The solar system consists of the eight planets and their satellites (or moon) and thousands of other smaller heavenly bodies such as asteroids, comets and meteors.
- The sun is the Solar System’s star.
- It is at the center of the Solar System.
- It is the largest member of stars.
- The Sun is made up of extremely hot gases. By mass: hydrogen 69.5%, helium 28%, carbon 2.5%, nitrogen, oxygen, sulfur, silicon, iron and magnesium, altogether and trace of other elements.
- It is the only source of heat and light for the entire solar system.
- The average distant of the sun from the earth is 149,597,900 km and its equatorial diameter is 1392520 km.
- Its surface is approximately 12000 times of the earth.
- The sun rotates on its axis in 25 days 9 hours & 7 minutes.
- It is 400,000 times as bright as the full moon and its rays travel at a tremendous speed of about 300,000 km per seconds and takes 8.5 minutes to reach the earth from the sun.
The eight planets in our solar system are commonly divided into two groups:
Inner Planets - Mercury, Venus, Earth, and Mars.
The inner planets are small and are composed primarily of silicate rock and iron.
Outer Planets - Jupiter, Saturn, Uranus and Neptune.
- Mercury is the closest planet to the Sun.
- It orbits in a highly elliptical orbit ranging from 46 million km (29 million miles) from the Sun out to 70 million km (43.5 million miles).
- It takes about 88 Earth days to orbit the Sun but rotates on its axis once every 59 Earth days. Because of the slow rotation, a single day on Mercury (mid-day to mid-day) takes 176 Earth days.
- Its axial tilt is remarkably small at 3/100ths of a degree. Much smaller than any other planet.
- Mercury is quite small with a diameter of 4,878km, (2/5ths that of Earth) and only 5% of Earth’s mass. Its gravity on the surface is 1/3rd of Earth’s.
- Mercury has almost no atmosphere and is blasted by the Sun during the day and exposed to cold space during the night. This means that it undergoes some of the widest temperature swings of anybody in the Solar System with temperatures reaching +430°
C and dipping down to -180°C.
- It has a highly cratered rocky surface and is known to have an iron core. However its magnetic field is much weaker than the Earth’s (1% as strong). Initially RADAR waves reflected from the surface of Mercury indicated that water ice might be present at the poles. It has recently been confirmed by the Messenger Spacecraft that ice water does indeed exist in deep craters at the poles the interiors of which are permanently shrouded in shadow.
- Because Mercury is so close to the Sun, it is only ever seen with the naked eye just before sunrise and just after sunset. At all other times it is masked by the brightness of the Sun.
- Venus is the second closest planet to the Sun and orbits in an almost circular orbit at 108 million km. As it orbits, Venus comes closer to Earth than any other planet in the solar system and can come to within about 40 million km.
- Venus takes about 225 Earth days to orbit the Sun and rotates at the incredibly slow rate of once every 243 days - and in a clockwise direction (as seen from looking down on the Suns north pole). Only Uranus (which almost spins on its side) also has a clockwise spin. A day on Venus (sunrise to sunrise) lasts 117 Earth days.
- Venus has a gentle axial tilt of 3 degrees.
- Venus, with a diameter of 12100 km, it is very nearly the same size as Earth (1000km smaller), and has 80% of Earth’s mass. its gravity on the surface is 90% that of Earth’s.
- Venus has a very dense atmosphere with pressures at the surface over 90 times that of Earth’s.
- The atmosphere is comprised of carbon dioxide with thick clouds of sulphur dioxide. This atmosphere has the strongest greenhouse effect known in the solar system which keeps the planet at a reasonably constant temperature of 460 degrees C. This makes Venus the hottest planet in the solar system, far hotter even then mercury which is twice as close to the Sun.
- Venus has a very weak magnetic field.
- The third closest planet to the Sun is Earth and is the largest and densest of the inner planets.
- Earth orbits in a reasonably circular at 150 million km and is the first of the planets to have a moon.
- Earth is of course the only place that we know of that has life. It has all essential elements like carbon, hydrogen, nitrogen and oxygen which act as building blocks for the origin of life.
- Earth takes 365.25 Earth days to orbit the Sun and rotates once every 23 hours, 56 minutes and 4 seconds. Because it rotates around the Sun the length of a day on Earth (sunrise to sunrise) takes 24 hours.
- The Earth has an axial tilt of 23.4 degrees and a diameter of 12742km.
- The Earth is thought to be 4.54 billion years old.
- Earth is known as the “watery planet” and “blue planet” due to the presence of huge amount of water.
- The earth is neither too hot nor too cold. It has ‘Goldilocks Zone’.
- The earth has a protective blanket of ozone layer high up in its atmosphere to save life from the harmful ultraviolet radiations coming from the sun.
- The moon is only satellite of the earth.
- The Moon is 3,474km in diameter (27% that of Earth) and orbits at a distance of between about 362,000 to 405,000 km.
- It takes 27days, 7 hours and 43 minutes to rotates on its own axis. This period of 27.5 days is called the sidereal month. Approximately the same period of time it takes to revolve around the earth. The moon’s period of revolution with reference to the sun is about 29.53mdays (29days, 12 hours, 44 minutes and 2.8 seconds). This period is called a synodic month.
- It has also been affected by the gravitational pull of the Earth which has over time caused the Moon's rotation to be slowed until it matches the time it takes to orbit the Earth. This is why the same side of the Moon always faces the Earth.
- Earth is protected from solar radiation by a strong magnetic field generated by movement of its core which is mainly comprised of molten iron.
