The Solar System

Overview of the Solar System Basics Source: Nine Planets - A Multimedia Tour of the Solar System * By Bill Arnett

The solar system consists of the Sun, the nine planets, about 90 satellites of the planets, a large number of small bodies (the comets and asteroids), and the interplanetary medium.

The nine planets are MercurySaturn VenusUranus EarthNeptune MarsPluto Jupiter

The orbits of the planets are ellipses with the Sun at one focus, though all except Mercury and Pluto are very nearly circular. The orbits of the planets are all more or less in the same plane (called the ecliptic and defined by the plane of the Earth's orbit).

How do they orbit? They all orbit in the same direction (counter-clockwise looking down from above the Sun's north pole); all but Venus, Uranus and Pluto also rotate in that same sense.

The Nine Planets

Classification Traditionally, the solar system has been divided into planets (the big bodies orbiting the Sun), their satellites (a.k.a. moons, variously sized objects orbiting the planets), asteroids (small dense objects orbiting the Sun) and comets (small icy objects with highly eccentric orbits).

The Big Questions What is the origin of the solar system? It is generally agreed that it condensed from a nebula of dust and gas. But the details are far from clear.

Big questions, continued What conditions allow the formation of terrestrial planets? It seems unlikely that the Earth is totally unique, but we still have no direct evidence one way or the other.

Big questions, continued  Is there life elsewhere in the solar system? If not, why is Earth special?  Is there life beyond the solar system? Intelligent life?  Is life a rare and unusual or even unique event in the evolution of the universe or is it adaptable, widespread and common?

The Earth

Earth is the third planet from the Sun and the fifth largest. orbit: 149,600,000 km (1.00 AU) from Sun diameter: 12,756.3 km mass: 5.972e24 kg

The Earth is divided into several layers which have distinct chemical and seismic properties

Earth’s division (depths in km): Crust Upper mantle Transition region Lower mantle D'' layer Outer core Inner core

The crust varies considerably in thickness; it is thinner under the oceans, thicker under the continents. The inner core and crust are solid; the outer core and mantle layers are plastic or semi-fluid.

Most of the mass of the Earth is in the mantle, most of the rest in the core; the part we inhabit is a tiny fraction of the whole

Parts of earth (values below x10^24 kilograms): atmosphere = oceans = crust = mantle = outer core = inner core =

71% of the Earth's surface is covered with water. Earth is the only planet on which water can exist in liquid form on the surface.

Liquid water is essential for life as we know it. The heat capacity of the oceans is also very important in keeping the Earth's temperature relatively stable.

Liquid water is also responsible for most of the erosion and weathering of the Earth's continents, a process unique in the solar system today (though it may have occurred on Mars in the past).

The Earth's atmosphere is 77% nitrogen, 21% oxygen, with traces of argon, carbon dioxide and water.

The tiny amount of carbon dioxide resident in the atmosphere at any time is extremely important to the maintenance of the Earth's surface temperature via the greenhouse effect.

The greenhouse effect raises the average surface temperature about 35 degrees C above what it would otherwise be (from a frigid -21 C to a comfortable +14 C); without it the oceans would freeze and life as we know it would be impossible.

The Moon

is the only natural satellite of Earth: orbit: 384,400 km from Earth diameter: 3476 km mass: 7.35e22 kg

The moon is the second brightest object in the sky after the Sun. As the Moon orbits around the Earth once per month, the angle between the Earth, the Moon and the Sun changes; we see this as the cycle of the Moon's phases. The time between successive new moons is 29.5 days (709 hours)

The Moon was first visited by the Soviet spacecraft Luna 2 in It is the only extraterrestrial body to have been visited by humans. The first landing was on July 20, 1969; the last was in December The Moon is also the only body from which samples have been returned to Earth.

The gravitational forces between the Earth and the Moon cause some interesting effects. The most obvious is the tides.

The Moon's gravitational attraction is stronger on the side of the Earth nearest to the Moon and weaker on the opposite side. Since the Earth, and particularly the oceans, is not perfectly rigid it is stretched out along the line toward the Moon.

The Moon has no atmosphere. But evidence from suggested that there may be water ice in some deep craters near the Moon's south pole which are permanently shaded. There is apparently ice at the north pole as well.

A piece of the moon

The End