Useful Notes / The Solar System

Mercury was near the sun so Janet dropped by - but the mercury on Mercury was much too high.
So Janet went to Venus, but on Venus she found: she couldn't see a thing for all the clouds around.
Earth looked exciting, Kind of green and inviting, So Janet thought she'd give it a go. But the creatures on that planet looked so very weird to Janet. She didn't even stop to say "hello".
Mars is red, And Jupiter's big. Saturn shows off it's rings. Uranus is built on a funny tilt, And Neptune is it's twin.
Pluto, little Pluto is the farthest planet from the sun
Schoolhouse Rock, "Interplanet Janet"note 

The things you can find in our star system. Generally speaking, it consists of a central star, four Earth-type planets, four (or five) 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.


  • The Sun: The star of the show, literally and figuratively. About 99.9% of all the mass in the Solar system is in the sun.


Most of these and their associated moons are named after characters from Classical Mythology; the planets themselves have Roman/Latin names, but the moons are more variable.

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 pressure is 90 bars, compared to 1 on Earth) and can reach a surface temperature of 470 °C/870 °F (although at the top of Maxwell Montes, almost 7 miles above the average surface level, it's "only" 380 °C/716 °F and 60 bars). The culprit for all this? The greenhouse effect—90 atmospheres of carbon dioxide with some helping of other greenhouse gases will be quite hot. note  Volcanoes on Earth have belched out 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 was vaporized (300 atmospheres worth) and created a super greenhouse effect with temperatures in the thousands of degrees. note  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. note  There is some argument over whether the proper adjective is 'Venusian', 'Venerean', or 'Cytherean'.
  • Earth (or in Latin: Terra): Mostly considered an Insignificant Little Blue Planet that despite its status holds extreme significance for some carbon-based lifeforms.
    • More seriously, Earth is the 6th most massive solar system object, the largest of the Rocky planets, and the most dense object. It is one of two worlds (or possibly three, with supersalty water on Mars) with liquids on the surface, forming rivers, lakes, and such (The other is Saturn's moon Titan, with methane and ethane as the surface liquids), the only one with life (that we know of), and the only one with plate tectonics. The atmosphere is unique in having a large proportion as free oxygen, which in addition to supporting most life, changes the structure in a number of subtle ways compared to other planets. Earth also generates a magnetic field, unlike Venus and Mars, that is much stronger than that of Mercury but weaker than that of the gas giants.
    • The Moon: Our nearest neighbor, and the only celestial body beyond Earth that has been explored by humans in person (allegedly).
  • Mars: Albedo features identified in the 19th century led to manic speculation about the potential presence of life. Today we know it's not very lively, but that doesn't make it any less interesting. It's still a favorite for space exploration, with a handful of unmanned probes sent there every two years. There are currently five working orbiters around it (2001 Mars Odyssey, Mars Express, Mars Reconnaissance Orbiter, MAVEN, and Mangalyaan), note  and two working landers on it (Opportunity and Curiosity).

Gas giants

  • Jupiter: The largest of them all, and thus fittingly named after the Roman God of Gods. Of course, the Romans named it that long before they had any idea just how big it was.
    • The Moons of Jupiter: Simon Marius and Galileo Galilei each had claims to the discovery of the first four. Though discovery credit ultimately went to Galileo (and thus, they are known as the Galilean satellites), Marius's names were ultimately used (from innermost out: Io, Europa, Ganymede and Callisto). All the others are named for the daughters and lovers of either Jupiter or his Greek counterpart, Zeus.
  • Saturn: Well known for its spectacular ring system. Its average density is less than that of waternote , 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: Originally named after Roman deities associated with harvests (Saturn itself is named after the Roman god of agriculture). When they ran out (Saturn, it turns out, has a lot of moons), they switched to Giants from various mythologies. Norse ice giants are used for newly-discovered moons beyond Phoebe.
    • The Rings of Saturn
  • Uranus: It had been detected by astronomers as early as 1690note , but Sir William Herschell actually identified it as a planet in 1789. It is named after the Greek god of the sky.note  It's 4 times the diameter of the Earth, which is still less than half the diameter of Jupiter. Seen as minty green in color observed from Earth by early telescopes, close-up observation showed it's more pale blue. 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 circles while wiping out Klingons.
  • 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 that it appeared to move and never followed up. Watery blue in color, befitting a planet named for the Roman god of the seas; 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, other than being shorter-lived; it disappeared sometime in the next five years, but a new one appeared in the northern hemisphere some years after that. 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.
  • Last but not least, in January 2016 a group of astronomers have suggested that the eccentric orbits of several large Kuiper Belt Objects (see the Detached Object category of small solar system bodies listed further down) could be explained by the presence of a ninth planet, that would be a bit smaller in mass than Neptune or Uranus, two to four times larger than Earth, and that would orbit the Sun in a highly eccentric and inclined orbit that would take many thousands of years to complete never approaching at less than around seven times the Sun-Neptune distance. Please note, however, that despite the claims of "a new planet discovered" there's at best just indirect proof of its existence and it's an entirely hypothetical object until it's finally imagednote .

