The Terrestrial Planets
The terrestrial planets are the four innermost planets in the solar system, Mercury, Venus, Earth and Mars. They are called terrestrial because they have a compact, rocky surface like the Earth's. The planets, Venus, Earth, and Mars have significant atmospheres while Mercury has almost none.
Mercury
As you travel outward from the Sun, Mercury is the closest planet. It orbits the Sun at an average distance of 58 million km. Mercury is airless, and so without any significant atmosphere to hold in the heat, it has dramatic temperature differences. The side that faces the Sun experiences temperatures as high as 420 ºC, and then the side in shadow goes down to -173 ºC. Mercury is also the smallest planet in the Solar System, measuring just 4879 km across at its equator.
Mercury has only been visited two times by spacecraft. The first was Mariner 10, back in the mid 1970s. It wasn’t until 2008 that another spacecraft from Earth made a close flyby of Mercury, taking new images of its surface with Caloris Basin as its largest surface (1350 km in diameter). It revolves around the sound for 0.24 Earth year (or almost 3 months)
Venus
Venus is the second planet in the Solar System, and it’s an almost virtual twin of Earth in terms of size and mass. Venus orbits at an average distance of 108 million km, and completes an orbit around the Sun every 224 days. Apart from the size, though, Venus is very different from Earth. It has an extremely thick atmosphere made almost entirely of carbon dioxide that cloaks the planet and helps heat it up to 460 °C. If you could stand on the surface of Venus, you would experience 92 times the pressure of Earth’s atmosphere, with incredibly high temperatures, and poisonous clouds of carbon dioxide and sulfuric acid rain.
Several spacecraft have visited Venus, and a few landers have actually made it down to the surface to send back images of its hellish landscape. Even though there were made of metal, these landers only survived a few hours at best. Maxwell Montes is Venus’ highest point of surface (17 km) and the planet’s revolution is 0.62 Earth year.
Earth
Earth is our home; the third planet from the Sun. It orbits the Sun at an average distance of 150 million km. Earth is the only planet in the Solar System known to support life. This is because our atmosphere keeps the planet warm from the vacuum of space, but it’s not so thick that we have a runaway greenhouse effect. The Earth has a solid core of iron surrounded by a liquid outer core that generates a magnetic field that also helps protect life on Earth from the radiation of space.
No planet in the Solar System has been studied as well as Earth, both on the ground and from space. Thousands of spacecraft have been launched to study the planet, measuring its atmosphere, land masses, vegetation, water, and human impact. It revolves around the Sun for 365.25 days. Mount Everest is the highest peak (8 km above sea level)
Earth has only a single moon… the Moon.
Mars
The 4th planet from the Sun is Mars, the second smallest planet in the Solar System. It orbits the Sun at an distance of 228 million km. You might think Mars is large, but it’s a tiny world, with about half the diameter of Earth, and just 1/10th the Mass. If you could stand on the surface of Mars, you’d experience about 1/3rd Earth’s gravity. Mars has almost no atmosphere to help trap heat from the Sun, and so temperatures can plunge below -140 °C in the Martian winter. Even at the height of summer, temperatures can get up to 20 °C in the day – just barely shirt sleeve weather.
Mars has been heavily studied by spacecraft. There are rovers and landers on the surface, and orbiters flying overhead. It’s probably the likeliest place to search for life in the Solar System. It has a 24-km peak surface called Olympus Mons and revolves around the Sun for 1.88 Earth years.
Mars has two tiny asteroid-sized moons: Phobos and Deimos.
The Jovian Planets
Jupiter, Saturn, Uranus, and Neptune are known as the Jovian (Jupiter-like) planets, because they are all gigantic compared with Earth, and they have a gaseous nature like Jupiter's. The Jovian planets are also referred to as the gas giants, although some or all of them might have small solid cores.
Jupiter
Mighty Jupiter is the biggest planet in our Solar System. It’s so large, in fact, that it has 2.5 times the mass of all the rest of the planets in the Solar System combined. Jupiter orbits from the Sun at an average distance of 779 million km. Its diameter at the equator is 142,984 km across; you could fit 11 Earths side by side and still have a little room. Jupiter is almost entirely made up of hydrogen and helium, with trace amounts of other elements.
Jupiter has been visited by several spacecraft, including NASA’s Pioneer and Voyager spacecraft; Cassini and New Horizons arrived more recently. Only the Galileo spacecraft has ever gone into orbit around Jupiter, and it was crashed into the planet in 2003 to prevent it from contaminating one of Jupiter’s icy moons. Jupiter has a faint ring.
Jupiter has the most moons in the Solar System – it has 63 moons at last count.
Saturn
Saturn is the 6th planet from the Sun, and the 2nd largest planet in the Solar System. It orbits the Sun at an average distance of 1.4 billion km. Saturn measures 120,000 km across; only a little less than Jupiter. But Saturn has much less mass, and do it has a low density. In fact, if you had a pool large enough, Saturn would float!
Of course, the most amazing feature of Saturn is its rings. These are made of particles of ice ranging in size from grains of sand to the size of a car. Some scientists think the rings are only a few hundred million years old, while others think they could be as old as the Solar System itself.
Saturn has been visited by spacecraft 4 times: Pioneer 11, Voyager 1 and 2 were just flybys, but Cassini has actually gone into orbit around Saturn and has captured thousands of images of the planet and its moons.
And speaking of moons, Saturn has a total of 60 moons discovered (so far).
Uranus
Next comes Uranus, the 7th planet from the Sun. It orbits the Sun at an average distance of 2.9 billion km. Uranus measures 51,000 km across, and is the 3rd largest planet in the Solar System. While all of the planets are tilted on their axes, Uranus is tilted over almost on its side. It has an axial tilt of 98° with a narrow faint ring made up of rocky or carbonaceous material. Uranus was the first planet to be discovered with a telescope; it was first recognized as a planet in 1781 by William Herschel.
Only one spacecraft, Voyager 2, has ever visited Uranus up close. It passed by the planet in 1986, and captured the first close images.
Uranus has 27 known moons.
Neptune
Neptune is the 8th and final planet in the Solar System, orbiting at an average distance of 4.5 billion km from the Sun. It’s the 4th largest planet, measuring about 49,000 km across. It might not be as big as Jupiter, but it’s still 3.8 times larger than Earth – you could fit 57 Earths inside Neptune. Neptune is the second planet discovered in modern times. It was discovered at the same time by both Urbain Le Verrier and John Couch Adams.
Neptune has only ever been visited by one spacecraft, Voyager 2, which made a fly by in August, 1989.
Neptune has 13 known moons and the planet has narrow rings that contain concentrations of icy particles called ring arcs.
Dwarf Planets
In 2006 the International Astronomical Union (IAU) approved a new classification scheme for planets and smaller objects in our Solar System. Their scheme includes three classes of objects: "small solar system bodies" (including most asteroids and comets), the much larger planets (including Earth, Jupiter, and so on), and the new category of in-between sized "dwarf planets".
There are currently five official dwarf planets. Pluto, formerly the smallest of the nine "traditional" planets, was demoted to dwarf planet status. Ceres, the largest asteroid in the main asteroid belt between Mars and Jupiter, was also declared a dwarf planet. The three other (for now!) dwarf planets are Eris, Makemake, and Haumea. Pluto, Makemake, and Haumea orbit the Sun on the frozen fringes of our Solar System in the Kuiper Belt. Eris, also a Trans-Neptunian Object, is even further from the Sun.
rlp2010 (source:IAU)