- Mars is the fourth closest planet to the Sun and orbits in an fairly eccentric orbit at around 230 (+-20) million km.
- Mars takes about 686 Earth days to orbit the Sun. It has a tilt (25.1 degrees) and rotational period (24 hour 37 minutes) which are both similar to the Earth with a day (sunrise to sunrise) lasting 24 hours, 39 mins. Because of the tilt it also has seasons in the same way as the Earth does.
- Mars is about half the size of the Earth with a diameter of 6,792km. However its mass is only a tenth of Earth’s with gravity on the surface being around 37% that of Earth’s.
- It is comprised of mostly (95%) carbon dioxide. Mars is very cold. Not only is it about 1.5 times further from the Sun than Earth, it also has a thin atmosphere which cannot store much heat. Because of this the temperature ranges from about -87 degrees C in winter up to a maximum of -5 degrees C in summer.
- Phobes and Demos are satellites of mars.
- Jupiter is the fifth closest planet to the Sun and is the first outer planet.
- It is the largest planet of the solar system.
- Jupiter orbits the Sun once every 12 years (at about 780 million km) and is comprised of gas (75% hydrogen and 24% helium) and is presumed to have a rocky core surrounded by a sea of liquid metallic hydrogen which forms a ball 110,000km in diameter. Jupiter’s total diameter is 142,984 km.
- Jupiter is also known as winter planet as its average temperature is very low (-148°C)
- Considering its size, Jupiter rotates very quickly at one rotation at just under once every 10 hours.
- Jupiter has many satellites (more than 60) but most of these are quite small (less than 10km diameter). The four largest satellites (Io, Europa, Ganymede and Callisto) which were discovered by Galileo Galilei in 1610 are named after the lovers of Zeus.
- Saturn is the sixth closest planet to the Sun. It is the second largest planet in the solar system having a radius 9 times that of Earth (57,000 km) and a mass 95 times that of Earth.
- Saturn orbits the Sun once very 29 years (at about 1400 million km) and is mainly comprised of gas (96% hydrogen and 3% helium) and is presumed to have a rocky core surrounded by a sea of liquid metallic hydrogen which forms a ball some 56,000km in diameter.
- Saturn's core is quite hot (11,700 degrees C) and it generates more heat than it receives from the Sun.
- Saturn rotates at around 10 hours 39 minutes.
- Saturn is of course best known for its ring system. These were first seen by Galileo Galilei in 1610.
- The rings extend from 7000 km to 120,000 km above the surface of Saturn. Incredibly, they are estimated at being between 1 km to 10 meters thick comprising of mainly ice particles ranging in size from dust to boulders a few meters across.
- The gaps in the rings are caused by the gravitational effects of Saturn's moons, and also by larger "moonlets" which inhabit the rings causing particles to be nudged into banded orbits.
- Saturn has 62 moons with only seven being large enough to become spherical in shape.
- Titan, the satellite of Saturn, is the largest satellite in the solar system.
- Uranus is the seventh closest planet to the Sun and the third largest and fourth heaviest of the planets.
- It diameter (50,000km) is four times that of Earth with a mass over 14 times that of Earth.
- Uranus orbits the Sun once very 84 years (at about 2900 million km) but is unusual in that it spins on its side (with an axial tilt of 97 degrees). This means that its moons and also its faint ring system also orbit in plane perpendicular to the plane of the ecliptic.
- Uranus is surrounded by 5 rings namely, alpha, beta, gamma, delta and epsilon.
- It is believed to be comprised of a small rocky core surrounded by a deep mantle of water, ammonia and methane. This is in turn surrounded by an atmosphere of hydrogen, helium and methane with an upper cloud layer.
- A temperature of -224 degrees C has been measured in Uranus' atmosphere - the coldest in the solar system.
Uranus has the second most extensive ring system of the solar system after Saturn. The rings, which are very difficult to see from ground based observations, were first discovered in 1977 by measuring the intensity of a star as Uranus and its rings passed in front of it. There are 13 known rings with radii of 38,000km to 98,000km. They are comprised of ice and some darker material which results in them being much darker than the rings of Saturn.
Uranus has 27 known moons with sizes ranging from over 1500 km diameter down to under 20km. The moons consist of ice, rock and other trace elements. Some of the inner moons undergo gravitation interactions with each other which may in many millions of years lead to instabilities and collisions.
- Neptune is the eighth closest planet to the Sun and is (since the relegation of Pluto) the last Planet in the Solar System. It is similar in size and composition to Uranus with a diameter of 49,000km and a mass of over 17 times that of Earth.
- Uranus orbits the Sun once every 165 years at about 4500 million km. This is 30 times the distance from the Sun to the Earth which means that the strength of sunlight at Neptune is 1% of its strength at Earth.
- Neptune spins on an axis with a tilt of 28 degrees, which is quite similar to that of the Earth, every 16 hours.
- Wind speeds of up to 600m/s have also been observed.
- Due to its great distance from the Sun, it is not surprising that the planet is one of the coldest with temperatures of -220 degrees C in the upper atmosphere. The core is thought to be at around 5200 degrees C.
Neptune has a ring system comprised of three main rings with radii between 63,000 km and 42,000km which appear to have a clumpy structure in which they form arcs rather than complete rings. It is believed that gravitational effects from Neptune's moons may cause the clumping.
Moons - Triton
Uranus has 13 known moons of which Triton (2700km diameter) is the most significant. The next largest moon, Proteus, is only 420km in diameter. Triton is the only large moon to orbit in retrograde (e.g. in the opposite direction to most orbits) and is thought to be a captured object rather than created in orbit.