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 four largest members of the asteroid belt in the 19th century. It, rather expectedly, ended in a massive Flame War among not just enthusiasts of astronomy, but astronomers themselves. For this reason, one of the planets that precipitated the kerfuffle was appropriately named for Eris, the goddess of discord.

To qualify as a dwarf planet, or "plutino", 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.) Even beyond the Pluto issue, the concept of dwarf planets is controversial among astronomers because of both the relatively arbitrary distinction between a dwarf planet and a planetnote , and because the terminology is inconsistent (i.e. a dwarf star is always a type of star rather than a separate category, so how would a dwarf planet not be type of planet?). To date, only 5 dwarf planets are known:

  • PlutoCharonnote note 
  • Haumeanote 
  • Makemake
  • Erisnote note 

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. It - along with the asteroids Pallas, Juno, and Vesta - was also the first "planet" to be demoted, similarly to Pluto, but it was first called an asteroid before the class of dwarf planets was invented for Pluto. Apparently some sources still classify it as an asteroid. It looks at first, by the way, as the typical celestial object pockmarked by lots of craters... until you look closer and find mysterious white spots on some of them, at least one of which have occasional hazes over it, as well as one big pyramid-shaped mountain with a height of 3 miles. Currentlynote  orbited by the Dawn spacecraft.

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, it's spread out over a volume of space bigger than Earth's entire orbital disc, and a full third of that mass is Ceres, so unmanned spacecraft generally pass through it without incident.
  • Comets
  • The Kuiper Belt: Named after astronomer Gerard P. Kuiper, who theorized its existence in 1951. Also known as the Edgeworth-Kuiper Belt.
  • The Scattered Disc
  • Detached Objects
  • The Oort Cloud: Named after astronomer Jan Oort.

The differences between the last four can be contentious. The Kuiper Belt mostly consists of objects locked into orbital resonance with Neptune. Pluto, along with many other objects known as "plutinos", is locked into a 3:2 resonance. Another class of objects, called "twotinos", are locked into a 2:1 resonance, and other, more arcane resonances exist. The scattered disc consists of objects pushed further out into space by Neptune's outward migration (relatively early in the life of the Solar System); Eris and several other dwarf planets are located here. The Oort cloud consists of objects upon which Neptune's gravity has little significant effect. The Detached Objects are trans-Neptunian bodies that are too close to the sun to be part of the Oort Cloud, yet too far away for Neptune to be responsible for their eccentric orbits. Since the dividing line for all four is the historical gravitational effect of another astronomical object, one really can't say "at XX AU, the Kuiper belt ends and the scattered disc begins."

The naming of minor Sun-orbiting objects in the Solar System depends on location. Objects in the Main Asteroid Belt are given names from Greek mythology. Objects in the same orbit as Jupiter (the Trojan asteroids) are given names associated with the Trojan War. Objects in similar orbits to Pluto (the "Plutoids") are named after deities of the underworld. Kuiper Belt objects (other objects beyond Neptune's or Pluto's orbits) are named after deities of creation that are not Greek or Roman.

Turning to nomenclature on a smaller scale, the New Horizons probe will reach Pluto and Charon in July 2015, giving cartographers at least two whole new worlds worth of craters, mountains, and other points of interest to name. Current plans are to name features after various underworld locations and their denizens, spacecraft and space scientists, explorers and their vessels (real and fictional), and artists and authors whose works have depicted exploration. A page has been set up to allow people to vote for names to be submitted to the IAU for official use, so cast your vote for a real-life Vallis Serenity, Colles Watership, and Regio Z'ha'dum! note note 

The official IAU definition for fictional names of features on Pluto is items from global underworld mythos, leading to the use of Balrog Macula and Cthulhu Regio. Most features are named for real astronomers (Tombaugh Regio, a.k.a. "Pluto's Heart"), real explorers (Hillary Montes and Norgay Montes), and real spacecraft (Sputnik Planum, Voyager Terra).

The official IAU definition for fictional names of features on Charon is items and milestones of "fictional space".note  With that one, the New Horizons team is really going to town. Vulcan Planum (with Spock Crater), Gallifrey Macula, Ripley Crater and Nostromo Chasma, Macross and Serenity Chasmae, Organa, Skywalker and Vader Craters...

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 embarrassing instances of Scifi 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 another walnut at 230 meters from the bowling ball, Uranus would be a peanut 450 meters from the bowling ball, and Neptune would be another peanut 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 at that scalenote . Bill Nye demonstrates this scale with an exhausting bicycle ride in this video. note 

You'll often hear, particularly when concerning the Voyager probes, that they have reached the "edge" of the Solar System; originally this meant moving beyond Pluto's orbit, a definition that now feels very "last century". Now, it generally means when the "atmosphere" generated by the Sun is pushed back by the atmosphere of interstellar space (a point called the heliopause). And while yes, the space beyond the heliopause is technically the same as the space between stars, it is NOWHERE NEAR the edge of the Sun's gravitational influence, which is a thousand times farther out.