How Planet Hunting Works

Planet Hunting Techniques and Technology

Hunting for planets outside our solar system is a little like trying to read a postage stamp stuck to a distant lighthouse's lamp: Parent stars shine so brightly that their glare drowns out everything else. To compensate, scientists have devised ingenious methods to detect exoplanets by measuring their effects on their parent stars.

A planet influences its star in two useful ways. First, its gravity tugs the star slightly to and fro as the planet orbits it. Second, the planet blocks a small amount of light as it passes in front of the star (from our point of view).

We can detect these effects using a few handy methods, each with its own strengths and weaknesses. Let's tackle astrometry first. As an orbiting planet's gravity tugs on its parent star, it causes the star to wobble in its path across the sky. We can discern this minuscule motion by precisely measuring the star's position. Based on the period, or time the star takes to complete a wobble, we can calculate the period and radius of the planet's orbit, along with the planet's mass. Astrometry is best at finding massive planets with orbits far from their suns.

Doppler spectroscopy also makes use of this gravitational push and pull, but whereas astrometry uses the relative side-to-side motion of the star, this method uses the Doppler shift that results from the planet pulling its star toward Earth, then away from it. As the star moves toward the Earth, its light is compressed, or "blue-shifted," toward the shorter wavelengths of the spectrum. As it travels away from us, we see the light waves stretch out toward the red (longer-wavelength) end of the spectrum. By measuring a star's spectrum over time, we can detect Doppler shifts caused by a planet or planets moving the star toward and away from us.

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Doppler shifts also tell us the star's radial velocity (how fast the star moves toward and away from us). As you might expect, larger radial velocities mean bigger planets. Based on the star's mass and the period of the shift, we can also calculate the planet's orbital radius. This method is best suited for detecting massive planets located near their parent star, and it can only estimate the minimum mass of such planets.

Photometry doesn't look for wobbles or shifts. Instead, it watches for the telltale dimming of a star's brightness that results when an orbiting exoplanet transits, or passes between it and us.

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Combining the three methods allows astronomers to develop a much clearer picture of these planets. Next, we'll explore how the Kepler mission is using photometry to perform a stellar census of potentially habitable planets.