# Solar System/Solar System

One of the most difficult and controversial fields of astronomy is the study of the way in which the Solar System was formed, as well as its past and its future evolution.
The Solar System, or more particularly, its larger bodies - all the planets with their satellites, orbit in the Sun close to one plane known as the ecliptic which is defined as the plane in which the Earth orbits, and all move in the same direction. It is helpful to remember that an observer viewing this from a position above the plane of the ecliptic, or from the direction of the terrestrial north pole, would see the planets move around the Sun in an counter-clockwise direction, also referred to as prograde motion. All the planets and their major satellites rotate in an counter-clockwise direction on their axis, with the exception of Venus and Uranus which, because of the position of their axes shows that they experienced some cataclysmic event in their far-off past.
The distance of the planets and their satellites from the Sun increases regularly as described by the Titius-Bode law. This regularity in the movements of revolution and rotation, and the disk shape of the whole system suggested a hypothesis to Immanuel Kant as long ago as 1755, and to Pierre-Simon de Laplace in 1796: that the Sun and the planets had been formed out of a cloud of rotating gas as a result of the combined action of the force of gravity and centrifugal force. This cloud has historically always been known as the "solar nebula."
Another aspect that must be borne in mind is the distribution of density within the system: the terrestrial planets have higher densities, varying from approximately 5.5 and 3.9 times the density of water, while the giants planets have consistently lower densities, varying from 0.7 to 1.6 times the density of water.

## The place of the Solar System in the universe

The universe contains an estimated 100 billion galaxies, of which our Milky Way is one. The Milky Way itself contains an estimated 100 billion stars, of which our Sun is one. The Milky Way is a spiral galaxy with a radius of around 50,000 light-years (a light-year being the distance that light travels in one year), and the Sun and its solar system are around 30,000 light-years from the center of the galaxy.

## The Age of the Solar System

Based on the age of the oldest meteorites, it has been calculated that the Solar System was formed approximately 4.5 billion years ago. Chondrules, spheres of crystal present in chondrite-type meteorites, contain radioactive elements and the proportions of these enable us to measure the time that has elapsed from the moment in which the crystals solidified: this time lapse provides us with an indication of the age of the Solar System.
A group of research scientists at the Institute of Physical Earth Sciences in Paris has carried out very precise measurements of the relationship that exists in certain phosphates between radioactive uranium-238 and the element which derives from it, lead 206. As these crystals are rich in uranium but have virtually no lead in their natural state, the lead isotope present in the phosphates has to result from the decay of the uranium. Working from these measurements, radiometric dating means that we can calculate the age of the crystals at about 4.56 billion years.

## The Sun and the Solar System

The sun is the only star in our solar system. The sun in the centre of our solar system is a medium sized star. The temperature at its surface is about 6000 degree C. Its size is so big that it could hold within itself 13 lakh earths like ours. All the objects around the sun revolve around it because of its gravitational force. Given the close resemblance in physical structure and chemical composition of the stars and particularly of those of solar type, it is a reasonable assumption that the Sun was formed in the same way as stars of later generations, those whose formation present-day observational techniques make it possible to witness. By the description "stars of the solar type" is meant those stars which have approximately the same mass as the Sun. The main physical features of a star and its evolution depend on its mass, as do the phases of its life: formation through the contraction of a cloud of dust and gas; maturity, when the star emits constant radiation for a length of time: the smaller the star, the longer the duration; and the star's end, with variations in radius and temperature and phenomena which may be more or less violent. It would appear that stars form from clouds of gas and dust, those of recent formation still being surrounded by these. The subsequent stages of stellar evolution are sufficiently well-known to make it possible to calculate their ages to within a very close approximation.

## How a star is formed

Of all the phases of a star's life, we know least about its formation. These clouds have very low temperatures and are very opaque; as a result they radiate only in the far-infrared which the terrestrial atmosphere absorbs completely, preventing us from seeing what is happening inside the cloud. As a result of observations from space using the IRAS (Infrared Astronomical Satellite) and data from microwave radio-astronomy which can even penetrate the opaque dust clouds, it has been possible to witness the first phases of condensation of protostars from the interstellar medium. This has led to the discovery that the protostar contracts and the matter accretes onto its equatorial region, while at the same time it expels matter violently along the two opposing directions of its polar axis. In this way a disk forms on the equatorial plane. The presence of a disk of solid matter in stars that have already formed was established by the same satellite but it is also possible to discern this from Earth if the image of the star is occulted with an opaque disk, so that the weak luminosity of the disk is not overwhelmed by the predominant stellar brilliance. These observations enable us to conclude that the young Sun must also have been surrounded by a disk of gas and dust: the solar nebula hypothesized by Kant and Laplace two centuries ago.
According to stellar evolutionary theories, the Sun took approximately 50 million years to reach the phase of stability in its evolution, that is to say, to reach its present values of radius and surface temperature and to radiate, at any given moment, the same quantity of energy that it emits today. Based on these same theories, it has been estimated that these values will remain virtually unchanged for another 5 billion years.

## Composition of the stellar nebula

Dust represents only 2% of the stellar nebula mass; the remainder is made up in the following proportions: gas, of which 78% is accounted for by hydrogen atoms, 20% by helium atoms and 2% by all the other elements. The dust accounts for the largest percentage of the heavier elements which are present in planetary conformation: carbon, oxygen, iron, silicon, magnesium. The denser clouds contain between 10,000 and one million molecules per cubic centimeter and have temperatures of approximately 10 degrees Kelvin, the equivalent of 263 C (440 F) below zero. They are unstable because at such low temperatures the force of gravity is greater than the thermal pressure exerted by the gas and dust particles.

## Origins

Of the numerous theories put forward to explain how the planets were formed, only two are still considered acceptable, although both of them pose many unsolved problems: the first is the planetesimal or accumulation theory, and the second is the protoplanetary or unstable disk theory. How can the formation of a planet from a disk of dust with particles measured in micrometers be explained? It is worth looking at the two possible answers.

## Formation through accretion

This is the most widely accepted hypothesis to be formulated to date. The particles collided with each other and stuck together, leading to processes known as "coagulation" and, subsequently, "accumulation" or "accretion." In this way larger bodies grew from the granules which collected into yet bigger, solid bodies, until they formed terrestrial planets and the nuclei of the giant planets. The gases and dust remained closely intermingled for as long as the turbulent movements typical of the interstellar medium continued in the solar nebula. As the turbulence in the solar nebula diminished, the particles began a process of sedimentation, accumulating towards the central plane of the nebula, forming a very thin disk. Given the relatively high density, it is estimated that the sedimentation process was very quick, perhaps as fast as a thousand years. As the sedimentation process progressed, the mass of the thin disk became unstable and the disk broke up into a large number of solid bodies, each having a diameter of approximately 1 kilometer (.62 miles), called planetesimals. It is estimated that approximately a thousand billion of these existed, orbiting around the Sun, within the region encompassing the orbit of Mars.

## Definition of a planet

The official definition of a solar "planet" (due to a 2006 resolution of the International Astronomical Union) is "a celestial body that (a) is in orbit around the Sun, (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and (c) has cleared the neighborhood around its orbit." By this definition the Sun has eight planets: in increasing order of distance from the Sun, they are Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune.