Solar system is a stable system which contains so many objects (celestial bodies) bounded by gravity to only one star - THE SUN. The sun contains 99.86% mass of the solar system and it is positioned in its center. Apart from its star, solar system also contains: 8 planets (Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune), their natural satellites, dwarf planets, asteroids, comets, meteors and interplanetary medium.
Planets and large number of asteroids circles around Sun on the pathways called orbits. These orbits slightly differ from perfect circle are positioned in the same plane inside radius of 30 Astronomical Units (One A.U. equals 149 597 871 km). However all dwarf planets and large amount of smaller objects are out of planetary orbits, so the radius of a sphere which contains everything inside the solar system is estimated to be 100 000 - 150 000 A.U.
All objects in solar system are orbiting around the Sun in the same direction of its rotation, and most of them actually rotates in the same direction (planet Venus is exception). Space between planets, satellites and other celestial bodies is not really empty. It is filed with interplanetary matter, which is composed of solar winds, cosmic rays and dust which mainly originates from comets.
Up until the end of XVIII century human kind knew only for 5 planets beside Earth: Mercury, Venus, Mars, Jupiter and Saturn.
When you look at the night all the stars have a constant path, but these 5 celestial bodies have weird movements across the sky and ancient Greeks gave them appropriate name planetes (wanderers). We still use names that originates from Greek and Roman mythology. Uranus was discovered by William Herschel in 1781. While Neptune's trajectory was first calculated by Le verrier and then finally discovered by Johann Galle in 1846. Uranus and Neptune are visible only using telescope while other planets we can observe with naked eye - given the clear night sky without light pollution.
Celestial bodies of the solar system shine with reflected light which is coming from the Sun. Up until 1920s these objects were observed to determine their position and movement in the solar system. With development of astrophysics and photometric, spectroscopic and polarimetric we can now research their surfaces and atmospheres. Analyzing spectra of reflected lights from the Sun we can estimate composition of the surface layers of an object in the solar system. Existence, density and depth of atmosphere can be determined measuring polarization and occultation.
During next period 1950-1970 solar system research methods were improved and new methods were implemented. Groundbreaking methods in astronomy in this period were infrared spectroscopy and radio astronomy. using these method scientists mapped relief, chemically analyzed samples of atmosphere and soil from different objects in Solar system (e.g. Moon, Venus, Mars, Titan).
Dividing planets on inner and outer is based on their distance from the Sun.
Inner planets have solid surface, low mass and small diameters. Their average densities are about 5000 kg/m3.
Outer planets are very big and massive which is the main reason why they are called gas giants. They contain 98% of all the planets in the solar system. They are made mostly of hydrogen and helium in gaseous or liquid aggregate state and they have significantly lower average density than inner planets, about 1300 kg/m3.
Gas giants have much faster rotation than terrestrial planets, they are also distinguished by their rings and large number of satellites around them. These satellites were created together with a planet or they represent caught asteroids. Some of the larger satellites show resemblance with terrestrial planets.
Terrestrial planets representatives have only 3 satellites. Mercury and Venus have no satellites, Earth has Moon and Mars has two satellites, Phobos and Deimos, they have small dimensions and most likely they are caught asteroids.
As most of things in astrophysics internal structure of planets can't be observed and researched directly. We research them using theoretical models which must correspond to observations and estimations, like average density, surface relief, chemical composition of atmosphere and intensity of magnetic field, etc...
Planets are in a state of mechanical equilibrium (thermal and Coulomb's forces are defying the force of gravity). Their interior is solid or liquid. In their structure we can separate several spherical layers which differs by their physical and chemical characteristics.
Temperature rises from surface to the center of a planet. Main source of energy is accretion (collection of material through gravity) in the period of planet formation, and later energy is accumulated through radioactive decay. Heat is transferred towards surface by convection and conduction (Read this post if you want to read more about heat transfer). Processes in lithosphere, which consists out of crust and upper layers of mantle, cause a tectonic phenomena and volcanic eruptions
Beside all these endogenic processes, surface of planets are under influences of meteor hits, as well as mechanical erosion (exogenic processes). In the earlier stage of planet formation and solar system meteor hits happened far more frequently. Nowadays most of these small objects are blown away by solar wind or Planets caught them with their gravitational pull.
We don't know much about structure of gas giant planets. For now we only have assumptions and estimations based on various models. It is considered that all gas giants have liquid mantle and a small solid core immersed in it.
Terrestrial and Jovian type planets have different chemical composition. Inside Terrestrial planets there are almost no hydrogen and inert gases and metals like iron (Fe), silicon (Si) and magnesium (Mg) are abundant, while Jovian planets contain hydrogen and helium in a similar proportion to Sun (70% hydrogen and 28% helium of total mass). Hydrogen in surface layers is liquid and neutral, while in deeper layers due to high pressure it becomes ionized (metallic).
Most of the planets have their own magnetic field which is created by dynamo mechanism (still assumption), due to rotation of conductive layers in their interiors. These fields are bipolar or quasi-bipolar and they extend to a considerable distance from the surface of the planet. In the interaction with solar winds magnetospheres are formed.
Mechanism of atmosphere formation is not completely understood. Theory for gas giant atmospheres is that they formed together with a planet and they didn't drastically changed over time. However, it is believed that terrestrial planets lost their primordial atmospheres, and for formation of new ones there are few theories. The leading one is that gases were separated during volcanic eruptions in the age of planet evolution or they were released with rock decomposition and destruction.
You remember what Bruce Lee said? "Be water my friend...", now I am not as popular nor wise, nor fast nor strong as him but I will say Be light my friend and enjoy this trip through solar system.
