Solar System
The solar system comprises the Earth's Sun and the retinue of celestial objects gravitationally bound to it. Traditionally, this is said to consist of the Sun, nine planets and their 158 currently known moons; however, a large number of other objects, including asteroids, meteoroids, planetoids, comets, and interplanetary dust orbit the Sun as well. Astronomers are debating the classification of a potential tenth planet and other trans-Neptunian objects.
Although the term "solar system" is frequently applied to other star systems and the planetary systems which may comprise them, it should strictly refer to Earth's system specifically: the word "solar" is derived from the Sun's Latin name, Sol, and thus the term sometimes appears as Solar System. When talking about another stellar system or planetary system, including the stars and bodies associated with them through gravity, it is usual to shorten it to drop the term "solar" and form names such as "the Alpha Centauri system" or "the 51 Pegasi system".
T h e p r i n c i p a l c o m p o n e n t o f t h e s o l a r s y s t e m i s t h e S u n ( a s t r o n o m i c a l s y m b o l & ) ; a m a i n s e q u e n c e G 2 s t a r t h a t c o n t a i n s 9 9 . 8 6 % o f t h e s y s t e m ' s k n o w n m a s s a n d d o m i n a t e s i t g r a v i t a t i o n a l l y .
I t s t w o l a r g e s t o r b i t i n g b o d i e s , J u p i t e r a n d S a t u r n , t o g e t h e r a c c o u n t f o r m o r e t h a n 9 0 % o f t h e r e m a i n i n g m a s s . ( T h e O o r t C l o u d t o o m i g h t h o l d a s u b s t a n t i a l p e r c e n t a g e , b u t a s y e t i t s e x i s t e n c e i s u n c o n f i r m e d ) .
B e c a u s e o f i t s l a r g e m a s s , t h e S u n a c h i e v e s t h e d e n s i t y i n i t s i n t e r i o r n e c e s s a r y t o s u s t a i n n u c l e a r f u s i o n , r e l e a s i n g e n o r m o u s a m o u n t s o f e n e r g y , m o s t o f w h i c h i s r a d i a t e d i n t o s p a c e i n t h e f o r m o f e l e c t r o m a g n e t i c r a d i a t i o n , w h i c h i n t u r n i s m o s t l y c o m p o s e d o f v i s i b l e l i g h t .
I n b r o a d t e r m s , t h e c h a r t e d r e g i o n s o f t h e s o l a r s y s t e m c o n s i s t o f t h e S u n a n d i t s p l a n e t a r y s y s t e m : t h e e i g h t b o d i e s i n r e l a t i v e l y u n i q u e o r b i t s c o m m o n l y c a l l e d p l a n e t s o r m a j o r p l a n e t s a n d t w o b e l t s o f s m a l l e r o b j e c t s , w h i c h c a n b e c a l l e d m i n o r p l a n e t s , p l a n e t o i d s , o r p l a n e t e s i m a l s .
P l u t o , t h e n i n t h p l a n e t , i s a l s o c o n s i d e r e d a m e m b e r o f t h e o u t e r b e l t , a n d i t s s t a t u s i s c u r r e n t l y u n c e r t a i n , p a r t i c u l a r l y s i n c e t h e d i s c o v e r y o f 2 0 0 3 U B 3 1 3 .
M o s t o b j e c t s i n o r b i t r o u n d t h e S u n a l l l i e w i t h i n t h e s a m e s h a l l o w p l a n e , c a l l e d t h e e c l i p t i c p l a n e , a n d o r b i t i n t h e s a m e d i r e c t i o n . M a n y a r e i n t u r n o r b i t e d b y m o o n s , a n d t h e l a r g e s t a r e e n c i r c l e d b y p l a n e t a r y r i n g s o f d u s t a n d o t h e r p a r t i c l e s .
T h e m a j o r p l a n e t s a r e , i n o r d e r , M e r c u r y, V e n u s , E a r t h , M a r s , J u p i t e r , S a t u r n , U r a n u s , N e p t u n e , a n d P l u t o . E i g h t o f t h e n i n e p l a n e t s a r e n a m e d a f t e r o r d e r i v e d f r o m g o d s a n d g o d d e s s e s f r o m G r e c o - R o m a n m y t h o l o g y .
