Brief Galactic Overview

The Milky Way is a galaxy that contains at least 200 billion stars. It is an average-size galaxy of the barred
spiral class. This means that there is a central bulge, with arms, which extend for many lightyears. However,
the term "barred" means that its central bulge is elongated; in the Milky Way, this elongation is at a ratio of
2:3, meaning that it is about one and a half times as long as it is wide.

The Milky Way has five distinct arms protruding from the central bulge. Their names are as follows: the Cygnus
Arm, Centarus Arm, Sagittarius Arm, Orion Arm, and Perseus Arm. Our Solar System lies in the Orion Arm.
The Milky Way belongs to a group of galaxies that is known as the Local Group. The Local Group contains about
30 galaxies, two of which are the Andromeda Galaxy and the Triangulum Galaxy. The Local Group is 6 million
light-years (5.676*1019 km) in diameter.

In turn, the Local Group belongs to a much larger group of galaxies known as the Virgo Supercluster. The Milky
Way lies on the outer edge of the supercluster. The Virgo Supercluster is about 100 million light-years
(9.46*1020 km) in diameter.

The Milky Way is 100,000 light-years (9.46*1017 km) in diameter. The Milky Way takes approximately 240 million
years to rotate, clockwise, even though the arm that we are in is whipping around the center at about 465,000
miles an hour. At that speed, a space ship from Earth would only need a half an hour to get to the moon!
The known universe consists of about 200 to 300 billion galaxies.


Solar System

The solar system formed from a giant cloud of dust and gas. Over 99.8% of the mass ended up in the Sun, our
nearest stellar neighbor. Most of the rest was distributed into the nine planets - Mercury, Venus, Earth, Mars,
Jupiter, Saturn, Uranus, Neptune and Pluto - with Jupiter comprising over twice as much mass as the rest of the
planets put together.

This was the view of our solar system until 1801 when the first asteroid was discovered. Over two hundred years
later, over 300,000 bits of rocky debris left over from the formation of our solar system have been found; some
have been discovered to lie over 13 times farther from the Sun than Pluto. Nearly a century before the first
asteroid was found, comets were realized to be phenomena that occur outside of Earth's atmosphere and that they
are small rock-ice balls that orbit the Sun just like everything else in the solar system.

Our understanding of our local region of space has evolved significantly over the past few centuries, and it
continues to change today. In 1992, the first object that spends its entire year beyond Pluto - a Kuiper Belt
Object - was found. In 2005, a Kuiper Belt Object larger than Pluto was discovered. Space probes operating at
and on Mars in the past few years have completely revised our understanding of Mars, and in 2004, the first probe
to sample particles from the Sun returned to Earth.

Our current picture of the solar system has one central star, eight main planets and their associated moons, a
large number of asteroids between Mars and Jupiter and more asteroids that lie throughout the solar system, and
then a gradually increasing disk of asteroids and comets beyond the orbit of Pluto in the Kuiper Belt. Beyond
that lies the Oort Cloud, which is a hypothesized cloud of comets that might extend half-way to the next star.


Our Star, The Sun.

Basic Statistics.
Mass                          1.989 * 1030 kg
Volume                        1.412 * 1018 kg3
Average radius                6.963 * 105 km
Average density               1,408 * kgm3
Surface gravity at equator    274.0 m/s2
Escape velocity               617.7 km/s
Rotational rate at equator    609.12 hours
Rotational rate at poles      ~936 hours


Overview of Planets

As the solar system emerged from its parent nebula, a young star was surrounded by a disk of dust and gas that
was starting to clump in various places. As these clumps grew larger, they collapsed under their mutual gravity
and to accrete (accumulate) more matter. Clumps closer to the sun were deprived of a lot of gas because the young
star's radiation forced most of the lighter materials outward into what would become the outer solar system.
While the inner planets became mostly rocky, the outer planets were able to accrete much more gas and to evolve
voluminous atmospheres.

Eventually, the sun's wind blew most of the remaining material away, and the new planets could grow no larger.
Mercury, Venus, Earth, and Mars became the rocky,terrestrial planets of the inner solar system, while Jupiter,
Saturn, Uranus, and Neptune became the gas giants of the outer solar system. Pluto, the "planet" beyond Neptune,
is not gaseous like the other planets of the outer solar system, nor is it rocky like those of the inner; it is
actually most like a giant comet.

