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Even though it seems likely that the galaxy is teeming with exoplanets, finding them isn't easy. Planets are millions of times dimmer than the stars they orbit and incredibly distant.

The challenges of observing extrasolar planets stem from three basic facts:

  • Planets don't produce any light of their own, except when young.
  • They are an enormous distance from us.
  • They are lost in the blinding glare of their parent stars.

For example, if there were a planet orbiting Proxima Centauri, the nearest star, it would be 7,000 times more distant than Pluto. Trying to observe this planet would be like standing in Boston and looking for a moth near a spotlight in San Diego.

The following is an overview of some of the planet detection methods that have thus far proved successful, as well as other methods currently in development.

Doppler Shift

Precise measurement of the velocity or change of position of stars tells us the extent of the star's movement induced by a planet's gravitational tug. From that information, scientists can deduce the planet's mass and orbit.

Why does a planet cause a star to sway? If a star has a single companion, both move in nearly circular orbits around their common center of mass. Even if one body is much smaller, the laws of physics dictate that both will orbit the center of the combined star and planet system. The center of mass is the point at which the two bodies balance each other.

Diagram showing how the doppler shift method finds planets.

The radial velocity method measures slight changes in a star's velocity as the star and the planet move about their common center of mass. In this case, however, the motion detected is toward the observer and away from the observer. Astronomers can detect these variances by analyzing the spectrum of starlight. In an effect known as Doppler shift, light waves from a star moving toward us are shifted toward the blue end of the spectrum. If the star is moving away, the light waves shift toward the red end of the spectrum.

This happens because the waves become compressed when the star is approaching the observer and spread out when the star is receding. The effect is similar to the change in pitch we hear in a train's whistle as it approaches and passes.

The larger the planet and the closer it is to the host star, the faster the star moves about the center of mass, causing a larger color shift in the spectrum of starlight. That's why many of the first planets discovered are Jupiter-class (300 times as massive as Earth), with orbits very close to their parent stars.

Astrometric Measurement

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Astrometric displacement of the Sun due to Jupiter as at it would be observed from 10 parsecs, or about 33 light-years.

As with the radial velocity technique, this methods depends on the slight motion of the star caused by the orbiting planet. In this case, however, astronomers are searching for the tiny displacements of the stars on the sky.

The planets of our solar system have this effect on the Sun, producing a to-and-fro motion that could be detected by an observer positioned several light years away. Astrometric instruments precisely measure the position of stars as compared to other stars around them, and are thus able to detect any movements in the star's position due to the "wobbling" caused by an orbiting exoplanet.

Transit Method

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If a planet passes directly between a star and an observer's line of sight, it blocks out a tiny portion of the star's light, thus reducing its apparent brightness.

Sensitive instruments can detect this periodic dip in brightness. From the period and depth of the transits, the orbit and size of the planetary companions can be calculated. Smaller planets will produce a smaller effect, and vice-versa. A terrestrial planet in an Earth-like orbit, for example, would produce a minute dip in stellar brightness that would last just a few hours.

Missions that use the transit method, such as the Kepler and CoRoT spacecraft, are able to monitor large numbers of stars at once for the dimming caused by a transit. The Kepler mission has discovered more than 1,000 potential exoplanets using this method.

Direct Imaging

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This Hubble Space Telescope image was one of the first direct images of a planet ever made.

Taking actual pictures of exoplanets is extremely difficult due to how much brighter a star is than its planet. However, specialized optics and clever observation methods have made a handful of exoplanet images possible, with the potential for many more to be made in the future.

One method of direct imaging, coronography, uses a special masking device to block out the light of a star so that the planets orbiting it can be seen more clearly. In space, this masking device could take the appearance of a giant starshade, precisely positioned in space in between a nearby telescope and the star that telescope is searching for exoplanets.

Another method of direct imaging, interferometry, uses specialized optics to combine light from multiple telescopes in such a way that the light waves from the star cancel each other out, leaving behind the light from any exoplanets that may be present. The Large Binocular Telescope Interferometer and Keck Interferometer both use this method to search for exoplanets.

Gravitational Microlensing

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This method derives from one of the insights of Einstein's theory of general relativity: gravity bends space. We normally think of light as traveling in a straight line, but light rays become bent when passing through space that is warped by the presence of a massive object such as a star. This effect has been proven by observations of the Sun's gravitational effect on starlight.

When a planet happens to pass in front of a star along our line of sight, the planet's gravity will behave like a lens. This focuses the light rays and causes a temporary sharp increase in brightness and change of the apparent position of the star.

Astronomers can use the gravitational microlensing effect to find objects that emit no light or are otherwise undetectable.