terrestrial planet finder
•  a Space telescope to find planets outside from our solar-system as small as Earth • 

 Main Page
 Mission Goal
 Project Status



Extrasolar Planets :

An extrasolar planet, or exoplanet, is a planet outside our Solar System. To date, Januarry 30, 2011 there have been 519 extrasolar planet discoveries. There are at least several hundreds of planet candidates awaiting to be investigated and confirmed as an exoplanet. Most of the exoplanets have been detected through radial velocity observations and other indirect methods rather than actual viewing or imaging of the objects. The planets detected so far are mostly giant planets resembling our Jupiter, since our current technology excludes detecting smaller planets than Jupiter. Altough there have been several discoveries of a "lightweight" extrasolar planets, only a few times more massive than our Earth, science shows that in the future, as our technology continues to develop, we will see more discoveries of smaller earth sized planets. It is now known, that a significant part of the stars have their own planetary systems, at least around 10%(truely this number can speculated to be much larger) of sun-like stars. This means, that there must be billions of planets in our own galaxy alone. There are also brown dwarfs that have planets orbiting around them, and also planets that are free-floating is space.

an artist's vision of an earth like planet with a moon

Extrasolar planets became an object of scientific investigation at the end of the nineteenth century. Many astronomers supposed that they existed, but there was no way of knowing how common they were or how similar they might be to the planets of our solar system. The first confirmed detection was made in 1992, with the discovery of several terrestrial-mass planets orbiting the pulsar PSR B1257+12. The first confirmed detection of an exoplanet orbiting a main-sequence star was made in 1995, when a giant planet, 51 Pegasi b, was found in a four-day orbit around the nearby G-type star 51 Pegasi. Since then, the detection frequency of extrasolar planets have increased every year.

PSR B1257+12
In 1992, Aleksander Wolszczan and Dale Frail found that the pulsar (PSR B1257+12) has two planets. These were the first extrasolar planets ever discovered orbiting a pulsar, they amazed many astronomers who expected to find planets only around main sequence stars.
click for a bigger resolution

How many stars are there with planets?
Planet-search programs have found planets orbiting a significant fraction of the stars they have looked at. However, the total fraction of stars with planets is uncertain because of observational selection effects. The radial-velocity method and the transit method (which are responsible for the vast majority of detections to-date) are most sensitive to big planets on small orbits. For that reason, many known exoplanets are "hot Jupiters": planets of approximately Jupiter mass on very small orbits with periods of only a few days. It is now known that 1% to 1.5% of sunlike stars possess such a planet, where "sunlike star" refers to any main-sequence star of spectral classes F, G, or K without a close stellar companion. It is further estimated that 3% to 4.5% of sunlike stars possess a giant planet with an orbital period of 100 days or less, where "giant planet" means a planet of at least 30 of Earth masses. The part of stars with smaller or more distant planets remains difficult to estimate. Extrapolation does suggest that small planets (of about Earth-like mass) are more common than giant planets. It also appears that planets on large orbits may be more common than ones on small orbits. Based on such extrapolation, it is estimated that perhaps 20% of sunlike stars have at least one giant planet while at least 40% may have planets of lower mass and size. Regardless of the exact fraction of stars with planets, the total number of extrasolar planet must be very large. Since our own Milky Way Galaxy has at least 200 billion stars, it must also contain billions of planets if not hundreds of billions of them.

earth sized planet
another artist's concept of a earth sized exoplanet.
click for a bigger resolution

  • Ton's of links to Exoplanet related sites and news: Planetas Extrasolares
  • The Extrasolar Planets Encyclopaedia: Exoplanet.eu

    back to topdisclaimerhome