Extrasolar Planets

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This artist concept depicts "multiple-transiting planet systems," which are stars with more than one planet. The planets eclipse, or transit, their host stars from the vantage point of the observer. This angle is called edge-on. NASA’s Kepler Space Telescope has found hundreds of these multiple-planet systems.Credit: NASA

Multiple-transiting Planet Systems This artist concept depicts "multiple-transiting planet systems," which are stars with more than one planet. The planets eclipse, or transit, their host stars from the vantage point of the observer. This angle is called edge-on. NASA's Kepler Space Telescope has found hundreds of these multiple-planet systems.

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

The artist’s concept depicts Kepler-62f, a super-Earth-size planet in the habitable zone of a star smaller and cooler than the sun, located about 1,200 light-years from Earth in the constellation Lyra.Credit: NASA Ames/JPL-Caltech

Kepler-62f. The artist's concept depicts Kepler-62f, a super-Earth-size planet in the habitable zone of a star smaller and cooler than the sun, located about 1,200 light-years from Earth in the constellation Lyra.
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

Beta Pictoris b. This artist’s impression shows how the planet inside the disc of Beta Pictoris may look. Only 12 million years old, or less than three-thousandths of the age of the Sun, Beta Pictoris is 75% more massive than our parent star. It is located about 60 light-years away towards the constellation of Pictor (the Painter) and is one of the best-known examples of a star surrounded by a dusty debris disc.Credit: ESO/L. Calçada

Beta Pictoris b. This artist’s impression shows how the planet inside the disc of Beta Pictoris may look. Only 12 million years old, or less than three-thousandths of the age of the Sun, Beta Pictoris is 75% more massive than our parent star. It is located about 60 light-years away towards the constellation of Pictor (the Painter) and is one of the best-known examples of a star surrounded by a dusty debris disc.
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.

Questions About Extrasolar Planets

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

    One of the posters from the Exoplanet Travel Bureau series on NASA’s PlanetQuest website. Like Luke Skywalker’s planet "Tatooine" in Star Wars, Kepler-16b orbits a pair of stars. Depicted here as a terrestrial planet, Kepler-16b might also be a gas giant like Saturn. Prospects for life on this unusual world aren’t good, as it has a temperature similar to that of dry ice. But the discovery indicates that the movie’s iconic double-sunset is anything but science fictionCredit: Courtesy NASA/JPL-Caltech

    Relax on Kepler-16b One of the posters from the Exoplanet Travel Bureau series on NASA's PlanetQuest website.

    • 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.
    • Exoplanet.org: The California and Carnegie Planet Search, a powerhouse in the field of extrasolar planet discovery.

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