Fact Sheet: How To Study Extrasolar Planets

Do Earth-like planets exist around nearby stars? If so, do they harbor life? Astronomers don't have the answers yet, but they're getting closer. Since the first discovery in 1995 of a planet orbiting a star outside our solar system, more than 140 extrasolar planets, some as small as Neptune, have been found.

Extrasolar planets are extremely difficult to spot for three main reasons: they are at immense distances from Earth; they are very faint; and they are overwhelmed by the blinding glare of their parent stars. To overcome these challenges, astronomers have developed a variety of indirect planet-hunting tools. The two most successful of these are described below, in addition to a method for the direct study of known extrasolar planets.

Wobble Technique

As a planet circles its star, its gravity tugs on the star a bit, wiggling the star back and forth. This rhythmic wobble, an indirect signature of a planet, can be seen by looking for changes in the wavelength of a star's light. As a wobbling star moves toward Earth, its light waves become squeezed together and shift to blue wavelengths; as it recedes from Earth, its light waves get stretched to red hues. The phenomenon, known as the Doppler shift, also explains why the pitch of a train's whistle changes as it comes and goes.

Various visible-light telescopes, both on the ground and in space, regularly search for these telltale wobbles. Once found, astronomers can measure a wobble's size and duration to estimate the mass and orbit of the unseen planet.

This planet-hunting approach, variously referred to as the "wobble," Doppler or radial velocity technique, was used to make the first discovery of an extrasolar planet, called 51 Pegasi b, in 1995. Since then, astronomers have employed the method to find more than 130 gas giant planets around other stars, including three as small as Neptune.

Another proposed planet-hunting tool called astrometric measurement would also take advantage of wobbling stars to search for planets. In this case, however, astronomers would precisely measure the positions and tiny displacements of stars in the sky.

NASA's SIM PlanetQuest mission, formerly known as the Space Interferometry Mission, will use this strategy to detect the presence of Earth-size planets orbiting nearby solar type stars. It is set to launch in 2011. Similarly, the Keck Interferometer will conduct an astrometric survey of hundreds of stars to search for planets as small as Uranus.

Transit Technique

Like a late moviegoer blocking a movie screen while trying to find a seat, a distant planet that happens to cross between us and its star will cause the star to slightly dim in brightness. This periodic dip can be detected by visible-light telescopes, both on the ground and in space. It not only reveals the presence of a planet, but also tells astronomers the planet's size. For example, a bigger planet will block more starlight in the same way that a hefty moviegoer would hide more screen.

This transit technique was first demonstrated in 1999 on an extrasolar planet, called HD 209458b, originally discovered via the wobble method earlier that year. Since then, the technique has led to the discovery of 6 extrasolar planets.

The transit technique applies to only those planets whose orbits we see edge-on, and whose paths take them between us and their suns. Large planets that stick closely to their stars, the so-called "hot Jupiters," are more likely to meet these criteria and be found with this method.

NASA's Kepler mission, set to launch in 2008, will use the transit technique to search for Earth-sized planets.

Secondary Eclipse Technique

This technique does not reveal never-before-seen extrasolar planets, but builds upon the transit method to directly study and quantify the light of known planets. It involves using an infrared-light telescope to watch a previously identified transiting planet cross not in front of but behind its parent star.

When a planet transits its star, it partially blocks the light of the star. When the planet continues on in its orbit behind the star, the star completely blocks its light. This "secondary eclipse" can be measured to determine exactly how much light is coming from just the planet.

The key to this approach is to observe a star system in infrared light. In visible light, the glare of a star can overwhelm its planetary companion and the little light the planet reflects by factors of tens of thousands or more. In infrared, a star shines less brightly and its planet gives off its own internal light, or heat radiation. As a result, the star may outshine the planet by a factor of only hundreds, making it possible to sift out the planet's light.

By observing these secondary eclipses at different infrared wavelengths, astronomers can obtain the planet's temperature, and, in the future, they may be able to pick out chemicals sprinkled throughout a planet's atmosphere. The technique also reveals whether a planet's orbit is elongated or circular.

So far, this strategy has been used successfully by only NASA's Spitzer Space Telescope. Future missions, like NASA's James Webb Space Telescope, will also be able to measure the light of extrasolar planets.

Eventually, astronomers will be able to directly image planets the size of Earth. NASA's Terrestrial Planet Finder coronagraph and interferometer missions, scheduled to launch in 2016 and 2019, respectively, will capture the first snapshots of worlds reminiscent of our own.

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