home observation science solar system stars our galaxy cosmology astrobiology exoplanets astrophotography
Welcome to the Solar System Basics:

Our Solar System now has eight planets - Pluto, Charon (actually still a moon of Pluto), Xena Eris and other soon to be discovered large KBO's and TNO's are now "dwarf planets." See my Blog for more details.

NASA Planetary Fact Sheet - Metric
NASA Planetary Fact Sheet - US Units
NASA Planetary Fact Sheet - Ratio to Earth Values
Planetary Fact Sheet Notes
Notes on the Fact Sheets

New members of our Solar System are still being discovered. Of recent notoriety: Eris (previously known as 2003 UB313)

The determination of this being an official dwarf planet has been determined by the International Astronomical Union - now officially called Eris - the unofficial name of this body is Xena. The image above is the discovery image.

On September 30, 2005, new data reveals Xena Eris has a moon - Gabrielle.

Eccentricity of Orbits: 0.4378
Semimajor Axis: 67.89 AU
Greatest Distance from Sun: 97.610 AU
Inclination: 43.993 degrees

As of right now, this object is designated as a Trans Neptunian Object (TNO) - similar to that of Sedna. The orbit is at least twice as large as Pluto - as seen in the simulated orbit above (care of NASA).

The Solar System is so named by its most prominent member - the Sun. Going from center outward, the Solar System consists of the Sun, 4 "terrestrial" planets, an asteroid belt, and the four gas giant planets. Pluto, the Kuiper Belt Objects (and Trans Neptunian Objects) and other dwarf planets, as well as the Oort cloud follow. The Solar System is HUGE but there is a lot of empty space. How big is it? Well, light travels at 186,000 miles per second (300,000 km/s) and it takes over 8 minutes for light to reach Earth from the Sun, and one hour for light to reach Jupiter from the Sun.

The drawing on the left shows the general orbital structures of our Solar System. This drawing is almost to scale, and demonstrates that the distance between the planets is actually quite large. Also notice how the orbit of Pluto is, for a relatively short time, closer to the Sun and Neptune.

The terrestrial planets: Mercury, Venus, Earth and Mars (in order from in to out). These are called terrestrial planets because they are a solid body, that is they have land. Their density is almost equal because they are made from rocky material.

Note: The images below, in order: Mercury, Venus, Mars, Jupiter, Saturn, Uranus, Neptune, and Pluto.

Mercury is closest to the Sun and is very hot and very cratered because of impact from debris (small rocks, comets, asteroids, and so on). Venus, Earth, and Mars have atmospheres; however Venus has a thick, acid atmosphere and is hot because of the greenhouse effect, and Mars has a very thin atmosphere.

Earth's atmosphere is "just right." After Mars, we have the asteroid belt. Do not be fooled by the movies because this area is not that dense. When NASA sends their probes, they pass through this region but they are not too concerned about impacts because their is great distances between each asteroid.

The asteroid belt exists because it is believed that the strong gravity from Jupiter prevents the debris to coalesce into a planet. It is unknown as to how many asteroids there are in this region, but speculation is that if coalesced, a planet about the size of Mars would form.

After the asteroid belt, we have the four gas giant planets (sometimes called Jovian planets which means "Jupiter-like"): Jupiter, Saturn, Uranus, and Neptune.

All four of the planets are made mostly of gas and their coloring is a result of varying concentrations of methane and other gases. They all have rings although Saturn is most famous for its prominent ring structure. Jupiter is the largest planet and sports a huge storm that has lasted centuries called the "Great Red Spot."

To dispel some science fiction, Jupiter is not a "failed star," that is if it were just a little more massive, it can collapse to form a star. In reality, Jupiter would have to be between 8 and 16 times as massive to begin contraction. The final "planet" we know to exist in the Solar System is Pluto.

There is actually some debate to this as some believe Pluto to be a captured Kuiper Belt Object - or KBO's. Gerard Kuiper speculated that between Pluto and the Oort cloud is a population of icy bodies, similar to Pluto. Sometimes these KBO's can be gravitationally captured by Neptune to become "Trans-Neptunian Objects"- or TNO's.

This debate has ended as the IAU, in 2006, declared Pluto a dwarf planet. In late 2005, there was a news article on a new planet called Sedna. This really isn't a planet, but an icy body. Depending on which group you ask, some say Sedna is a KBO, others a TNO. So we'll let that go.

The final target of our voyage is the Oort cloud. Jan Oort speculated that a cloud of icy debris surround the entire Solar System, and that this is the starting point for comets. As some nearby object disturbs the Oort cloud, an icy rock is "thrown" our way in the form of a comet.

It is the random nature of the comets orbit that is the evidence of the Oort cloud. Had a comet come from within the Solar System or from KBO's, comets would only appear on the same plane as the planets.


There are eight planets in the Solar System: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune (and there is the dwarf planet Pluto). The Solar System is also host to asteroids, comets, Kuiper Belt Object, Trans Neptunian Object, and the Oort cloud.

The image above is designed to illustrate the apparent sizes of each planet as related to each other (montage created from NASA JPL images).

To learn more about a specific planet, please make your selection on the upper left.

Back to Top

Search | Site Map | Appendix
©2004 - 2024 Astronomy Online. All rights reserved. Contact Us. Legal. Creative Commons License
The works within is licensed under a Creative Commons Attribution-ShareAlike 3.0 Unported License.