The first couple were thought to be planetish, but as the total number climbed however, astronomers realized asteroid belt was a new organization of matter in the solar system. The Little Prince refers to all the asteroids as planets and worlds IIRC. <div class="Discussion_UserSignature"> <p><font color="#ff0000"><strong>TPTB went to Dallas and all I got was Plucked !!</strong></font></p><p><font color="#339966"><strong>So many people, so few recipes !!</strong></font></p><p><font color="#0000ff"><strong>Let's clean up this stinkhole !!</strong></font> </p> </div>
Actually, there is no agreement on this. There is a fuzzy border that defines planets and non-planets. <br /><br />Two things can be planets (according to my unique definition): <br />Any celestial object orbiting a star (first and foremost): which is larger than the smallest celestial body with atmospheric clouds. <br />Any celestial object orbiting a planet (first and foremost): which has clouds in a thick atmosphere (or a frozen or liquid atmosphere that would have clouds and precipitation and evaporation if heated to the proper temperature). <br /><br />Under these definitions, Triton and Titan are considered planets. The moon and Charon are not considered planets because both of them do not contain an cloudy atmosphere. Pluto is a planet because it is an icy object with a cloudy atmosphere. Sedna might not be a planet unless if it is either large enough to have an cloudy atmosphere or large enough to have moon. Eros and other asteriods can have moons but they cannot be planets because they are not large enough to have clouds.
Naming of Planets and SEDNA<br /><br />Definition of a Planet<br /><br />The IAU notes the very rapid pace of discovery of bodies within the Solar system over the last decade and so our understanding of the Trans-Neptunian Region is therefore still evolving very rapidly. This is in serious contrast to the situation when Pluto was discovered. As a consequence, The IAU has established a Working Group to consider the definition of a minimum size for a Planet. Until the report of this Working Group is received, all objects discovered at a distance from the Sun greater than 40 AU will continue to be regarded as part of the Trans-Neptunian population.<br /><br /><br />The object 2003 VB_12<br /><br />The object designated 2003 VB_12 was discovered with the Palomar 1.2-m Schmidt telescope on November 14, 2003 and has also been found on sky photographs taken in 2001 and 2002. Orbit computations show that the object's distance from the Sun varies between 76 and about 900 Astronomical Units (1 AU is close to 150 million kilometers). Its current distance is about 86 AU, about twice Pluto's distance. A very uncertain diameter of around 1800 km has been estimated from the brightness of the object. See http://www.gps.caltech.edu/~mbrown/sedna/.<br /><br />Since the object appears to be somewhat smaller than Pluto and has an extremely elongated orbit with a period of around 10,000 years, compared to 248 years for Pluto, it seems appropriate to designate the object as an asteroid and not as the 10th planet.<br /><br />The name Sedna has been proposed after an Inuit ocean goddess, but the name has not yet been endorsed by the IAU and needs to be reviewed first by the IAU Committee on Small Bodies Nomenclature.<br /><br />......................<br /><br />Three of my friends have minor planets named after them. They are (11941) Archinal for Brent Archinal, (author of Star Clusters) and (22338) Janemojo for Jane Houston Jones <div class="Discussion_UserSignature"> <font size="1" color="#3366ff">www.siriuslookers.org</font> </div>
Exactly what constitutes a planet is still a hot topic in astronomy, although in my opinion the arguments seem more philosophical and semantic at times than scientific.<br /><br />I don't think there can be any non-arbitrary definition of a planet.<br /><br />If I were going to define a planet, I'd call any body independently orbiting a star with an average diameter greater than or equal to 1000 km, perhaps with a fudge factor of 100 km thrown in on the small side, in the case of uncertainty. Why?<br /><br />--Because it keeps with the powers of ten notation of the metric system.<br /><br />--Because it's larger than the largest asteroid, Ceres.<br /><br />--Because the number of bodies larger than 1000 km is probably not large.<br /><br />To keep away from the "So, does this mean we'd have to memorize 25-30 planets?" argument, I'd limit the definition of a "major planet" to one larger than or equal in size to the Moon, which would hopefully keep the "Pluto is a comet" crowd happy.<br /><br />--So, 1000 km to Moon-size = minor planet<br />--Moon-size and up = major planet
"orbiting a star" So if Mars is ejected from the solar system, it is no longer a planet? What should we call planets that orbit the galaxy instead of a star or two?<br /> I guess Mercury is not a planet (by your definition) as Mercury has negligible atmosphere and negligible ice. Most comets have a cloudy atmosphere when close to the sun and ice when far from the sun Neil
<font color="yellow">"orbiting a star" So if Mars is ejected from the solar system, it is no longer a planet? What should we call planets that orbit the galaxy instead of a star or two?</font><br /><img src="/images/icons/crazy.gif" /><br /><br /><font color="yellow">I guess Mercury is not a planet (by your definition) as Mercury has negligible atmosphere and negligible ice.</font><br /><br />Oh it's a planet alright, as long as it's larger than the largest possible celestial body with atmospheric clouds. Mercury is too close to the sun to have a good atmosphere. Triton (2700 km) and Titan (2575 km) are both larger than Mercury (2440 km).