Left-over building blocks were herded into the region between Mars and Jupiter to become the asteroid belt. Other
material, farther from the Sun so able to retain more volatile components, formed the comets of the Kuiper Belt
and Oort cloud

Even though there was no longer much planet-building material left, the solar system looked different than it
does today. Rogue asteroids roamed the solar system, as evidenced by the crater patterns on the Earths Moon. The
interplanetary asteroids frequently struck planets and moons until approximately 3.8 billion years ago. After
that period, most of the rogue asteroids had either been ejected from the solar system, captured as moons, struck
planets or moons, or been shepherded into relatively stable orbits, mostly between Mars and Jupiter in the
asteroid belt.

Since then, the solar system has been relatively calm, though each object in it has had a unique evolution that
is explored in the pages of this section.

What is a Planet?

Historically, the solar system was composed of Mercury, Venus, Earth, Mars, Jupiter, Saturn, the Sun, and a
smattering of moons. Then comets were realized and asteroids were discovered, and Uranus and Neptune were found.
The solar system was still pretty cut-and-dry: There was a star, around which there were eight planets. Between
the fourth and fifth was a belt of asteroids that were tiny bodies significantly smaller than any planet. Around
most planets were moons.

Pluto's discovery in 1930 and its subsequent classification as a planet was not questioned because another planet
was expected to be there. But lately, many more objects have been discovered around, within, and outside of
Pluto's orbit. For this reason, and for revised theories of how the solar system formed and what objects it
should contain far from the Sun, many astronomers throughout the 1990s and early 2000s no longer considered Pluto
to be a planet, but rather was the "King Comet" of the Kuiper Belt. Then, it was announced in Summer 2005 by a
group led by Michael Brown that an object they had discovered in 2003 (2003 UB313, now named "Eris") was in fact
larger than Pluto.

Consequently, after two years of review by a sub-committee, the August 2006 meeting of International Astronomical
Union (IAU) passed a resolution that defines a lower limit for what a planet is. The definition is liked by very
few, it has many issues with it, and it will probably be changed within the next few years.
The new definition, first of all, is really only applicable to our Solar System. It also only sets a lower limit,
not an upper limit, which means that there is still an issue with how large a planet can get before it is no
longer considered to be a planet*. Originally, the definition listed four criteria for an object to be classified
as a planet:

      1.	The object must be large enough to have spherized due to its own gravity.
      2.	It must orbit the Sun.

However, this proposed definition led to a huge backlash from mainly the astronomers who study solar system
dynamics. It also increased the number of planets from 9 to 12 by adding the largest astroid, Ceres, keeping
Pluto, adding its moon Charon, and adding Eris. Also, weird things could happen over time, such as Earth’s
moonwould eventually become a planet because, since the moon is slowly moving away from Earth, the center of mass
of the Earth-Moon system would eventually be outside Earth's surface, so the moon would be considered to be
orbiting the Sun, not Earth. Thus, another criterion was added:

      3.	The object must dominate its orbit.
 
This removes Pluto because it crosses Neptune's orbit - a planet over 10,000 times more massive than it. This
would also mean that Earth's moon would never become a planet because Earth dominates the orbit.

But, there are still problems with this definition, mainly with the first and third point. It is not defined how
spherical an object must be to be considered a "sphere" in this sense. Ceres is now considered a Dwarf planet,
but its axes only differ by about 30 km (about 3%). Jupiter, however, is very squashed, where its axes differ by
a factor of about 6.5%, and the bulge at its equator is clearly visible in a telescope or even high-powered
binoculars.

A problem with the third aspect of this definition is what does it mean to "dominate?" All planets have asteroids
that cross their path, so no planet orbits alone. Jupiter, which must be a planet by any sense of the word, has
over 600 Trojan asteroidsthat precede and follow it in its orbit, so it has not cleared its orbit. And Pluto
crosses Neptune's orbit, so should Neptune not be a planet because it has this giant ball of icy rock that
crosses its path.

These are all questions that have led many to denounce the Resolution. But besides astronomers, there was much
public outcry, with polls showing that up to 75% of the population believed that Pluto should still be a planet.
The public likes an underdog, and Pluto had always (at least for 75 years, anyway) been that. Childrens' letters
to NASA begged them not to change Pluto's status, and many believed that Pluto should be "grandfathered" in as a
planet because it had been considered one since its discovery. And, every encyclopedia and textbook was made out
of date because of the new classification, making only eight planets in the solar system.
 
But others see this as a learning opportunity, and it is a chance to educate people on what it means to actually
have a "definition" (after all, Pluto hasn't changed in any way, shape, nor form just because the IAU says it is
no longer a planet), and about the dynamic nature of astronomy.
 
*Extra Solar planets that have been discovered since 1995 beg for an upper-mass cut-off as well. This has been,
relatively unofficially, put at about 13 times the mass of Jupiter. Anything larger is generally considered to
be a brown dwarf - a "failed star" that wasn't able to accrete enough mass to start nuclear fusion in its core.












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