Introduction to Planet

Planet, a large, solid heavenly body that revolves around a star and shines by reflected light. The sun has eight known major planets. Many other stars are known to have one or more planets, but very little is known about these bodies. The major planets of the sun, in order of increasing mean, or average, distance from the sun, are Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune. More than 2,000 smaller planets have been cataloged; most have orbits between the orbits of Mars and Jupiter. Dwarf planets, including Pluto and Ceres and many smaller planetlike bodies, also orbit the sun.

The word planet comes from the Greek word for wanderer. The ancient Greeks considered the sun, the moon, Mercury, Venus, Mars, Jupiter, and Saturn to be planets because they appear to move among the stars. (They did not know of Uranus, which is difficult to see with the unaided eye, nor of Neptune, which cannot be seen without the aid of a telescope.) The Greeks thought that the planets move around Earth. This idea about planets remained more or less unchanged until the Polish astronomer Copernicus developed his theory, published in 1543, that Earth and the other planets revolve around the sun and that only the moon travels around Earth. Uranus was discovered in 1781 and Neptune in 1846. There is no formal definition for the term planet. Recently, the scholars have come up with a simple classification system that distinguishes the smallest worlds from the largest comets, asteroids, and other bodies.

In 2006, the International Astronomical Union (IAU), a recognized authority in assigning designations to heavenly bodies, defined the term planet. Some astronomers welcomed it while others considered the definition incomplete and therefore refused to adopt it. The IAU standard divides objects that orbit the sun into three major categories: planets, dwarf planets, and small solar system bodies. A planet orbits the sun and no other body. Due to its copious mass it gets compacted into a round shape by its own gravitational pull. A planet has a strong enough gravitational pull to remove most objects from the region of its orbit. A dwarf planet also orbits the sun and is large enough to be round. But it does not have a strong enough gravitational pull to clear its orbit of other objects. Thus many planets, dwarf planets, and other bodies have smaller objects called satellites or moons orbiting them. Smaller solar bodies, including most asteroids and comets, have too little mass for gravity to round their irregular shapes.

Since its discovery in 1930, Pluto was considered a planet. However, because of its small size and irregular orbit scientists debated whether Pluto should be grouped with bodies such as Earth and Jupiter. Pluto is similar in many respects to other icy objects found in a region of the outer solar system called the Kuiper belt. In the early 21st century, astronomers discovered several such Kuiper belt objects (KBOs) comparable in size to Pluto. The IAU thus formulated the dwarf planet classification to describe Pluto and other planet-sized objects.

Planetary Motion

Seen from a point north of the plane of Earth's orbitfor example, from the North Starthe planets travel counterclockwise in their orbits. All the orbits are nearly level with the plane of Earth's orbit, except those of Mercury and Pluto, which are more tilted. Seen with the unaided eye, the five clearly visible planets look much like stars. However, the planets do not twinkle like stars when overhead in the sky, although they may twinkle near the horizon. Through a telescope these planets can be easily distinguished from stars because they appear as discs while the stars are so far away that they appear as mere points of light.

While Copernicus correctly deduced that the planets revolve around the sun, he otherwise retained the Ptolemaic system. Planetary motion was first correctly described by Johannes Kepler.

The first two of Kepler's Laws were published in 1609, the third in 1619. They state that:

1. The orbit of every planet is an ellipse that has the sun as one of its foci.2. A straight line from the planet to the sun sweeps over equal areas of the ellipse in equal periods of time. (This means that the planet's speed is greatest when the planet is nearest the sun and lowest when it is at the point in its orbit farthest from the sun.) 3. The squares of the periods of any two planets are in the same proportion as the cubes of their mean distances from the sun. (A planet's period is the time it takes the planet to complete one orbit. Its mean distance from the sun is defined as half the length of the major axis of the ellipse.)

Kepler's Laws described planetary motion, but did not explain it. The explanation came in Isaac Newton's Principia (1687). His law of gravitation states:

Every particle of matter attracts every other particle of matter with a force proportional to the product of their masses and inversely proportional to the square of their distance apart.

