The Planets (plus the Dwarf Planet Pluto)

Our solar system consists of the sun, eight planets, moons, many dwarf planets (or plutoids), an asteroid belt, comets, meteors, and others. The sun is the center of our solar system; the planets, their moons, a belt of asteroids, comets, and other rocks and gas orbit the sun.

The eight planets that orbit the sun are (in order from the sun): Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune. Another large body is Pluto, now classified as a dwarf planet or plutoid. A belt of asteroids (minor planets made of rock and metal) lies between Mars and Jupiter. These objects all orbit the sun in roughly circular orbits that lie in the same plane, the ecliptic (Pluto is an exception; it has an elliptical orbit tilted over 17° from the ecliptic).

Easy ways to remember the order of the planets (plus Pluto) are the mnemonics: “My Very Excellent Mother Just Sent Us Nine Pizzas” and “My Very Easy Method Just Simplifies Us Naming Planets” The first letter of each of these words represents a planet - in the correct order.

The largest planet is Jupiter. It is followed by Saturn, Uranus, Neptune, Earth, Venus, Mars, Mercury, and finally, tiny Pluto (the largest of the dwarf planets). Jupiter is so big that all the other planets could fit inside it.

The Inner Planets vs. the Outer Planets

The inner planets (those planets that orbit close to the sun) are quite different from the outer planets (those planets that orbit far from the sun).

• The inner planets are: Mercury, Venus, Earth, and Mars. They are relatively small, composed mostly of rock, and have few or no moons.
• The outer planets include: Jupiter, Saturn, Uranus, Neptune, and Pluto (a dwarf planet). They are mostly huge, mostly gaseous, ringed, and have many moons (again, the exception is Pluto, the dwarf planet, which is small, rocky, and has four moons).

Temperatures on the Planets

Generally, the farther from the Sun, the cooler the planet. Differences occur when the greenhouse effect warms a planet (like Venus) surrounded by a thick atmosphere.

Density of the Planets

The outer, gaseous planets are much less dense than the inner, rocky planets.

The Earth is the densest planet. Saturn is the least dense planet; it would float on water.

The Mass of the Planets

Jupiter is by far the most massive planet; Saturn trails it. Uranus, Neptune, Earth, Venus, Mars, and Pluto are orders of magnitude less massive.

Gravitational Forces on the Planets

The planet with the strongest gravitational attraction at its surface is Jupiter. Although Saturn, Uranus, and Neptune are also very massive planets, their gravitational forces are about the same as Earth. This is because the gravitational force a planet exerts upon an object at the planet’s surface is proportional to its mass and to the inverse of the planet’s radius squared.

A Day on Each of the Planets

A day is the length of time that it takes a planet to rotate on its axis (360°). A day on Earth takes almost 24 hours.

The planet with the longest day is Venus; a day on Venus takes 243 Earth days. (A day on Venus is longer than its year; a year on Venus takes only 224.7 Earth days).

The planet with the shortest day is Jupiter; a day on Jupiter only takes 9.8 Earth hours! When you observe Jupiter from Earth, you can see some of its features change.

The Average Orbital Speed of the Planets

As the planets orbit the Sun, they travel at different speeds. Each planet speeds up when it is nearer the Sun and travels more slowly when it is far from the Sun (this is Kepler’s Second Law of Planetary Motion).

The Planets in Our Solar System

Planet (or Dwarf Planet)	Distance from the Sun (Astronomical Units/miles/km)	Period of Revolution Around the Sun (1 planetary year)	Period of Rotation (1 planetary day)	Mass (kg)	Diameter (miles/km)	Apparent size from Earth	Temperature (Kelvin)	Number of Moons
Mercury	0.39 AU 36 million miles 57.9 million km	87.96 Earth days	58.7 Earth days	3.3 x 1023	3,031 mi 4,878 km	5-13 arc seconds	100-700 K mean: 452 K	0
Venus	0.723 AU 67.2 million mi 108.2 million km	224.68 Earth days	243 Earth days	4.87 x 1024	7,521 mi 12,104 km	10-64 arc seconds	726 K	0
Earth	1 AU 93 million mi 149.6 million km	365.26 days	24 hours	5.98 x 1024	7,926 mi 12,756 km	N/A	260-310 K	1
Mars	1.524 AU 141.6 million mi 227.9 million km	686.98 Earth days	24.6 Earth hours (1.026 Earth days)	6.42 x 1023	4,222 miles 6,787 km	4-25 arc seconds	150-310 K	2
Jupiter	5.203 AU 483.6 million mi 778.3 million km	11.862 Earth years	9.84 Earth hours	1.90 x 1027	88,729 mi 142,796 km	31-48 arc seconds	120 K (cloud tops)	67 (18 named plus many smaller ones)
Saturn	9.539 AU 886.7 million mi 1,427.0 million km	29.456 Earth years	10.2 Earth hours	5.69 x 1026	74,600 mi 120,660 km	15-21 arc seconds (excluding rings)	88 K	62 (30 unnamed)
Uranus	19.18 AU 1,784.0 million mi 2,871.0 million km	84.07 Earth years	17.9 Earth hours	8.68 x 1025	32,600 mi 51,118 km	3-4 arc seconds	59 K	27 (6 unnamed)
Neptune	30.06 AU 2,794.4 million mi 4,497.1 million km	164.81 Earth years	19. Earth hours	1.02 x 1026	30,200 mi 48,600 km	2.5 arc seconds	48 K	13
Pluto (a dwarf planet)	39.53 AU 3,674.5 million mi 5,913 million km	247.7 years	6.39 Earth days	1.29 x 1022	1,413 mi 2,274 km	0.04 arc seconds	37 K	4

Another Planet?

In 2005, a large object beyond Pluto was observed in the Kuiper belt.

A few astronomers think that there might be another planet or companion star orbiting the Sun far beyond the orbit of Pluto. This distant planet/companion star may or may not exist. The hypothesized origin of this hypothetical object is that a celestial object, perhaps a hard-to-detect cool, brown dwarf star (called Nemesis), was captured by the Sun’s gravitational field. This planet is hypothesized to exist because of the unexplained clumping of some long-period comet’s orbits. The orbits of these far-reaching comets seem to be affected by the gravitational pull of a distant, Sun-orbiting object.

Planet Activities and Quizzes

• Planet Coloring pages
• An interactive puzzle on the Solar System.
• Find It!, a quiz on the planets.
• A fill-in-the-blank (cloze) activity on the Solar System - or go to the answers.
• Solar System Model to make.
• Solar System calendar to print out and color.
• Solar System Crafts
• How to write a report on a planet - plus a rubric.