Mercury is the smallest and fastest planet in the solar system. It is also the closest planet to the sun. It has no moons. Mercury is the smallest of the eight planets in our solar system. It is only a little bigger than Earth's moon. It would take more than 18 Mercurys to be as big as Earth.
Venus is the second planet from the sun and our closest planetary neighbor. Similar in structure and size to Earth, Venus spins slowly in the opposite direction most planets do. Its thick atmosphere traps heat in a runaway greenhouse effect, making it the hottest planet in our solar system with surface temperatures hot enough to melt lead. Glimpses below the clouds reveal volcanoes and deformed mountains.
Earth is the third planet from the sun and the fifth largest in the solar system. Just slightly larger than nearby Venus, Earth is the biggest of the terrestrial planets. Our home planet is the only planet in our solar system known to harbor living things. The name Earth is at least 1,000 years old. All of the planets, except for Earth, were named after Greek and Roman gods and goddesses. However, the name Earth is an English/German word, which simply means the ground.
Mars is a cold desert world. It is half the diameter of Earth and has the same amount of dry land. Like Earth, Mars has seasons, polar ice caps, volcanoes, canyons and weather, but its atmosphere is too thin for liquid water to exist for long on the surface. There are signs of ancient floods on Mars, but evidence for water now exists mainly in icy soil and thin clouds.
Jupiter is the fifth planet from our sun and the largest planet in the solar system. Jupiter's stripes and swirls are cold, windy clouds of ammonia and water. The atmosphere is mostly hydrogen and helium, and its iconic Great Red Spot is a giant storm bigger than Earth that has raged for hundreds of years.
The second largest planet in our solar system, adorned with thousands of beautiful ringlets, Saturn is unique among the planets. It is not the only planet to have rings -- made of chunks of ice and rock -- but none are as spectacular or as complicated as Saturn's. Like fellow gas giant Jupiter, Saturn is a massive ball of mostly hydrogen and helium.
Ptolemy develops a geocentric model that has the planets moving around the Earth. However, in order to account for retrograde motion, he put the planets on circles that move in circles.
Copernicus suggests a heliocentric model. His model has the planets moving around the Sun in circular orbits. This can explain retrograde motion, but his model doesn't fit all the planetary position data that well. Really, it's no better than Ptolemy's geocentric model.
Kepler proposes that the planets do not orbit in circles. Instead, they have elliptical orbits. This agrees with the observational data very well.
Newton develops a model for gravity that also says planets would have elliptical orbits.
Galileo gets a telescope and looks at the sky. He see stuff that suggests the Earth orbits the Sun.