Origin and Age
Using radiometric dating, scientists can estimate that the solar system is 4.6 billion years old. The oldest rocks on Earth are approximately 3.9 billion years old. Rocks this old are rare, as the Earth's surface is constantly being reshaped by erosion, vulcanism and plate tectonics.
To estimate the age of the solar system scientists must use meteorites, which were formed during the early condensation of the solar nebula.
The oldest meteorites (such as the Canyon Diablo meteorite) are found to have an age of 4.6 billion years, hence the solar system must be at least 4.6 billion years old.
The current theory of solar system formation is the nebular hypothesis, first proposed in 1755 by Immanuel Kant and independently formulated by Pierre-Simon Laplace. The nebular theory has been refined over many years and now has a great deal of evidence supporting it.
To briefly summarize, the nebular theory holds that the solar system was formed from the gravitational collapse of a gaseous cloud called the solar nebula. It had a diameter of 100 AU and was 2-3 times the mass of the Sun. Over time a disturbance (possibly a nearby supernova) squeezed the nebula, pushing matter inward until gravitational forces overcame the internal gas pressure and it began to collapse.
As the nebula collapsed it began to spin faster to conserve angular momentum, and became warmer. As the competing forces associated with gravity, gas pressure, magnetic fields, and rotation acted on it the contracting nebula began to flatten into a spinning protoplanetary disk with a gradually contracting protostar at the center.
Grains of dust (silicates and metals) and ice (hydrogen compounds) condensed from the gas, and began to accrete into larger and larger clumps, forming planetesimals. Inside the frost line, planetesimals were composed of rock and metal, because those are the only grains that can condense at those temperatures, and remained relatively small because they were only 0.6% the mass of the disk.
The larger icy planetesimals beyond the frost line became massive enough to capture and hold onto helium and then hydrogen gases, which caused them to rapidly grow into jovian protoplanets.
After 100 million years, the pressure and density of hydrogen in the centre of the collapsing nebula became great enough for the protosun to begin thermonuclear fusion, which increased until hydrostatic equilibrium was achieved. The young Sun's solar wind then cleared away all the gas and dust in the protoplanetary disk, blowing it into interstellar space, thus ending the growth of the planets.
Regions
According to their location, the objects in the solar system are divided into three zones: the inner solar system, including terrestrial planets and the Main belt of asteroids; a middle region including the giant planets, their satellites and the centaurs, and the outer solar system, comprising the area of the Trans-Neptunian objects including the Kuiper Belt, the Oort cloud, and the vast region in between. This region is occasionally referred to as the solar system's "third zone".
Interplanetary Medium
T h e e n v i r o n m e n t i n w h i c h t h e s o l a r s y s t e m r e s i d e s i s c a l l e d t h e i n t e r p l a n e t a r y m e d i u m . T h e S u n r a d i a t e s a c o n t i n u o u s s t r e a m o f c h a r g e d p a r t i c l e s , a p l a s m a k n o w n a s s o l a r w i n d , w h i c h f o r m s a v e r y t e n u o u s " a t m o s p h e r e " ( t h e h e l i o s p h e r e ) , p e r m e a t i n g t h e i n t e r p l a n e t a r y m e d i u m i n a l l d i r e c t i o n s f o r a t l e a s t 1 6 T m o r 1 6 × 1 0 9 k m i n t o s p a c e . S m a l l q u a n t i t i e s o f d u s t a r e a l s o p r e s e n t i n t h e i n t e r p l a n e t a r y m e d i u m a n d a r e r e s p o n s i b l e f o r t h e p h e n o m e n o n o f z o d i a c a l l i g h t . S o m e o f t h e d u s t i s l i k e l y i n t e r s t e l l a r d u s t f r o m o u t s i d e t h e s o l a r s y s t e m . T h e i n f l u e n c e o f t h e S u n ' s r o t a t i n g m a g n e t i c f i e l d o n t h e i n t e r p l a n e t a r y m e d i u m c r e a t e s t h e l a r g e s t s t r u c t u r e i n t h e s o l a r s y s t e m , t h e h e l i o s p h e r i c c u r r e n t s h e e t .
Continued ... References and Links
Solar System's 'look-alike' found BBC - May 6, 2008
First Sunset Outside Our Solar System Glimpsed Live Science - December 11, 2007
Our Solar System is asymetrical Live Science - December 10, 2007
Scientists find the dent in our solar system MSNBC - December 10, 2007
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