Our solar system
consists of the Sun , the planets
It includes: the satellites of the planets; numerous
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
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% ?
Minor Planets: 0.0000002% ?
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
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,
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 Sun.
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.
The following table
lists statistical information for the Sun and
* 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.