Inner Planets


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 427 ºC, and then the side in shadow goes down to -183 ºC. Mercury is also the smallest planet in the Solar System, measuring just 4879 km across at its equator.


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 480 °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.


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. About 70 percent of the Earth's surface is water-covered, and the oceans hold about 96.5 percent of all Earth's water.
Earth has only a single moon… the Moon.


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.