What's an Exoplanet?

- Describe how astronomers obtain evidence for the existence of exoplanets (including astrometry, transit observations and use of Doppler-shifts)
- Discuss the difficulties associated with the detection of individual planets

Although planets orbiting other stars (exoplanets, sometimes called extra solar planets) had long been theorised, there was no evidence for their existence until the mid-1990s.

Since then several hundred have been discovered and confirmed. They are normally described as ‘Hot Jupiters’, large bodies orbiting very closely to their parent star. More of these bodies have been discovered because they are easier to detect.

At least 2 exoplanets, Gliese 581c and d lie within a habitable zone where water can exist as a liquid. Exoplanet MOA-2007-BLG-192Lb w is also notable as it has a mass of 3.3 of Earth. These are known as ‘Super-Earths’.

Detecting exoplanets is difficult for a number of reasons:

  • They orbit stars and the brightness of the star may prevent us from finding them.
  • Planets may orbit too far away from our line of sight for some methods to detect them e.g. when their ecliptic is too eccentric for them to be noticed.


Different methods exist to detect them:

By measuring the position of a star very accurately, any minute wobble in the stars position can be due to the tiny pull of a planet on the parent star.  

Transit Methods
As a planet moves in front of a star minute changes in light occur. Scientist can then work out the size and orbit of a planet.

Radial velocity
Changes in the movement of a star towards and away from Earth can be detected using the Doppler effect.


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Finding Exoplanets

Fomalhaut - Exoplanets discovered Life

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Planet Quest Ways to find an exoplanet

Kepler - NASA Space Observatory detecting Exoplanets



- Describe different methods used to detect exoplanets
- Why are sceintists interested in discovering Earth-like planets elsewhere?


Research exoplanets and discover some of their unusual properties.