|The Solar System
For I dipped into the Future, far
as human eye could see; saw the vision of the world, and
all the wonder that would be. -Alfred Lord Tennyson, 1842
Our solar system
consists of an average star we call the Sun, the planets Mercury,
Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune, and Pluto.
It includes:the satellites of the planets; numerous comets, asteroids, and meteoroids; and the interplanetary medium. The Sun is the richest source of electromagnetic energy (mostly in the form of heat and light) in the solar system.
The Sun's nearest known stellar neighbor is a red dwarf star called Proxima Centauri, at a distance of 4.3 light years away. The whole solar system,together with the local stars visible on a clear night, orbits the center of our home galaxy, a spiral disk of 200 billion stars we call the Milky Way. The Milky Way has two small Galaxies orbiting it nearby, which are visible from the southern hemisphere. They are called the Large Magellanic Cloud and the Small Magellanic Cloud. The nearest large galaxy is the Andromeda Galaxy. It is a spiral galaxy like the Milky Way but is 4 times as massive
and is 2 million light years away. Our galaxy, one of billions of
galaxies known, is traveling through intergalactic space.
The planets, most of the satellites of the planets
and the asteroids revolve around the Sun in the same direction,
in nearly circular orbits. When looking down from above the Sun's
north pole, the planets orbit in a counter-clockwise direction.
The planets orbit the Sun in or near the same plane, called the
ecliptic. Pluto is a special case in that its orbit is the
most highly inclined (18 degrees) and the most highly elliptical
of all the planets. Because of this, for part of its orbit, Pluto
is closer to the Sun than is Neptune. The axis of rotation for most
of the planets is nearly perpendicular to the ecliptic. The exceptions
are Uranus and Pluto, which are tipped on their sides.
The Sun contains 99.85% of all the matter in the
Solar System. The planets, which condensed out of the same disk
of material that formed the Sun, contain only 0.135% of the mass
of the solar system. Jupiter contains more than twice the matter
of all the other planets combined. Satellites of the planets, comets,
asteroids, meteoroids, and the interplanetary medium constitute
the remaining 0.015%. The following table is a list of the mass
distribution within our Solar System.
- Sun: 99.85%
- Planets: 0.135%
- Comets: 0.01% ?
- Satellites: 0.00005%
- Minor Planets: 0.0000002% ?
- Meteoroids: 0.0000001% ?
- Interplanetary Medium: 0.0000001%
Nearly all the solar system by volume appears to
be an empty void. Far from being nothingness, this vacuum of "space"
comprises the interplanetary medium. It includes various forms of
energy and at least two material components: interplanetary dust
and interplanetary gas. Interplanetary dust consists of microscopic
solid particles. Interplanetary gas is a tenuous flow of gas and
charged particles, mostly protons and electrons -- plasma -- which
stream from the Sun, called the solar wind.
The solar wind can be measured by spacecraft, and
it has a large effect on comet tails. It also has a measurable effect
on the motion of spacecraft. The speed of the solar wind is about
400 kilometers (250 miles) per second in the vicinity of Earth's
orbit. The point at which the solar wind meets the interstellar
medium, which is the "solar" wind from other stars, is called the
heliopause. It is a boundary theorized to be roughly circular or
teardrop-shaped, marking the edge of the Sun's influence perhaps
100 AU from the Sun. The space within the boundary of the heliopause,
containing the Sun and solar system, is referred to as the heliosphere.
The solar magnetic field extends outward into interplanetary
space; it can be measured on Earth and by spacecraft. The solar
magnetic field is the dominating magnetic field throughout the interplanetary
regions of the solar system, except in the immediate environment
of planets which have their own magnetic fields.
The terrestrial planets are the four innermost
planets in the solar system, Mercury, Venus
, Earth and Mars. They
are called terrestrial because they have a compact, rocky surface
like the Earth's. The planets, Venus, Earth, and Mars have significant
atmospheres while Mercury has almost none. The following diagram
shows the approximate distance of the terrestrial planets to the
, Saturn, Uranus,
and Neptune are known as the Jovian (Jupiter-like) planets, because
they are all gigantic compared with Earth, and they have a gaseous
nature like Jupiter's. The Jovian planets are also referred to as
the gas giants
, although some or all of them might have small
solid cores. The following diagram shows the approximate distance
of the Jovian planets to the Sun.
