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Planet(Redirected from Planets)
A planet (from the Greek planetes or "wanderers") is a body of considerable mass that orbits a star and that doesn't produce energy through nuclear fusion. Prior to the 1990s only nine were known (all of them in our own solar system); as of 2004, 118 are known, with all of the new discoveries being extrasolar planets, aka "exoplanets."
Planets are thought to form from the collapsing nebula that a planet's star formed out of, aggregating from gas and dust that orbits the protostar in a dense protostellar disk before the star's core ignites and its solar wind blows the remaining material away.
Table of contents showTocToggle("show","hide")
1 Within the Solar System
1.1 Accepted Planets
1.2 Other Objects
1.3 Classification
2 Extrasolar Planets
3 See Also
4 External Links
Within the Solar System
Except for Earth, all of the accepted planets in the solar system are named after Roman gods. Moons are also named after gods and characters from classical mythology or from the plays of Shakespeare. Asteroids can be named, at the discretion of their discoverers, after anybody or anything (subject to approval by the International Astronomical Union's panel on nomenclature). The act of naming planets and their features is known as planetary nomenclature.
Accepted Planets
The commonly accepted list of major planets of our solar system (in increasing distance from the Sun) are
English-speaking schoolchildren often use a variety of mnemonics to remember the planets in this order:
- Mary's Violet Eyes Make John Stay Up Nights, Period. (Or Poor John to include the Planetoids.)
- Mother Very Thoughtfully Made Jelly Sandwiches Under No Protest. (Or A Jelly Sandwich to include Asteroids.)
- My Very Energetic Mother Just Served Us Nine Pizzas. (Or "Pistachio Nuts" for those rare times when the orbit of Pluto is within the orbit of Neptune).
Other Objects
Recently an object, Sedna has been discovered orbiting the sun 2 billion miles beyond Pluto. Sedna, after the Inuit goddess of the sea, is the provisional name given to this 1,180–2,360km (730-1,470 miles) diameter object by NASA. Its official name for now is 2003 VB12. The diameter is still uncertain but believed to be between 1/2 and 3/4 of Pluto's. Several news sources have already reported Sedna as the tenth planet [1], but that is not generally accepted by astronomers. Another possible planet is "2004 DW", an object with an orbit and mass similar to Pluto. Other candidates include Quaoar and Varuna.
Several hypothetical planets, like Planet X (supposedly beyond the orbit of Pluto) or Vulcan (thought to orbit inside the orbit of Mercury), were posited, and were subjects of intense searches that found nothing.
Classification
Astronomers distinguish between minor planets, such as asteroids, comets, and trans-Neptunian objects; and major (or true) planets.
Planets within Earth's solar system can be divided into categories according to composition.
- Terrestrial or rocky: Planets that are similar to Earth—with bodies largely composed of rock: Mercury, Venus, Earth, Mars
- Jovian or gas giant: Those with a composition largely made up of gaseous material: Jupiter, Saturn, Uranus, Neptune
- Icy: Sometimes a third category is added to include bodies like Pluto, whose composition is primarily ice; this category of "icy" bodies also includes many non-planetary bodies such as the icy moons of the outer planets of our solar system (e.g. Titan).
The eight rocky and gaseous planets are universally recognized as major planets. For consistency's sake, some believe that due to size and composition Pluto (like Sedna) should be classified a minor planet, as the largest of the Trans-Neptunian objects in the Kuiper belt. For example, Mike Brown of Caltech defines a planet to be
any body in the solar system that is more massive than the total mass of all of the other bodies in a similar orbit. [2]
Under this definition, neither Pluto nor Sedna would be a major planet.
Many consider the Earth and its Moon to be a double planet, given that the Moon is rocky and almost the same size as Mercury.
Extrasolar Planets
Almost all extrasolar planets (those outside our solar system) discovered to date have masses which are about the same or larger than the gas giants within the solar system. (The only exception is three planets discovered orbiting a burned-out star, or supernova remnant, called a pulsar. These are comparable in size to the terrestrial planets). This is largely because the gravitational effect of massive planets is larger, making them easier to detect. However, it is far from clear if the newly discovered planets would resemble gas giants in our solar system or if they are of an entirely different type or types which are unknown in our solar system. In particular, some of the newly discovered planets orbit extremely closely to their parent star sometimes in highly elliptical orbits. They therefore receive much more stellar radiation than the gas giants in our solar system, which makes it questionable whether they are the same type of planet at all.
The National Aeronautics and Space Administration of the United States has a program underway to develop a Terrestrial Planet Finder artificial satellite, which would be capable of detecting the planets with masses comparable to terrestrial planets. The frequency of occurrence of these planets is one of the variables in the Drake equation which estimates the possibility of extraterrestrial intelligence.
Interstellar planets are rogues in the interstellar space, not gravitationally linked to any given solar system. No interstellar planet is known to date, but they may exist.
See Also
External Links
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