Astrology - Planets

Welcome back to Astronomy, students! In this lesson, we will be moving on to planets. What is a Planet?
This seemingly simple question doesn't have a simple answer. Everyone knows that Earth, Mars and Jupiter are planets. At least, they are for now. Both Pluto and Ceres were once considered planets until new discoveries triggered scientific debate about how to best describe them—a vigorous debate that continues to this day. The most recent definition of a planet was adopted by the International Astronomical Union in 2006. It says a planet must do three things:

It must orbit a star (in our cosmic neighborhood, the Sun).
It must be big enough to have enough gravity to force it into a spherical shape.
It must be big enough that its gravity cleared away any other objects of a similar size near its orbit around the Sun.

Discussion — and debate — will continue as our view of the cosmos continues to expand. For brevity's sake, we will only discuss the planets in our own solar system during this class.
The smallest planet in our solar system and nearest to the Sun, Mercury is only slightly larger than Earth's moon. From the surface of Mercury, the Sun would appear more than three times as large as it does when viewed from Earth, and the sunlight would be as much as 11 times brighter. Despite its proximity to the Sun, Mercury is not the hottest planet in our solar system—that title belongs to nearby Venus—but it is the fastest, zipping around the Sun every 88 Earth days. The Muggle's National Aeronautics and Space Administration states that Mercury was not discovered, but rather, "known by the ancients." Mercury is approximately 37.2 million miles from the Sun.

The second planet from the Sun and our closest planetary neighbor, Venus is similar in structure and size to Earth, but it is now a very different world. 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. Venus is approximately 67 million miles from the Sun, and has a year length of 225 Earth days.

If the Sun were as tall as a typical front door, Earth would be the size of a dime, and Mars would be about as big as an aspirin tablet. Mars is the fourth planet from the Sun at an average distance of about 142 million miles. One day on Mars takes a little over 24 hours. Mars makes a complete orbit around the Sun (a year in Martian time) in 687 Earth days.

The fifth planet from the Sun, and the most massive in our solar system, Jupiter has a long history surprising scientists—all the way back to 1610 when Galileo Galilei found the first moons beyond Earth. That discovery changed the way we see the universe. Jupiter is 501.8 million miles from the sun, and orbits the sun in 4,333 Earth days.

Saturn is the sixth planet from the sun and 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 made mostly of hydrogen and helium. Saturn is approximately 935.6 million miles from the sun. One year on Saturn is equivalent to 10,759 Earth days.

The first planet found with the aid of a telescope, Uranus was discovered in 1781 by astronomer William Herschel, although he originally thought it was either a comet or a star. It was two years later that the object was universally accepted as a new planet, in part because of observations by astronomer Johann Elert Bode. Herschel tried unsuccessfully to name his discovery Georgium Sidus after King George III. Instead the planet was named for Uranus, the Greek god of the sky. Uranus is approximately 1.8 billion miles from the Sun and has a year length of 30,687 Earth days.

The last planet in our solar system is Neptune. Dark, cold and whipped by supersonic winds, ice giant Neptune is the eighth and most distant planet in our solar system. More than 30 times as far from the Sun as Earth, Neptune is the only planet in our solar system not visible to the naked eye. In 2011 Neptune completed its first 165-year orbit since its discovery in 1846. On average, Neptune is 2.7 billion miles from the Sun, and one year consists of 60,190 Earth days.