How Planet Hunting Works

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.

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.

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.

Kepler is the first NASA mission capable of finding Earth-sized planets around other stars. Its main goal is to generate a base estimate, or census, of the number of such planets orbiting within habitable zones, where conditions are right for liquid water to exist.

The instrument package doesn't orbit the Earth in a satellite: It's housed within a spacecraft 9 feet (2.7 meters) in diameter and 15.3 feet (4.7 meters) high that orbits the sun, trailing our home planet.

Kepler uses a very wide field telescope and a photometer (light meter) to measure brightness variations in more than 156,000 stars simultaneously [source: Ames Research Center, NASA Finds Earth-size Planet Candidates]. It takes these readings every 30 minutes because transits can require from an hour to half a day, depending on the planet's orbit and the type of star involved.

Mission scientists also employ spectroscopic data from ground-based observatories to help confirm planet candidates and use stellar observations to remove other confounding factors, such as binary stars (a pair of stars that revolve around a common center of mass).

The Cygnus-Lyra neighborhood was chosen as the study area because it's well-populated with stars and lies high enough above the Earth's orbital plane that the sun, Earth and moon won't get in the way of Kepler's observations. The stars are between 600 and 3,000 light-years away. From our perspective, they cover an area equivalent to 1/400 of the sky [source: Harwood].

Kepler detects planets via the photometric or transit method, which means that it detects the small drop-off in a star's brightness that occurs when an orbiting planet passes between its star and us. Once the data analysis identifies a dimming event, scientists look for further dips of the same magnitude, duration and period to confirm the planet's existence.

This is no mean feat: An Earth-sized planet crossing in front of a sun-sized star dims its light by a mere 0.01 percent. NASA folks like to say that detecting such a tiny dip is like spotting a flea crawling across a headlight from several miles away. Jupiter-sized planets cast a bigger shadow. Even so, viewed from outside our solar system, Jupiter's transit only diminishes our sun's brightness by 1 to 2 percent [source: Ames Research Center, FAQ].

There's more. For the transit method to work, a planet must pass almost perfectly along our line of sight, the chances of which are around 0.5 percent for an Earth-sized planet (in an Earth-sized orbit) and 10 percent for a Jupiter-sized planet (if it orbits near its star) [source: Ames Research Center, FAQ].

To put it another way: Even if we checked out 100,000 stars that actually had Earthlike planets, we would only be able to "see" 500 of them via the transit method. Using probabilities like these, scientists can estimate the planet population of our galaxy from Kepler's observations.

Before Kepler came along, the stable of distant planets located by astronomers numbered in the tens and hundreds, not thousands. Nevertheless, this was an extraordinary number considering the limitations faced by scientists using available instruments -- particularly ground-based telescopes, which require researchers to compensate for atmospheric distortions.

Between 2005 and 2008, researchers discovered five super-Earths, each boasting masses between five and 10 times that of the Earth.

In 2008, astronomers using the Hubble Space Telescope's Near Infrared Camera and Multi-Object Spectrometer detected carbon dioxide on an exoplanet for the first time. The method involved subtracting the parent star's spectroscopic data from the combined data of star and planet. Unfortunately, the Jupiter-sized exoplanet HD 189733 b orbits too close to its star to be habitable, but the technique could provide valuable information if applied to other habitable candidates. Scientists are interested in carbon dioxide because it, like methane, can point to biological processes.

In 2009, astronomers reported the first exoplanet ever found through astrometry, adding it to the list of 350 planets previously found by the Doppler shift method. Had it been confirmed, VB 10b would have tipped the scales at six times more massive than Jupiter. However, subsequent Doppler spectroscopy observations failed to detect the expected radial velocity shifts in its parent star, VB 10, and the claim was refuted [source: Bean].

That same year, using six months of observations from ground-based amateur-style telescopes, scientists announced GJ 1214b, a planet 6.5 times more massive than Earth and 2.7 times wider. Researchers believe that the planet might be made mostly of water. GJ 1214b orbits a red dwarf star more than 40 light-years from Earth at a distance equivalent to one-fortieth the space between Mercury and our sun.

What discoveries were made in 2010 and 2011?

In March 2010, researchers announced another milestone: a Jupiter-like planet 1,500 light-years from Earth that was relatively cool and that could be studied in detail. Because the COROT satellite discovered it, it was dubbed COROT-9b. Previous work had already found other cool planets, but COROT-9b was the first that transits between its star and Earth. This meant that scientists could study both its size (from the amount it diminished the light of its parent star) and its atmospheric composition (from the way starlight interacted with it as it passed through its atmosphere) [source: ESA].

COROT-9b lies in its star's habitable zone but, because it's a gaseous world, scientists don't consider it likely to be hospitable to life. Its atmosphere might contain water, however, and such a large planet could also sport a habitable moon [source: ESA].

In late September 2010, a group of astronomers in the United States using spectroscopic data from ground-based instruments announced the discovery of a potentially hospitable planet, Gliese 581g, orbiting the star Gliese 581 just 20 light-years away. The announcement sparked widespread excitement because the planet was found so close to Earth, and only 15 years after astronomers identified the first exoplanets. Soon after the announcement, however, scientific groups began raising doubts about the discovery [source: Wall].

Researchers had already found evidence for other planets in the same red dwarf system, two of which (Gliese 581d and Gliese 581e) orbited on the fringes of the habitable zone. So, which of Gliese 581's children would take the crown as the best candidate yet found for supporting life? The issue was too complicated to resolve easily. Detecting planets spectroscopically requires toning down the noise inherent in observational data and then determining which assumptions to use. The same data can argue for different numbers of planets depending on whether you assume eccentric (highly elliptical) orbits or nearly circular ones. Scientists had yet to reach a consensus at the time this article was written.

January 2011 saw the Kepler mission confirm finding its first rocky planet, estimated at 1.4 times the size of Earth. Located well outside the habitable zone, Kepler-10b stands out as the smallest planet discovered outside our solar system so far.

And in February 2011, Kepler scientists announced the discovery of five planets, each orbiting in the habitable zones of stars smaller and cooler than our sun. If confirmed, these will represent the first planets of Earthlike size found in habitable zones.That same month, Kepler located six confirmed planets orbiting a sunlike star, Kepler-11, 2,000 light-years from Earth. This constitutes the largest group of transiting planets orbiting a single star ever discovered outside our solar system [source: NASA].

While those discoveries have been momentous, it's important to remember that Kepler so far has only searched a small fraction of the known universe. It may well be that in the years ahead, scientists will make even more amazing finds -- including, perhaps, an Earthlike planet that's home to living things.