How Do We Find Exoplanets?
image of Beta Pictoris, dust disc, and Beta Pictoris b courtesy the Europan Southern Observatory
Astronomers tried for years to discover evidence that worlds existed around stars other than our Sun. By the mid 1990's, technology finally caught up with desire for discovery and lead to the first discovery of a planet orbiting another sun-like star, Pegasi 51b. Why did it take so long to discover these distant worlds, and what techniques do astronomers use to find them?
Even as far back as the late 19th century, a few astronomers thought that they had spotted evidence of planets orbiting other distant stars. The problem with their observations and data was that other astronomers were unable to duplicate their results. Since confirmation of such discoveries requires a different individual or team to be able to duplicate their results, these "exoplanets" were found to be the result of inconclusive data or data analysis. it is only recently that our equipment and data analysis methods have become advanced enough for multiple teams of astronomers, working from observatories around the world, to be able to independently confirm discoveries of exoplanets.
Planets, unlike stars, do not generate their own visible light, making them hard to spot against the extremely bright light of their host star. They only reflect the light that the star shines on them. In addition, planets are very small - much smaller than their host stars. However, astronomers have come up some clever methods to tease planets out of hiding.
Some of the surprisingly many methods that have been successfully used to find extrasolar planets are: transits, radial velocity, astrometry, direct imaging, and even microlensing. How do astronomers use these techniques to find planets?
Graph illustrating the results from various exoplanet detection methods
Graph illustrating the results from various exoplanet detection methods, created by WIkimedia user Aldaron.
Color Key:  
radial velocity (dark blue)
  transit (green)
  timing (dark yellow)
  direct imaging (dark red)
  microlensing (dark orange)

Image of a transit
One of the most famous exoplanet detection methods is the transit method, used by Kepler and other observatories. When a planet crosses in front of its host star, the light from the star dips very slightly in brightness. By repeatedly detecting these incredibly tiny dips in brightness scientists can confirm that a planet is in orbit around a star. The instruments needed to pick up this tiny blip are extraordinarily sensitive. If you can imagine trying to detect the dip in light from a massive searchlight when a tiny ant crosses it, a a distance of tens of miles away you can begin to see how difficult it can be to spot a planet from light-years away! An other drawback to the transit method is that the distant solar system must be at a favorable angle to our own point of view here on Earth-if the distant system's angle is just slightly askew, there will be no transits! Even in our own system, a transit is very rare. For example there were two transits of Venus visible across our Sun from Earth in this century. But the next time Venus transit the Sun as seen from Earth will be more than a century from now, even though it will have completed hundred of orbits!
Image of Radial Velocity method
Spotting the Doppler shift of a star's spectra is another handy exoplanet detection technique and it was the technique used to find the Pegasi 51b, the first planet detected around a Sun-like star. Until the launch of Kepler it was the method that had found the most planets. This technique is called the radial velocity or "wobble" method. Astronomers split up the visible light emitted by a star into a rainbow. This is called the spectra of the star, and gaps in between the normally smooth bands of light help determine the elements that make up the star. However, if there is a planet orbiting the star, it causes the star to wobble ever so slightly back and forth. This will in turn cause the lines within the spectra to shift ever so slightly towards the blue and red ends of the spectrum as the star wobbles slightly away and towards us. This is caused by the blue and red shifts of the planet's light. (insert link to "What is the blue and red shift?") By carefully measuring the amount of shift in the star's spectra astronomers can determine the size of the object puling on the host star and if the companion is indeed a planet. By tracking the variation in this periodic shift of the spectra they can also determine the time it takes the planet to orbit it's parent star.
Image of the astrometry exoplanet detection method in action
Image courtesy Rich Townsend/University College London
Astrometry is the science and technique of very precisely measuring the positions of stars. With some of our nearest stars it is possible to detect the presence of a planet by taking in the very tiny shifts in the position of a star compared to the stars around it. If it shifts back and forth in a tiny, repeatable fashion, then that star must have a companion in orbit around it, tugging the star just enough to be detectable. This technique requires a very sensitive telescope, and so far very few planets have been detected this way - though the Hubble Space Telescope did prove that this method could work by confirming the discovery of a planet around Gliese 876 in 2002. The upcoming GAIA space telescope is expected to use this method to find thousands of exoplanets using this method. 
Microlensing is technique that uses the power of gravity to bend light to help reveal exoplanets and other companions around stars. If a star is aligned in front of another star, it will give a temporary boost to that stars light-and if a planet is also present and aligned, its own gravity will boost the light further, in a slight but detectable way. Approximately a dozen planets have been found via this method.
Image if three exoplanets orbiting HR 8799
Three exoplanets orbtiing HR 8799. Credit: Gemini Observatory/NRC/AURA/Christian Marois, et al.
Finally, exoplanets can be revealed by directly imaging them, such as in our image of Beta Pictoris b found at the top of this article, or this image of three planets found orbiting the star HR 8799! It is still very rare to be able to directly image an exoplanet but there have been a few planets who have been imaged by our most advanced telescopes. These telescopes use an instrument called a coronograph to block the bright light from the host star to capture the dim light from planets. Hubble Space Telescope has captured images of giant planets orbiting a few nearby systems. Ground based observatories such as the Keck, Hale, and the Gemini observatories have captured images of exoplanets as well. Due to the difficulties in using this method only about a dozen planets have been imaged. However, the upcoming James Webb Space Telescope should boost this number significantly, as well being powerful enough to gather spectral information from these exoplanet atmospheres. 
These methods continue to be refined and improved upon as our understanding and instrumentation improves. Upcoming space missions such as TESS and the James Webb Space Telescope, as well as new, powerful ground-based telescopes like the Thirty Meter Telescope, giant Magellan Telescope, and European Extremely Large Telescope promise to unlock a torrent of new information about these distant worlds. These missions will not just find more exoplanets, but they will also discover more details about these planets: the colors and spectra of their atmospheres, maps of their weather and temperatures, detecting possible exomoons, and even scanning their atmospheres for signs of life! 
image of TESS mission looking for exoplanets
Artists concept of the upcoming  TESS mission looking for planets around other stars.
Image  courtesy MIT/TESS team
There is even more information on exoplanets, activities, and more on the NASA PlanetQuest website and Kepler's official site, usch as the interactive series of exoplanet activites and some of the latest exoplanet news and articles. NASA's Night Sky Network has further resources available in our Outreach Resources section, including a clever demo on how astronomers use the wobble method to detect planets!

The future of exoplanet discovery is only just beginning, and promises rich rewards in humanity's understanding of our place in the universe, where we are from, and if there is life elsewhere in our cosmos.

Last Updated: July 2015
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