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14.15: Exoplanets

Difficulty Level: At Grade Created by: CK-12
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Is there life on other planets?

Newly discovered planet Kepler-22b is the habitable zone of a sun-like star. This means that the planet could have liquid water, which is necessary for life on Earth. Even though this is the most Earth-like planet yet, the chances that it harbors life are slim. But there are likely many more Earth-like planets in the universe.

Extrasolar Planets or Exoplanets

Since the early 1990s, astronomers have discovered other solar systems, with planets orbiting stars other than our own Sun. These are called "extrasolar planets" or simply exoplanets (see Figure below). Exoplanets are not in our solar system, but are found in other solar systems.

The extrasolar planet Fomalhaut is surrounded by a large disk of gas. The disk is not centered on the planet, suggesting that another planet may be pulling on the gas as well.

Some extrasolar planets have been directly imaged, but most have been discovered by indirect methods. One technique involves detecting the very slight motion of a star periodically moving toward and away from us along our line-of-sight (also known as a star’s "radial velocity"). This periodic motion can be attributed to the gravitational pull of a planet or, sometimes, another star orbiting the star.

An animation showing how the orbit of a smaller body, such as a planet or small star, can be identified by the effect it has on the orbit of a larger star is seen here from above: http://upload.wikimedia.org/wikipedia/commons/5/59/Orbit3.gif. This is in line with the plane of the system: http://en.wikipedia.org/wiki/File:Dopspec-inline.gif.

A planet may also be identified by measuring a star’s brightness over time. A temporary, periodic decrease in light emitted from a star can occur when a planet crosses in front of, or "transits," the star it is orbiting, momentarily blocking out some of the starlight.

More than 700 extrasolar planets have been identified and confirmed and the rate of discovery is increasing rapidly.

Extrasolar Planet from the ESA discusses extrasolar planets and particularly a planetary system very similar to our solar system (1g): http://www.youtube.com/watch?v=ouJahDONTWc (3:29).

An introduction to extrasolar planets from NASA is available at (1g): http://www.youtube.com/watch?feature=player_profilepage&v=oeeZCHDNTvQ (3:14).

According to NASA, a statistical analysis shows that the Milky Way galaxy contains 100 million planets. That's a lot of exoplanets!


Hundreds of exoplanets have now been discovered. To learn something about how planet hunters find these balls of rock they usually can't even see, watch this QUEST video at http://www.kqed.org/quest/television/the-planet-hunters.


  • Now that scientists know how to identify extrasolar planets, the numbers of confirmed examples are increasing rapidly.
  • An exoplanet may decrease a star's brightness as it passes in front of it.
  • The gravitational pull of a planet may be detected in the slight motion of a star.


Use this resource to answer the questions that follow.


1. How many total exoplanets have been found? How many have been confirmed?

2. What are the sizes of the exoplanets that have been found so far?

3. What are the challenges to finding exoplanets?

4. List the methods for detecting planets.

5. Why is direct imaging of exoplanets so difficult?


1. What is an exoplanet?

2. Where are explanets located? How are exoplanets discovered?

3. Why are scientists so interested in exoplanets?

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Short for extrasolar planet. A planet that exists in a solar system different from our own.

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Difficulty Level:
At Grade
Date Created:
Feb 24, 2012
Last Modified:
Aug 07, 2016
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