The Solar System

The things you can find in our star system. Generally speaking, it consists of a central star, four Earth-type planets, four gas giants, and a large amount of smaller objects, including dwarf planets, moons, asteroids, comets and space junk, mostly clustered in rings around the Sun or other planets.

If you're interested in things in space farther away than our star system, you might wish to consult Local Stars. If you're interested in what planets might, or might not, be orbiting other stars, you might wish to consult the more general Useful Notes page on Planets.

Stars

 * The Sun: The star of the show, literally and figuratively. About 99% of all the mass in the Solar system is in the sun.

Planets
Most of these and their associated moons are named after characters from Greek or Roman mythology.

Terrestrial ("Earth-type") planets

 * Mercury: smallest of the planets, closest to the Sun. It is small and very hot (apart from some permanently shadowed craters at the poles which may contain ice), with no the merest whisper of an atmosphere. Early scientists believed it to be tide-locked (one side permanently faces the sun), but it turns out it rotates 3 times for every 2 times it circles the sun (which, when combined with an elliptical orbit causes weird effects like "hot" and "cold" poles on the equator, and the Sun doing a slow loop-the-loop in the sky once each 88-day Mercurian year). When Mariner 10 flew by in 1974, it found the planet to be unexpectedly dense; scientists now believe it was originally similar in size and composition to Venus and Earth, but a massive impact with a leftover planetesimal tore away the atmosphere and most of the relatively light mantle, leaving the metal-heavy core behind.
 * Venus: sometimes referred to as Earth's sister planet due to their similar sizes. It has an extremely dense atmosphere (surface presure is 90 times that of Earth) and can reach a surface temperature of 470 °C/870 °F (although the top of Maxwell Montes, almost 7 miles above the average surface level, it's a chilly 380 °C/716 °F and a...less squishy 60 bars of pressure). The culprit for all this? The greenhouse effect - most of that atmosphere is carbon dioxide. Earth started with the same amount, but it ended up trapped in carbonate rock. Venus also started with the same amount of water as the earth had, but it remained in vapor form (300 atmospheres worth) and created a super greenhouse effect with temperatures in the thousands of degrees . Eventually the water molecules dissociated into hydrogen and oxygen and escaped into space, leaving Venus high and dry. Interestingly, the zone between 50 and 65 kilometers above the surface has pressures and temperatures right around Earth normal. Add to that the fact that an 80/20 nitrogen/oxygen mix would act like a lifting gas and Cloud City would be right at home. Due to Venus being mythologically associated with femininity, by convention all geographic features there are named after women or female entities, except for Maxwell Montes and Alpha and Beta Regio. There is some argument over whether the proper adjective is 'Venusian', 'Venerean', or 'Cytherean'.
 * Planet Earth: This planet holds extreme significance for some underdeveloped carbon-based lifeforms despite being just an Insignificant Little Blue Planet.
 * The Moon
 * Mars

Gas giants

 * Jupiter
 * The Moons of Jupiter
 * Saturn: Well known for its spectacular ring system. It's average density is less than that of water, and despite its bland butterscotch appearance it has storms that rival any found on Jupiter. Plus it has a polar hexagon. How cool is that?
 * The Moons of Saturn
 * The Rings Of Saturn
 * Uranus: It had been detected by astronomers as early as 1690, but Sir William Herschell actually identified it as a planet in 1789. It's 4 times the diameter of the Earth, which is still less than half the diameter of Jupiter. Minty green in color; it's denser than Jupiter and Saturn with a higher proportion of methane, ammonia and water. Voyager 2 passed by it in 1986 and observed few distinct clouds, but later observations from Earth have revealed more. It has a set of coal-black rings (discovered in 1977) and is tilted 98 degrees on its axis - each pole spends 42 years in light and 42 in darkness. Also known for being the planet which the Enterprise, like toilet paper, circles while wiping out Klingons.
 * The Moons Of Uranus (The Bard In Space): Fittingly for an English-discovered planet, Uranus' moons are all named after characters from Shakespeare and The Rape of the Lock.
 * Neptune: Discovered in 1846 by three different astronomers (John Couch Adams of the UK and Urban Leverrier of France predicted its location independently based on changes in the orbit of Uranus, and Johann Gottfried Galle of Germany found it based on Leverrier's data.). Note, however, that Galileo actually observed Neptune twice in the winter of 1612/13, but merely noted it appeared to move and never followed up. Watery blue in color; its composition is similar to that of Uranus. Voyager 2 detected some noticeable cloud features when it flew by in 1989, including the "Great Dark Spot" which is an almost perfect analogue to Jupiter's Great Red Spot. It has an unstable ring system that clumps into arcs at some longitudes. From 1979 to 1999 it was further away from the Sun than Pluto, and with a nearly 165-year-long orbital period it has only completed one orbit since its discovery -- and that in 2010.
 * The Moons Of Neptune (The Little Mermaids)