Our Milkyway Galaxy
This image of our galaxy, the Milky Way, was taken with NASA's Cosmic
Background Explorer's (COBE) Diffuse Infrared Background Experiment
(DIRBE). This never-before-seen view shows the Milky Way from an
edge-on perspective with the galactic north pole at the top, the
south pole at the bottom and the galactic center at the center.
The picture combines images obtained at several near-infrared wavelengths.
Stars within our galaxy are the dominant source of light at these
wavelengths. Even though our solar system is part of the Milky Way,
the view looks distant because most of the light comes from the
population of stars that are closer to the galactic center than
our own Sun.
Andromeda Galaxy, M31
The Andromeda Galaxy, M31, is located 2.3 million light years away,
making it the nearest major galaxy to our own Milky Way. M31 dominates
the small group of galaxies (of which our own Milky Way is a member),
and can be seen with the naked eye as a spindle-shaped "cloud" the
width of the full Moon. Like the Milky Way, M31 is a giant spiral-shaped
disk of stars, with a bulbous central hub of older stars. M31 has
long been known to have a bright and extremely dense grouping of
a few million stars clustered at the very center of its spherical
Obliquity of the Nine Planets
This illustration shows the obliquity of the nine planets. Obliquity
is the angle between a planet's equatorial plane and its orbital
plane. By International Astronomical Union (IAU) convention, a planet's
north pole lies above the ecliptic plane. By this convention, Venus,
Uranus, and Pluto have a retrograde rotation, or a rotation that
is in the opposite direction from the other planets. (Copyright
1999 by Calvin J. Hamilton)
The Solar System
During the past three decades a myriad of space explorers have escaped
the confines of planet Earth and have set out to discover our planetary
neighbors. This picture shows the Sun and all nine planets of the
solar system as seen by the space explorers. Starting at the top-left
corner is the Sun followed by the planets Mercury, Venus, Earth,
Mars, Jupiter, Saturn, Uranus, Neptune, and Pluto. (Copyright
1998 by Calvin J. Hamilton)
Sun and Planets
This image shows the Sun and nine planets approximately to scale.
The order of these bodies are: Sun, Mercury, Venus, Earth, Mars,
Jupiter, Saturn, Uranus, Neptune, and Pluto. (Copyright Calvin
This image shows the Jovian planets Jupiter, Saturn, Uranus and
Neptune approximately to scale. The Jovian planets are named because
of their gigantic Jupiter-like appearance.
The Largest Moons and Smallest Planets
This image shows the relative sizes of the largest moons and the
smallest planets in the solarsystem. The largest satellites pictured
in this image are: Ganymede (5262 km), Titan (5150 km), Callisto
(4806 km), Io (3642 km), the Moon (3476 km), Europa (3138 km), Triton
(2706 km), and Titania (1580 km). Both Ganymede and Titan are larger
than planet Mercury followed by Io, the Moon, Europa, and Triton
which are larger than the planet Pluto.
Diagram of Portrait Frames
On February 14, 1990, the cameras of Voyager 1 pointed back toward
the Sun and took a series of pictures of the Sun and the planets,
making the first ever "portrait" of our solar system as seen from
the outside. This image is a diagram of how the frames for the solar
system portrait were taken.
All Frames from the Family Portrait
This image shows the series of pictures of the Sun and the planets
taken on February 14, 1990, for the solar system family portrait
as seen from the outside. In the course of taking this mosaic consisting
of a total of 60 frames, Voyager 1 made several images of the inner
solar system from a distance of approximately 6.4 billion kilometers
(4 billion miles) and about 32° above the ecliptic plane. Thirty-nine
wide angle frames link together six of the planets of our solar
system in this mosaic. Outermost Neptune is 30 times further from
the Sun than Earth. Our Sun is seen as the bright object in the
center of the circle of frames. The insets show the planets magnified
Portrait of the Solar System
These six narrow-angle color images were made from the first ever
"portrait" of the solar system taken by Voyager 1, which was more
than 6.4 billion kilometers (4 billion miles) from Earth and about
32° above the ecliptic. Mercury is too close to the Sun to be seen.
Mars was not detectable by the Voyager cameras due to scattered
sunlight in the optics, and Pluto was not included in the mosaic
because of its small size and distance from the Sun. These blown-up
images, left to right and top to bottom are Venus, Earth, Jupiter,
Saturn, Uranus, and Neptune.
The following table lists statistical information
for the Sun and planets:
* The Sun's period of rotation at the surface varies
from approximately 25 days at the equator to 36 days at the poles.
Deep down, below the convective zone, everything appears to rotate
with a period of 27 days.