Solar system

simple:Solar System zh-cn:太阳系zh-tw:太陽系

A generic solar system (or planetary system) consists of at least one star and various orbiting objects (such as asteroids, comets, moonss, and planets). The term originated to describe the plantary system around Sol, the Latin name for our sun. The planet Earth is located within our solar system, which is usually just called the Solar system; others being referred to as planetary systems to avoid confusion. This terminology will be used below.

Table of contents

1 Solar system objects 2 Origin and evolution of planetary systems 3 Orbit of the solar system 4 Discovery and exploration of the solar system 5 The solar system and other planetary systems 6 Attributes of Major Planets 7 Other facts 8 See also

Solar system objects

There are a wide variety of objects present in the solar system that fall under various different categories. In recent years many of these categories have been found to be less clear-cut than once thought. This encyclopedia employs the following divisions:

• The Sun, a spectral class G2 star that contains 99.86% of system's mass.
• The planets of the solar system are those nine bodies traditionally labelled as such; Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune and Pluto.
  • Sizeable objects that orbit these planets are moonss. For a complete listing, see that article.
  • dust and other small particles that orbit these planets form planetary rings
  • Space debris of artificial origin can be found in orbit around Earth.
  • The planets were originally formed from planetesimals, sub-planetary bodies that accreted together during the first years of the solar system and no longer exist. The name is also sometimes used to refer to asteroids and comets in general, or to asteroids below 10km in diameter
• Asteroids are objects smaller than planets that lie roughly within the orbit of Jupiter and are composed in significant part by non-volatile minerals. They are subdivided into the near-Earth asteroids, which are further subdivided into Atens, Apolloss, and Amors.
  • Asteroid moons are asteroids that orbit larger asteroids. They are not as clearly distinguished as planetary moons, sometimes being almost as large as their partner.
  • Trojan asteroids are located in either of Jupiter's L₄ or L₅ points, though the term is also sometimes used for asteroids in any other planetary Lagrange point as well.
  • Meteoroids are asteroids roughly boulder-sized or smaller, all the way down to dust. Dust within the solar system is responsible for the phenomenon of zodiacal light.
• Comets, which are composed largely of volatile ices and whose orbits are highly eccentric, generally having a periapsis within the orbit of the inner planets and an apoapsis out past Pluto. Short-period comets exist with apoapses closer than this, however, and old comets that have had most of their volatiles driven out by solar warming are often categorized as asteroids. Some comets with hyperbolic orbits may also originate outside the solar system.
• Centaurss, icy comet-like bodies that have less eccentric orbits that remain in the region between Jupiter and Neptune.
• Trans-Neptunian objects, icy bodies whose mean orbital radius lies beyond Neptune's. These are further subdivided:
  • Kuiper belt objects, with orbits lying between 30 and 50 AU. Thought to be the origin for short-period comets. Pluto is sometimes classified as a Kuiper belt object in addition to being a planet, and a class of Kuiper belt objects with Pluto-like orbits are called Plutinos. The remaining Kuiper belt objects are classified as Cubewanos in the main belt and scattered disk objects in the outer fringes.
  • Oort cloud objects, currently hypothetical, with orbits lying between 50,000 and 100,000 AU. This region is thought to be the origin of long-period comets.
• Small quantities of dust are present throughout the solar system, and is responsible for the zodiacal light. Some of the dust is likely interstellar dust from outside the solar system.

Origin and evolution of planetary systems

Planetary systems are generally believed to form as part of the same process which results in star formation; although, some argue that systems are formed by some kind of accidental "stellar near-collison". The more common theory argues that the objects of a planetary system developed from a solar nebula.

Orbit of the solar system

The solar system is part of the Milky Way galaxy, a spiral galaxy with a diameter of about 100,000 light years containing approximately 200 billion stars, of which our Sun is fairly typical.

Estimates place the solar system at between 25,000 and 28,000 light years from the galactic center. Its speed is about 220 kilometers per second, and it completes one revolution every 226 million years.

The solar system appears to have a very unusual orbit. It is both extremely close to being circular, and at nearly the exact distance at which the orbital speed matches the speed of the compression waves that form the spiral arms. The solar system appears to have remained between spiral arms for most of the existence of life on Earth. The radiation from supernovas in spiral arms could theoretically sterilize planetary surfaces, preventing the formation of large animal life on land. By remaining out of the spiral arms, Earth may be unusually free to form large animal life on its surface.

Discovery and exploration of the solar system

Because of the geocentric perspective from which humans viewed the solar system, its nature and structure were long misperceived. The apparent motions of solar system objects as viewed from a moving Earth were believed to be their actual motions about a stationary Earth. In addition, many solar system objects and phenomena are not directly sensible by humans without technical aids. Thus both conceptual and technological advances were required in order for the solar system to be correctly understood.

The first and most fundamental of these advances was the Copernican Revolution, which adopted a heliocentric model for the motions of the planets. Indeed, the term "solar system" itself derives from this perspective. But the most important consequences of this new perception came not from the central position of the Sun, but from the orbital position of the Earth, which suggested that the Earth was itself a planet, and the planets other Earths. This was the first indication of the true nature of the planets. Also, the lack of perceptible stellar parallax despite the Earth's orbital motion indicated the extreme remoteness of the fixed stars, which prompted the speculation that they could be objects similar to the Sun, perhaps with planets of their own.

The solar system and other planetary systems

Until recently, the solar system was the only known example of a planetary system, although it was widely believed that other comparable systems did exist. A number of such systems have now been detected, although the information available about them is very limited. The technique employed involves the detection through the Doppler effect of periodic variations in the motion of parent stars which is attributed to the presence of planets. This allows the mass and orbital characteristics of the unseen planets to be determined. Unfortunately the sensitivity of these techniques currently does not permit the detection of planets of mass and orbit comparable to the Earth.

Attributes of Major Planets

All attributes below are measured relative to the Earth:

Planet	Equatorial diameter	Mass	Orbital Radius
Mercury	0.382	0.06	0.38
Venus	0.949	0.82	0.72
Earth	1.00	1.00	1.00
Mars	0.53	0.11	1.52
Jupiter	11.2	318	5.20
Saturn	9.41	95	9.54
Uranus	3.98	14.6	19.22
Neptune	3.81	17.2	30.06
Pluto*	0.24	0.0017	39.5

^*Pluto traditionally has been considered a planet. However now that we have learned more about the variety of bodies in the solar system we can see that its composition and orbit indicate that it has much more in common with Kuiper belt objects than it has with the rest of the planets. Therefore some astronomers no longer class Pluto as a planet.

Other facts

The total surface area of the solar system's objects that have solid surfaces and diameter > 1km is ~ 1.7 × 10⁹ km². ([1])

It has been suggested that the Sun may be part of a binary star system, with a distant companion named Nemesis. Nemesis was proposed to explain some regularities of the great extinctions of life on Earth. The theory says that Nemesis creates periodical perturbations in the asteroids and comets of the solar system causing a shower of large bodies and some of them hit Earth causing destruction of life. After this theory was stated, a search was undertaken for such a dim, distant companion; it found no visible star within one light-year of the Sun.

See also

• Laws of Kepler
• Titius-Bode law
• Planetary nomenclature
• Zodiacal light
• Timeline of solar system astronomy

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