Dwarf Planets
The reason behind the introduction of this category of celestial bodies was a discovery of several Kuiper Belt Objects that rivaled or exceeded Pluto in size and thus strained the definition of planet. It was decided that it'd be simpler to demote Pluto than to make all of them planets - a similar course of events took place after the discovery of the first asteroid belt in the 19th century. It, rather expectedly, ended in a massive Flame War among not just enthusiasts of astronomy, but astronomers themselves.

To qualify as a dwarf planet, the object must be big enough that its own gravity has pulled it into a more-or-less round shape. (It also can't be orbiting another planet, since then it would be a moon.) To date, only 5 dwarf planets are known:


 * Pluto-Charon
 * Haumea
 * Makemake
 * Eris

All these are dark and freezing cold, absolutely no life (except for, probably, Mi-go) could exist or survive here.


 * Ceres, the biggest in the asteroid belt, and the only main-belt asteroid big enough to qualify as a dwarf planet.

Everything Else

 * The Asteroid Belt -- Also known as the Piazzi Belt (after the discoverer of Ceres) to distinguish it from the Kuiper Belt, it can be found between Mars and Jupiter. It's not an Asteroid Thicket -- the belt's combined mass is only 4% of the Moon's, and it's spread out over a volume of space bigger than Earth's entire orbital disc, so unmanned spacecraft generally pass through it without incident.
 * Comets
 * Halley's Comet
 * The Kuiper Belt - named after astronomer Gerald Kuiper who theorized its existence in 1951. Also known as the Edgeworth-Kuiper Belt.
 * The Scattered Disc
 * The Oort Cloud - named after astronomer Jan Oort.

The differences between the last three can be contentious. Basically, the Kuiper belt consists of objects locked into orbital resonance with Neptune (Pluto and other plutinos have a 3:2 resonance, other objects called "twotinos" have a 2:1 resonance), the scattered disc consists of objects pushed out further into space by Neptune (Eris is out here), and the Oort cloud consists of objects too far away for Neptune's gravity to affect.

Interplanetary distances
Most popular depictions of the Solar system, even in science classes, tend to emphasize the relative sizes of the sun and planets and gloss over the scale of the immense distances between them. This can lead to embarrasing instances of Sci-Fi Writers Have No Sense of Scale.

Look at the page image for Conveniently Close Planet. That's just the Earth-moon system to scale. The planets are much, much farther apart than this.

If the sun were the size of a bowling ball, the Earth would be roughly the size of a peppercorn, and the distance between them would be nearly 25 meters. Jupiter would be the size of a walnut (still in its shell), and would be over 120 meters from the sun-bowling-ball. Saturn would be 230 meters from the bowling ball, Uranus would be a peanut 450 meters from the bowling ball, and Neptune would be a whopping 700 meters from the bowling ball (that's nearly half a mile away from it). Light (remember, the fastest thing in the Universe and whose velocity cannot be exceeded) at this scale would move at the impressive speed of 166 meters per hour, and just try to imagine the speed of our current interplanetary probes. Bill Nye demonstrates this scale with an exhausting bicycle ride in this video.