A planetary system comprises of various non-stellar objects orbiting a star such as planets, moons, asteroids, meteoroids, comets, and cosmic dust. Collectively, one or more stars and their planetary systems form a star system. Our own planetary system, which contains Earth, together with the Sun build up a unique star system: the solar system.
Origin and evolution of planetary systems
Planetary systems around sun-like stars are generally believed to form as part of the same process which results in star formation. Some early theories involved another star passing extremely close to the sun, and drawing material out from it which then coalesced to form the planets. However, the probability of such a near collision is now known to be far too low to make this a viable model. Accepted theories today argue that planetary systems form from a solar nebula.
Some planetary systems are very unlike our own, however: planetary systems around pulsars have been inferred from slight variations in the period of the pulses of electromagnetic radiation. Pulsars are formed in violent supernova explosions, and a normal planetary system could not possibly survive such a blast - planets would either evaporate, or the sudden loss of most of the mass of the central star would see them escape the gravitational hold of the star. One theory is that existing stellar companions were almost entirely evaporated by the supernova blast, leaving behind planet-sized bodies. Alternatively, planets may somehow form in the accretion disk surrounding pulsars.
Significant planetary systems
- Solar System - The Sun and its planetary system, the first such systems discovered
- PSR 1257+12 - the first extrasolar planetary system discovered, the first pulsar planetary system discovered, the first multi exoplanet system discovered, the first multi planet system with a pulsar discovered
- Upsilon Andromedae - the first multiplanet extrasolar planetary system discovered around a main sequence star, found to be so in April 1999
- PSR B1620-26 - the first multistar planetary system discovered
- 55 Cancri - the largest extra solar planetary system discovered (4 planets, as of August 2004, along with a distant stellar companion) 
- Gliese 876 - the first system around a red dwarf star and the first discovered to be in an orbital resonance
- HD 69830 - found to have three Neptune-mass planets and an asteroid belt, all within 1 AU  
- 2M1207 - the first imaged system and the first brown dwarf system with a planet discovered 
- Cha 110913 - the first substellar/planetary system discovered 
- ↑ Wired News. "Found: Solar System Like Our Own", 2002-06-13.
- ↑ Whitney Clavin. "NASA's Spitzer Telescope Sees Signs of Alien Asteroid Belt", 2005-04-20.
- ↑ Christophe Lovis (2006-05-18). "Trio of Neptunes and their Belt".
- ↑ Robert Roy Britt. "Likely First Photo of Planet Beyond the Solar System", 2004-09-10.
- ↑ "Planet-sized Brown Dwarf May Yield Miniature Solar System", 2005-11-29.
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