His laws of motion, the three basic laws of classical mechanics, state:

1. A body at rest remains at rest, and a body in motion continues to move in a straight line at a uniform speed unless it is acted upon by some external force. 2. Change in the motion of a body is in proportion to, and in the direction of, the force causing the change. 3. The action of every force is accompanied by an equal action in the opposite direction (the reaction).

Newton's laws explain perturbations, or irregularities in orbits. They also show that Kepler's third law, to be strictly accurate, must be modified by multiplying the square of each period by the sum of the planet's mass and the sun's mass. Even without the modification, however, the law is reasonably accurate, because the planets' masses are so small compared to the sun's mass.

Terrestrial and Giant Planets

The planets are divided into two groups by their size and structurethe terrestrial planets and the giant planets.The innermost four planetsMercury, Venus, Earth, and Marsare known as terrestrial (Earth-like) planets, from the Latin word for Earth, terra. These planets, of which Earth is the largest, are rocky and comparatively small. The other Earth-like planets have 5.5 to 82 percent of Earth's mass and 38 to 95 percent of Earth's diameter. The densities of the terrestrial planets range from about four to about five and one-half times that of water. Diameters range from about 3,000 to about 8,000 miles (4,800 to 12,900 km). For the most part, the terrestrial planets are solid, being made up of stone and metals, with a relatively thin or nonexistent layer of atmosphere.

The outer four planetsJupiter, Saturn, Uranus, and Neptuneare called gas giants or Jovian (Jupiter-like) planets. These planets have no solid surfaces and consist mainly of gaseous atmospheres made up of hydrogen and helium. Their densities are low, ranging from about three-fourths to about one and one-half times that of water, and their diameters range from about 30,000 to about 89,000 miles (48,000 to 143,000 km). All but Neptune show a flattening at their poles and a bulge at their equators. In general, the giant planets seem to have small, dense cores surrounded by extremely thick layers of liquid and gas. Jupiter, Saturn, and Neptune give off more energy than they receive from the sun. Most of this extra energy takes the form of heat or infrared radiation, instead of visible light. According to some scientists, the source of some of the energy is probably the slow compression of the planets due to their own gravity.

Inferior and Superior Planets

The inferior (or inner) planets are those whose orbits are between Earth's orbit and the sun. The superior (or outer) planets have orbits beyond Earth's.

The inferior planets (Mercury and Venus) are never seen very far from the sun in the sky; Mercury's greatest elongation is about 28, Venus' is about 46. (A planet's elongation is the angle made at Earth by straight lines drawn to the planet and the sun.) When a planet is between Earth and the sun, it is at inferior conjunction; when the sun is between it and Earth, it is at superior conjunction. At both conjunctions the elongation is 0. The inferior planets show phases like the moon's. The phases cannot be seen without a telescope.

The superior planets (Mars, Jupiter, Saturn, Uranus, and Neptune) are said to be at conjunction (elongation 0) when the sun is between Earth and the planet. They are at opposition (elongation 180) when Earth is between the sun and the planet. At eastern and western quadrature, the elongation is 90. Superior planets do not show phases like the moon's. They are full both at conjunction and at opposition, and show only a slight flattening of one side at other times.

Superior planets usually move eastward as seen against the background of stars. When opposition approaches, they appear to slow down, stop, and begin moving westward. The westward motion is called retrograde motion. After a time, the planet appears to reverse its direction again and it then moves eastward until the next opposition approaches. The retrograde motion is due to Earth passing the superior planet as each travels in its orbit.

Bode's Law

Bode's Law, also known as the Bode-Titius Law, was worked out by Johann Titius in the 1760's and published in 1772 by Johann Bode. It gives the approximate distances of all the planets known at that time and of the planet Uranus, discovered in 1781. The law also indicates a planet should exist between the orbits of Mars and Jupiter, and in the early 1800's the asteroids were discovered in that region. However, the distances of the last bodies to be discoveredNeptune and Plutodeviate greatly from Bode's Law and most astronomers today do not believe that the law has any physical justification.

Bode's Law
ObjectDistance by Bode's LawActual Distance
Mercury 0.40.39
Venus 0.70.72
Earth 1.01.00
Mars 1.61.52
Main Belt of asteroids 2.82 to 3
Jupiter 5.25.20
Saturn 10.09.54
Uranus 19.619.18
Neptune 38.830.06