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Artist's Concpetion of Extrasolar Planets
An artist's vision of another solar system. This is a painting that shows an artist's conception of what some recently discovered extrasolar planets and their sun might look like. Most of the planets that have been discovered so far are giant planets, close to their stars.

Extrasolar Planets

Extrasolar planets are planets that orbit stars other than our Sun. The first extrasolar planet discovered around a sunlike star was announced on October 6, 1995. Since then, astronomers have been discovering extrasolar planets at a dizzying rate, and the list of all the known extrasolar planets contains more than 500 new worlds!

Planets around Pulsars

The first extrasolar planets to be discovered in 1991 were (to most astronomers' amazement) around a pulsar! Why this amazed astronomers was that no one could imagine how a planet could get to be around a pulsar. Any planets around the original star should have been destroyed in the supernova explosion which created the pulsing neutron star. Since then various theories about how the planets could be there have been passed around. In any case, radiation from the neutron star would make this stellar system very hostile to life as we know it. At present we can only detect Earth sized planets if they are around pulsars.

Hot Jupiters

Many of the extrasolar planets known are so-called hot Jupiters. This just means that they are Jupiter sized planets very close in to the central star. The reason that so many of these types of planets have been found is that the method used to discover extrasolar planets is most sensitive to heavy planets close in to the central star. Most extrasolar planets cannot yet be directly imaged because they are very small compared to their stars. Instead astronomers "see" that they are around a star by the gravitational influence they have on that star, causing it to wobble ever so slightly around the centre of mass of the whole system. Since the gravitational force depends both on the mass of the object and how close it is, massive planets close to the star have the biggest effect and so are the easiest to detect.

Atmospheres of extrasolar planets

In November 2001, it was announced that the first chemicals had been detected in the atmosphere of an extrasolar planet. This was done by looking at the system as the planet transited (or passed in front of) the star. The light from the star will change by a tiny amount when part of it passes through the atmosphere of the planet and by making extremely sensitive observations this tiny change can be detected. The element that was detected was sodium, and from prior observation the planet is known to be about 70% the mass of Jupiter and so close to its star that its atmosphere is heated to 1100 Celsius (2000 Fahrenheit)! There are now over two dozen exoplanets for which we have detections of chemicals in their atmospheres.

Prospects for detecting Earth-like planets

NASA's Kepler mission is a space-mission launched in 2009 optimised for the discovery of Earth-like planets around other stars by looking for transits. The Kepler team has announced over 2,300 exoplanet candidates, including the confirmation of an Earth-sized planet in the habitable zone of a Sun-like star. Recent advances in optical interferometry (for example at the Keck Telescope in Hawaii) are also likely to help.

The Ask an Astronomer team's favorite links about Extrasolar Planets:

  • The Extrasolar Planets Encyclopaedia: The site to keep you up-to-date with the latest discoveries. It's quite technical in places and used by professional Astronomers, so this might not be the place to go for an introduction to the subject.
  • Planet Quest: Jet Propulsion Laboratory's site on the search for extrasolar planets, with a lot of background information as well as the latest news.
  • The California and Carnegie Planet Search, a powerhouse in the field of extrasolar planet discovery.

Previously asked questions about Extrasolar Planets:

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