Solar system

A generic solar system (or planetary system) consists of at least one star and various orbiting objects (such as asteroids, comets, moons, and planets). The term originated to describe the planetary system around Sol, the Latin name for our sun. The planet Earth is located within our solar system, which is usually called simply the Solar system; others are called planetary systems to avoid confusion. This article uses this terminology.

Solar system objects

The wide variety of objects that exist in the solar system fall into several 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 is a spectral class G2 star that contains 99.86% of the 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 moons. For a complete listing, see that article.
  • Dust and other small particles that orbit these planets form planetary rings.
  • Space debris of artificial origin that can be found in orbit around Earth.
  • Planetesimals, from which the planets were originally formed, are sub-planetary bodies that accreted during the first years of the solar system and that no longer exist. The name is also sometimes used to refer to asteroids and comets in general, or to asteroids below 10 km in diameter.
• Asteroids are objects smaller than planets that lie roughly within the orbit of Jupiter and are composed in significant part of non-volatile minerals. They are subdivided into asteroid groups and families based on their specific orbital characteristics.
  • Asteroid moons are asteroids that orbit larger asteroids. They are not as clearly distinguished as planetary moons, sometimes being almost as large as their partners.
  • 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 that range in size from roughly boulder sized to particles as small as dust.
• Comets are composed largely of volatile ices and have highly eccentric orbits, generally having a perihelion within the orbit of the inner planets and an aphelion beyond 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.
• Centaurs are icy comet-like bodies that have less-eccentric orbits so that they remain in the region between Jupiter and Neptune.
• Trans-Neptunian objects, which are icy bodies whose semi-major axes lie beyond Neptune's. These are further subdivided:
  • Kuiper belt objects have orbits lying between 30 and 50 AU. This is 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 the 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, have orbits lying between 50,000 and 100,000 AU. This region is thought to be the origin of long-period comets.
  • The newly discovered object 2003 VB₁₂ (Sedna), with a highly elliptical orbit extending from about 76 to 850 AU, does not obviously fit in either category, although its discoverers argue that it should be considered a part of the Oort cloud.
• Small quantities of dust are present throughout the solar system and are responsible for the phenomenon of zodiacal light. Some of the dust is likely interstellar dust from outside the solar system.

Jupiter constitutes most of the mass of the solar system outside the Sun: 0.1% of the mass of the solar system. In turn, Saturn constitutes most of the remaining mass, then Uranus ans Neptune, then Earth and Venus (see also below).

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.

At the solar system, the escape velocity w.r.t. the Milky Way's gravity is ca. 1000 km/s.

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 supernovae 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. See extrasolar planet for more information.

Attributes of major planets

All attributes below are measured relative to the Earth:

Planet	Equatorial diameter	Mass	Orbital Radius	Year	Day
Mercury	0.382	0.06	0.38	0.241	58.6
Venus	0.949	0.82	0.72	0.615	-243
Earth	1.00	1.00	1.00	1.00	1.00
Mars	0.53	0.11	1.52	1.88	1.03
Jupiter	11.2	318	5.20	11.86	0.414
Saturn	9.41	95	9.54	29.46	0.426
Uranus	3.98	14.6	19.22	84.01	0.718
Neptune	3.81	17.2	30.06	164.79	0.671
Pluto*	0.24	0.0017	39.5	248.5	6.5

Of the other objects, Ganymede has the largest mass (0.02).

^*Soon after its discovery in 1930, Pluto was classified a planet by the International Astronomical Union. However, based on additional discoveries since that time, some astronomers have suggested reconsideration of that decision.

Other facts

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

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, although this theory is no longer taken seriously by most scientists.

Extension

Outer boundaries, in that order, are the termination shock, the heliosheath, and the heliopause.

Scale models

Because of the large range of significant distances (e.g., the distance between the Earth and the Sun is almost 12,000 times the diameter of the Earth), constructing a scale model of the solar system is a challenging task. Generally, to make the planetary sizes reasonably perceptible requires an outdoor location with the interplanetary distances being represented by significant human-scale distances.

Scale Models of the Solar System

Location	Scale	Sol dia.	Earth dia.	Sol-Earth	Sol-Pluto
The Real Thing	1:1	1.392 Gm	12.76 Mm	149.6 Gm	5.914 Tm
University of Maine at Presque Isle	1:93,000,000	15 m	140 mm?	1.6 km	64 km
Peoria, Illinois	1:125,000,000	11 m	100 mm	1.2 km	64 km
York	1:575,872,239	2.417 m	22.1 mm	259.73 m	10.2679 km
Eugene, Oregon	1:1,000,000,000	1.39 m	12 mm	150 m	5.9 km
Jodrell Bank	1:5,000,000,000?	300 mm?	2.5 mm?	30 m?	1 km?
Saint-Louis-du-Ha! Ha!, Quebec	1:10,000,000,000	13.9 cm	1.2 mm	15 m	590 m

See also

• Laws of Kepler
• Titius-Bode law
• Planetary system
• Planetary pairs
• Planetary nomenclature
• Zodiacal light
• Timeline of solar system astronomy
• Astronomical symbols
• Geological features of the Solar System