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Useful Notes: 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 (P)lanets.

Stars

  • 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.

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 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 adiabtic lapse rate - 90 atmospheres of anything will be quite hot.note  Volcanos 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: This planet holds extreme significance for some underdeveloped carbon-based lifeforms despite being just an Insignificant Little Blue Planet.
    • 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 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 three working orbiters around it (2001 Mars Odyssey, Mars Express and Mars Reconnaissance Orbiter), 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. 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'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 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 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.

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:

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 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.

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
  • 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. 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. Since the dividing line for all three 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 Asteriod 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.

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 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 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 probesnote . Bill Nye demonstrates this scale with an exhausting bicycle ride in this video.

You'll often here, 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.


Snow Means ColdUseful NotesThe Sun

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