7 Ways to Discover Alien Planets

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Finding Alien Worlds

Scientists have discovered nearly 2,000 alien planets since spotting the first ones orbiting a star beyond our sun back in 1992. Here's a brief rundown of the main techniques they use to find these far-flung worlds.

FIRST STOP: The Transit Method

Old Faithful: The Transit Method

Photo Credit: NASA/Ames/JPL-Caltech

Old Faithful: The Transit Method

This technique watches for the tiny, telltale dips in a star's brightness caused when a planet crosses (or transits) the star's face, blocking some of its light. NASA's Kepler spacecraft has employed this method to great effect, spotting more than 2,700 potential planets since its March 2009 launch.

Astronomers also look for variations in the timing of a particular planet's transit, because these can reveal the presence of additional worlds orbiting the same star.

NEXT: Wobbling Stars

Wobbling Stars: Radial Velocity

Photo Credit: ESO/M. Kornmesser

Wobbling Stars: Radial Velocity

The radial velocity method picks up on the tiny wobbles an orbiting planet induces in its parent's star's motion toward or away from Earth. This technique is also known as the Doppler method because it measures shifts in the star's light caused by these gravitational tugs.

Several teams of astronomers have discovered many exoplanets using this method and such Earth-based instruments as the HARPS spectrograph, on a telescope at the European Southern Observatory's La Silla Observatory in Chile, and the HIRES spectrograph, on Hawaii's Keck telescope.

NEXT: Gravitational Microlensing

Gravitational Microlensing

Photo Credit: NASA/JPL-Caltech/R. Hurt

Gravitational Microlensing

In gravitational microlensing, astronomers watch what happens when a massive object passes in front of a star from our perspective on Earth. The nearby object's gravitational field bends and magnifies the light from the distant star, acting like a lens.

This produces a light curve — a brightening and fading of the faraway star's light over time — whose characteristics tell astronomers a lot about the foreground object, which is often a star. If this star has any planets, these can generate secondary light curves, alerting researchers to their presence.

Gravitational microlensing is less biased toward planets that orbit relatively far from their stars than the transit or radial velocity methods. Researchers have even used it to find so-called "rogue planets," which cruise through the depths of space without a parent star.

NEXT: Direct Imaging

Say Cheese! Direct Imaging

Photo Credit: NASA/ESA/T. Currie, U. Toronto