted to the blue end of the spectrum because the star is moving toward us. At bottom, the lines are shifted to the red end of the spectrum because the star is moving toward us. The size of this shift reveals the mass of the orbiting planet, and the length of time required to shift from red to blue and back again reveals how long it takes the planet to complete one orbit. [Tim Jones]

Each chemical element produces an imprint on the spectrum, like the barcode on any product you buy at the supermarket. These chemical “fingerprints” reveal a star’s composition.

As the star moves toward or away from us, its light is shifted to shorter or longer wavelengths in a process called the Doppler effect.

In everyday life, examples of the Doppler effect include the shift in the pitch of an ambulance siren or a train whistle. When the sound moves toward you, it is compressed to shorter wavelengths, so the pitch goes up; when it moves away from you, it is stretched to longer wavelengths, so the pitch goes down.

When a star moves away from us, its light is stretched to longer wavelengths, which appear redder, so this effect is described as a “redshift.” And when the star moves toward us, it produces a “blueshift.” The shift shows up as a slight displacement in the wavelength of the star’s chemical elements.

As a companion planet orbits a star, its gravity pulls the star back and forth, producing a back-and-forth shift in the star’s spectrum. The size of the shift reveals how fast the star is moving in response to the planet’s pull. And the length of time required to complete one back-and-forth shift in the spectrum reveals the planet’s orbital period. Mathematical calculations then reveal the minimum mass of the orbiting planet.

So far, most of the planets discovered with this technique are both massive and close to their parent stars, so they exert a stronger gravitational tug. Over time, though, the technique will reveal less-massive planets in more-distant orbits — perhaps including planets like Earth.

But the radial-velocity technique doesn’t tell the whole story about a star and its companions.


While the radial velocity technique is quite effective at discovering companions, it cannot produce a precise measurement of a companion’s mass. It reveals a minimum mass, but the true number could be a lot bigger.

The problem is that the radial-velocity measurements alone don’t reveal a system’s geometry and its orientation in space. Without that information, it’s like watching only one end of a teeter-totter and trying to guess what’s on the other end. If you see one end plus the fulcrum, you can deduce what’s on the other end, even if you can’t see it. If you see an adult on the one visible end, for example, and he’s sitting close to the fulcrum, then you know there must be a child on the other end.

The team led by Fritz Benedict is nailing down the masses of the companions in several star systems by adding a second technique, called astrometry, which measures an object’s position in space. This technique basically fills out the view of the teeter-totter, letting astronomers know where the fulcrum is located (in this case, the center of mass for the visible star and its unseen companion) and helping them calculate what’s on the other end.

To do that, the team is using Hubble Space Telescope’s Fine Guidance Sensors to precisely measure the motions of stars.

Instead of measuring the star’s spectrum, this technique plots the star’s exact position in the sky as it moves in response to the gravitational tug of its companions. By measuring the size and shape of the wiggle, the astronomers can determine the orientation of a target system and measure the distance between the star and its unseen companion. Combining that information with radial velocity measurements made from the ground reveals the precise masses of both the target star and its companion.

Such observations are crucial because they confirm the suspected extrasolar planets, and they provide important details that theorists use to refine their models of how planetary systems form and evolve.

Sometimes, the observations show that a suspected planet isn’t a planet at all.

Consider a system known as HD 33636. Astronomers discovered a possible planet in the system several years ago and calculated that it was at least nine times as massive as Jupiter, the giant of our own solar system.

In 2006, though, the Benedict team looked at the system with HST. When combined with observations from telescopes on the ground, the astrometry technique revealed that the companion is more than 140 times as massive as Jupiter. An object that heavy isn’t a planet — it’s a star. So instead of a star with a big planet, HD 33636 is a binary star system.

Ongoing observations will provide more-accurate measurements of the masses of several possible planets, allowing astronomers to confirm the identity of these unseen companions.