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The Solar System
Origins:
Many ideas have been put forward to explain how the solar system came to be. Despite the great variety
of objects in the solar system, the Sun, the planets and their moons, the asteroids, and the comets are not a
random collection of bodies but a family with a common origin. We have no evidence that anything in
our solar system came from outside except starlight and other kinds of radiation.
The most compelling theories speak of a great cloud of gas and dust that connects the formation of the
planets, the asteroids, and the comets with the formation of the Sun itself some 4.5 billion years ago.
As the gas and dust began to collapse under the force of gravity, the Sun began to form in the dense
central region. Smaller collapsing balls of gas formed at various distances away from this central region,
giving birth to the planets.
The Asteroid Belt can be thought of as the heap of raw material of a planet that never came to be. It might
have formed, if not for the gravitational tug of Jupiter. Jupiter’s hefty influence also explains why Mars is
quite a bit smaller than Earth and Venus.
The connection between star and planet formation, as well as observations of other stars with disks of
matter surrounding them, hints that billions of other stars in our galaxy might have planets. An
international planet hunt is underway and since the first discovery, evidence of over 100 so-called extrasolar planets has been found. Taken together, these two lines of thought give credence to the idea that
collapsing gas and dust form both stars and their associated kingdoms at the same time.
Bode’s Law:
The orbits traced by the paths of the planets around the Sun are not equally spaced, but they do form a
pattern. Noticing this pattern, J. B. Titius (1729-96) and J. E. Bode (1747-1826) developed a rule that
predicts the spacing of the planets in our solar system. The spacing is roughly doubled from planet to
planet.
A simple way to use Bode’s law to predict the average distance of the planets from the Sun is to begin with
the following sequence of numbers. (Note that after the number 3, each number in the sequence is double
that of its predecessor.)
0, 3, 6, 12, 24, 48, 96, 192, 384
Add 4 to each number:
4, 7, 10, 16, 28, 52, 100, 196, 388
Divide each by 10:
0.4, 0.7, 1.0, 1.6, 2.8, 5.2, 10.0, 19.6, 38.8
The Solar System
This gives a sequence of numbers representing each planet’s average distance from the Sun in
Astronomical Units (AU). One astronomical unit is equal to the average distance of the Earth from the Sun
or 93,000,000 miles. The following table shows how these predictions compare to the actual distance to
each planet.
Bode’s Law
(AU)
Actual distance
(AU)
Mercury
0.4
0.39
Venus
0.7
0.72
Earth
1.0
1.00
Mars
1.6
1.52
The Main Asteroid Belt
2.8
2.8
Jupiter
5.2
5.20
Saturn
10.0
9.54
Uranus
19.6
19.19
Neptune
38.8
30.1
Planet
As you can see, Bode’s Law quite accurately describes the distances of the planets from the Sun, with the
exception of Neptune. You might also have noticed that Bode’s Law predicts the existence of another
planet between Mars and Jupiter. Instead, this is where we find the Main Asteroid Belt. Astronomers once
thought that the Main Belt was the debris ring left over from a destroyed planet. But it’s much more
likely that such a planet was, in fact, prevented from ever forming by Jupiter’s strong gravitational pull.
There is no scientific explanation for Bode’s law, and it may well be a simple mathematical coincidence.
But it does provide a good and fun way to learn the relative distances between the planets.
Until recently our solar system was the only planetary system known to exist. This changed in 1995 when
Astronomers discovered a planet roughly half the mass of Jupiter orbiting a Sun-like star called 51 Pegasi.
Since the initial discovery, astronomers have found over 100 planets around other stars. Astronomers now
believe that planetary systems are plentiful throughout the universe.
The Solar System
Scale Model of the Solar System:
The solar system is big!
You and I are tiny compared to even the smallest planet, but the planets are very small compared to the
huge distances between them.
To help us understand these great expanses of space, it’s convenient to use a measurement called the
Astronomical Unit or AU for short. An AU is the average distance between the Earth and the Sun—about
150 million kilometers (93 million miles).
With the AU we can begin to gauge the scale of the solar system. Pluto is about 40 AU from the Sun.
Or think of it this way: If the Earth were the size of a basketball, the Sun would be a 26-meter diameter
(85-foot) sphere about three kilometers (two miles) away, and Pluto would be the size of a golf ball 110
kilometers (68 miles) away. Our solar system is a pretty big place.
Taking it one step further, the nearest star outside our solar system—Alpha Centauri—is 272,000 AU away,
or about 760,000 kilometers (470,000 miles) away using our basketball scale.
When astronomers talk about the distance to the stars, however, they prefer to measure things in lightyears. A light-year is the distance light travels in one Earth year, about 9.5 trillion kilometers (5.9 trillion
miles) or about 63,000 AU.
Alpha Centauri is 4.3 light-years from Earth.
Orbital Plane:
Our solar system is centered on a single star—the Sun—that contains almost a thousand times more stuff
than all of the other objects in the solar system put together!
If you could look down from high above the north pole of the Sun you would see planets revolving
around it in a counterclockwise direction. By the way, that’s the same direction the Sun rotates. You’ll see
that each planet’s orbit is actually an oval—or ellipse. Most of the planets, in turn, rotate around their
axes in the same counterclockwise direction as the Sun. But Venus and Uranus rotate clockwise from this
perspective.
The solar system is remarkably flat when seen edge on. Most of the planets are neatly spread out along an
imaginary, flattened disk. Astronomers say they all orbit nearly in the same plane.
Mercury deviates a little from this flat plane. And the further away from the Sun you go, the further out of
the plane of the solar system these little bodies orbit.
Most of the planets have one or more moons, the exceptions being Mercury and Venus, which don’t have
moons. The planet with the most moons in the solar system is Jupiter, with over 60 of them.
The Solar System
The planets in our solar system can be divided into three groups. The Inner Solar System consisting of
Mercury, Venus, Earth and, Mars; the Outer Solar System consisting of Jupiter, Saturn, Uranus, and
Neptune; and finally, Pluto, Eris and Ceres which are a whole new type of object called a Dwarf Planet.
The four worlds of the Inner Solar System (including Earth) are relatively small, rocky bodies. The three
worlds of the Outer Solar System are large gassy spheres and don’t have solid surfaces.
But beginning around the orbit of Neptune, a very large number of very small, very cold “dwarf planets”
wander in elongated eccentric orbits. Pluto is the largest of these. Astronomers are discovering new
members all the time.
Planets closer to the Sun move faster than those farther away. Mercury, the closest planet makes a
complete trip around the Sun in 88 days, whereas Dwarf Planet Pluto takes a whopping 248 years. The
Earth takes about 365 days—the definition of a year