گِرهُہ جمع (گِرَهَہ)
- بين الاقوامي نجمياتي يونين پٽاندر، any of the objects Mercury، Venus، ڌرتي، مريخ، وسپت، Saturn، Uranus، Neptune, ۽ Pluto, مان هر هڪَ گِرَهہ آهي؛ نہ انهن کان علاوه ڪو ٻيو، ۽ نہ ئي انهن مان ڪو ڇڏي: بنا اهڙي ڪنهن بہ مختصر يا سليس وصف جي، جيڪا انهن مڙني تي پوري لهندي هجي.
- ڪا اهڙي وَٿَ جيڪا 1) سڌو سنئون ڪنهن مرڪزي تاري کي مداريندي هجي، 2) ايترو گھڻو مايو نہ رکندي هجي جو ان جي پيٽَ ۾ ناڀ ضماءُ پيش اچي سگھي، 3) گھٽ ۾ گھٽ ايترو مايو ضرور رکندي هجي جيڪو ان کي ڪم و بيش قاعديوار ڊول ڏيندو هجي، 4) اهڙين وَٿُن جي ڪنهن گروهہ جو حصو نہ هجي جن جا مدار هڪ ٻي کي ڪَٽيندا (اورانگھيندا) هُجَن؛ جهڙوڪ گِرَهڙين جو پَٽو.
- A working definition by the International Astronomical Union states that a companion to a star that is smaller than required for the fusion of deuterium, estimated at 13 Jupiter masses is termed a planet. (But see usage note below).
- (Colloquial or Jargon) A celestial object that is like a planet, and unlike the typical moon etc., in one’s field of study, without concern as to its orbit. (See usage notes below.) Careful speakers use the term world instead.
Any simple definition of “planet” will not name only and exactly the nine planets in our solar system. This is partly historical accident and long precedent, giving rise to the current controversy over Pluto. For example, when Ceres was discovered it was called a planet. But, when it was discovered not to be alone, a new term was invented—asteroid—and the designation of planet revoked. When Pluto was discovered to be not alone, it had been called a planet for generations and cultural inertia gives us the explicit first definition.
Sometimes other celestrial bodies are called planets either colloquially or as technical jargon in some discipline. For example, a geologist would care about the physical properties of the object and not care anything at all about how to get there. For example,
- Our angle is that Titan is basically another terrestrial planet, but it’s often not recognized as one because it happens to be orbiting Saturn. —Dr. Vikki Meadows, an astrobiologist at NASA’s Jet Propulsion Laboratory.
Dr. Meadows was pointing out that the term terrestrial planet as it is used in astrobiology applies to Titan, even though that is counterintuitive from the name of the term and the proper meaning of planet. Titan, Europa, and other large moons with interesting active geologic processes are sometimes accidentally called planets in this way.
Meanwhile, the Working Group on Extrasolar Planets distinguishes planets from the larger brown dwarfs and objects that are free-floating rather than companions. But for a lower limit it specifies “The minimum mass/size required for an extrasolar object to be considered a planet should be the same as that used in our Solar System.”. However, there is no accepted definition of planet for our solar system that is based on size. When another Kuiper belt object is found that is larger than Pluto, this issue may come to a head. It’s moot for extrasolar planets anyway since only very large ones can currently be detected.