Overview of the Solar System

The solar system consists of the sun and anything orbiting around it.

The Sun:

The sun has over 99.8% of the mass of the entire solar system.
The sun is ¾ hydrogen and ¼ helium (by weight).
It is a hot gas ... the layer you see below is called the photosphere and is over 6,000°C (11,000°F).
Sunspots occur on the photosphere ... these can be thought of as a "magnetic storm" which are cooler than the surrounding photosphere. Each individual spot lasts about a week but about every 11 years lots of spots appear on the sun. This is known as the sunspot cycle.
Click here or here to see a current image of the sun.

Taken April 28, 2000

Click here or here to see where we are now in the sunspot cycle.
Sunspots are closely associated with eruptions on the sun known as a flares, prominences, and CME's (coronal mass ejections).
Material ejected by the sun during an active period can occasionally interact with the earth. Fortunately, the earth's magnetic field shields us from this deadly particle radiation and it gets deflected along the magnetic field towards the earth's poles. This can produce aurora (northern lights) and disrupt sensitive electronics.

from sun

(animation)

Taken in November 1992

Another aurora image

From 1645 - 1715 the sunspot cycle shut off ... known as the Maunder minimum. No one knows why! However, during that same time period, Europe when through a cooling period known as the "mini ice age" ... coincidence? Much has to be learned about a possible connection between these events.

The Terrestrial Planets

The 4 inner planets are all small, rocky and known collectively as the Terrestrial Planets.
The terrestrial planets (Mercury, Venus, Earth and Mars) have a high density due to their rocky makeup.

Mercury

Mercury looks very much like the moon and not very interesting.

Venus

Once thought to be "earth's twin" because it was about the same size as earth ... but U.S.S.R. probes in the 1960's showed it was very much different from the earth.
Venus has a perpetual cloud cover ... the Magellan space probe used radar to probe these clouds and obtain images of the surface.
Venus has no water! Its surface is over 900 degrees F due to a dense atmosphere of CO₂ (a run away greenhouse effect). Could the earth be heading towards these conditions? We discussed that possibility when we discussed the carbon cycle.
Venus shows no evidence of plate tectonics! Most of the surface shows evidence of major volcanic activity.
- The mystery is ... is the surface still active? The evidence indicates that the surface hasn't changed much in several hundred million years (based on a very uniform meteor impact pattern) ... why? How, then does it release the heat building up under the crust?
- Venus has steep mountain ranges ... it was always a mystery how they "hold themselves up" because it was always assumed the surface rocks would be more plastic due the their high temperature. But recent experiments show that even through the surface temperatures are high ... most of the water has been driven out of the surface rocks making them extremely rigid! So why doesn't plate tectonics work on Venus?
- One idea suggests that Venus has a very thick rigid crust. This acts to insulate the heat that must be trapped inside the planet (remember radioactivity?). If that is the case, this trapped heat will build to a point where the crust eventually cracks and the surface undergoes intense volcanic activity (very much like a pressure cooker blowing its lid). Once a large amount of heat escapes, the crust solidifies and goes dormant for a long time. If that is the case, Venus will re-awaken and the surface will become very active.

Mars

Mars is red (rusty) and about half the size of the earth. It has a thin atmosphere of CO2 and no liquid water at the surface. Mars is an arid planet. Mars is -63° C (-81° F) with a maximum temperature of 20° C (68° F) and a minimum of -140° C (-220° F).
Mars has a similar tilt to its axis (so it has ice caps that change in size with the seasons), and it rotates on its axis in 24.6 hours.
- This led early astronomers to speculate that "Martians" were drooling over the prospect of invading earth.
In 1976, the US landed 2 Viking probes on the surface of Mars. It took some good pictures but found no life.
Mars has huge inactive shield volcanoes ... the largest is Olympus Mons which is three time the height of Mt. Everest and has a base the area of Wisconsin.
Mars has Valles Marineris: a canyon that would stretch from LA to NY. This canyon was not formed by running water or plate tectonics but rather, as a result of a stretching crust.
Mars once had running water on its surface. This is only possible if the atmosphere of Mars was much thicker than it is today. What happened to Mars?
In 1996 a team of astronomers examined a rock from Mars (which was blasted off the surface by an impact and flung toward the earth). They found tiny nanofossils which suggest that life may have existed on Mars in its past. The debate continues today due to the discovery of similar nanofossils in earth rocks. We really will not be able to answer this question until we can return rocks from the surface of Mars (without contamination with the earth of any kind).
Does life exist on Mars today? A good place to look would be the Martian ice caps but a probe scheduled to land there in 1999 never made it :(

Asteroids

Asteroids are small rocky fragments which remind us of the state of the solar system 4.6 billion years ago. The asteroid belt is just beyond the orbit of Mars but on occasion, some asteroids get dislodged to orbits that take them close to the earth. Then they become dangerous! Space probes have close-up images of asteroids Eros, Gaspra , and Ida

The Jovian Planets

The next 4 planets are all very large and mostly gaseous. They are collectively known as the Jovian Planets (Jupiter, Saturn, Uranus, Neptune). These gas worlds have a large mass but have a low density. At such a great distance from the sun, icy planetesimals were able to exist in great numbers and the Jovian planets formed from these low density objects. Although there is probably a solid surface deep inside, these planets are essentially all atmosphere. All the Jovian planes have ring systems and many satellites.

Jupiter

The largest planet with a turbulent atmosphere.
Jupiter has a giant "red spot" ... a storm which has been seen for over 300 years.
Jupiter's moon IO has active volcanoes (so does Neptune's moon, Triton).
The moon Europa is ice covered but may have liquid water oceans flowing below the ice.

Saturn

Spectacular ring system
Saturn's moon, Titan, has an atmosphere and will be the target of a probe called Cassini due to arrive in 2004.

Uranus & Neptune

Cold, gaseous, distant worlds. We are short on time!

Pluto (err sorry Pluto ... you are now off the list)

Last planet discovered (1930) and very small (even smaller than our moon).
It doesn't fit the "mold" of any other planet .... it isn't small and rocky like the 4 inner (terrestrial) planets ... it isn't big and gassy like the next 4 (Jovian) planets ... Pluto is small and icy!
In 1992, astronomers started finding all kinds of small icy "mini-plutos" just beyond Neptune. It became apparent that Pluto actually did fit somewhere ... as the largest one in this new class. This group of small icy worlds is called the Kuiper Belt. Should we now take Pluto off the list of planets and just make it the largest Kuiper belt object? The vote is in ... Pluto is officially off the list of planets but reclassified as a "dwarf planet" as of 8-24-2006 (sort of like an honorary member).

The Kuiper Belt

Recently discovered small icy worlds just beyond Neptune. 1992 QB1 was the first found (or was Pluto?) but the list is constantly growing. Hey, 2003 UB313 (nicknamed Xena) is even larger than Pluto ... maybe that is why Pluto got the ax.

The Oort Comet Cloud

Comets are dirty ice balls the size of mountains. The ices are frozen water and carbon dioxide which are intermixed with small rocky fragments. Where do they come from? Jan Oort in 1950 speculated that they reside at the extreme limits of the solar system ... well beyond the planets. Although astronomers can not observe objects this small at such as great distance, it is assumed that comets do indeed reside there. If a nearby star gravitationally nudges some of these comets, they could find themselves propelled toward the sun.

Comet Hale Bopp

Comet Hale Bopp

The tail of the comet forms when it gets close to the sun and gasses evaporate and rocky debris "flakes off" the comet.

Check these great sites on the solar system:

The Nine Planets
Views of the Solar System
Astronomy Picture of the Day