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Useful Notes / The Solar System

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"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 its rings. Uranus is built on a funny tilt, And Neptune is its 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 terrestrial planets, two gas giants, two ice 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.

Despite years and years of Science Fiction stories about planets around other suns, we actually lacked any real scientific proof of them until the early 1990s when exoplanets were first detected by their wobble on their parent star. Until then, it was quite possible that our solar system was simply a fluke. For example, one theory on the formation of our solar system, "Tidal Theory" — 1917, was that a passing star came close to our sun, drawing a filament of solar matter out of it which coalesced into the planets. Wikipedia has a page on those theories here if you are interested.

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 (popularly formally named in Science Fiction as Sol): The star of the show, literally and figuratively. About 99.9% of all the mass in the Solar system is in the sun. Our Sun is a wonderfully stable one, unusually so even for a G-type, which is beneficial for us. Other stars big and small often emit dangerous-to-life x-ray flares. She has the odd sunspot here and there too, and thank goodness; last time our Sun didn't, we experienced an ice age. Rather than red dwarfs, which shine out mostly infrared light, she emits a lot of light in the visible spectrum, which is just great for photosynthesising plants. She does emit ultraviolet radiation too, so put on that sunblock before you step outside. Sadly, all good things must come to an end. The Sun is already 4.6 billion years old, and when it uses up too much of its Hydrogen reserves, it'll get hotter and hotter, eventually becoming a humongous Red Giant that'll no doubt swallow up Earth and the other inner planets.


Most of these and their associated moons are named after characters from Classical Mythology; most of the planets themselves have Roman/Latin names, but Uranus has a Greek name and Earth has a Germanic name. The names of 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. Among the inner planets Mercury's orbit is the most unstable and susceptible to Jupiter's gravitational influence.
  • 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. It oddly rotates in the opposite direction to most other planets. 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. Our world used to have a similar atmosphere with its "twin" Venus, but lost it in the giant impact event.
    • The Moon (AKA, Luna): Our nearest neighbor, and the only celestial body beyond Earth that has been explored by humans in person (allegedly). It is theorized that the Moon is the left over debris from a giant impact event with a proto-Earth and a Mars-sized object called Theia. Another theory holds that both Theia and the proto-Earth were completely destroyed by the impact, then the debris coalesced to form Earth and the Moon. Or even that they collided twice before ending up in their current forms. The Moon is a very unusual object (while not the largest moon in the solar system, relative to the size of its planet it's enormous) and the high angular momentum of the Earth-Moon system is even more unusual, and dramatic events were needed to bring it into existence. It's also Tidal Locked, which means it's in a synchronous rotation with Earth, such that the same face is always pointing towards the Earth at all times. This is not as remarkable or unusual as it may appearnote .
  • 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, ExoMars Trace Gas Orbiter, MAVEN,Hope, and Tianwen 1), note  and two working landers on it (Perseverence 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. Jupiter is vast, often referred to as a "failed brown dwarf" and while not as dense as Earth, it has a magnetic field some 20,000 times stronger than our world does. Jupiter is... odd, and by extension makes our solar system unusual. Gas giants have the tendency to migrate inwards towards their stars, becoming "hot jupiters". This happened in our system, and it would have come in like a wrecking ball... until the formation of Saturn. Saturn pulled Jupiter outwards, allowing the rocky terrestrial worlds to form. Jupiter has been described as a cosmic vacuum protecting us from comets and other asteroids that would otherwise obliterate all life on Earth.
    • 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

Ice Giants

  • Uranusnote : 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. Uranus is even colder than Neptune. 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. Scientists have reason to believe it's on its unique side-ways tilt due to a massive impact that depleted its internal core temperature. 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.

And on that note...

  • 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 (so-called Planet Nine), 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 closer than around seven times the Sun-Neptune distancenote . 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 imaged.note .

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.note 

To qualify as a dwarf planet, or "plutoid", 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 a type of planet?).

The term "clearing the neighborhood" is the biggest source of confusion to laypeople; even astronomers who support the current definition generally think it's a clunky term and that something like "gravitational dominance" or "dynamic dominance" would be more appropriate. A sufficiently massive object will have three effects on the debris in its orbit. The first is that any objects will be captured and become moons. This is the least likely of the three effects to happen. The second effect, and the one most frequently thought of when people hear "clear the neighborhood" is expulsion: small bodies will be launched into eccentric orbits or out of the solar system entirely. The third, and most frequently misunderstood, is that any small bodies that are not captured or expelled will be locked in an orbital resonance. Neptune and Pluto are a prime example: every time Neptune completes three full revolutions around the sun, Pluto completes two. It was discovered in the 1990s that Pluto is the largest of many objects that were locked into such a relationship with Neptune. Other examples include the Trojan Asteroids that inhabit the regions governed by Earth, Mars, and Jupiter. In short, a small object does not need to directly orbit a major planet to be controlled by its gravity. Where major planets control entire regions, dwarf planets only control their moons at best.

To date, only 5 dwarf planets are known, with another forty or so pending confirmation:

  • Pluto– Charonnote 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
    • Halley's Comet: The best-known of all comets, because it's large enough to be seen from Earth and (unlike most large-ish comets) has an orbital period short and predictable enough that the average human has at least one opportunity in their lifetime to see it. If you weren't around for it's last appearance in 1986, you can look forward to seeing it in 2061.
  • 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. Could be the home of yet another hypothetical planet, a body as massive as Mars that would orbit at twice Neptune's distance to the Sun in an inclined orbit, and whose detection if it was real at all would be as challenging as Planet IX's one.
  • Detached Objects
    • Sedna: Almost certainly the sixth dwarf planet. Its orbit is so distant that it never comes within 46 AU of Neptune (which for the record is more than 1.5 times distance between Neptune and the sun) and thus no planet's gravity exerts any influence on it. Its orbit is also extremely elliptical (its closest approach to the sun is 76 AU while the furthest it gets is 936 AU), something that is normally caused by interaction with Neptune and thus causing renewed speculation that a large planet must exist beyond Neptune's orbit. Its orbital period is estimated to be 11,400 years, while it's only physically observable from Earth for 25 years, or 0.22%, of its orbit. This means astronomers were very lucky to find it.
  • The Oort Cloud: Named after astronomer Jan Oort.

The differences between the last four can be contentious. The Kuiper Belt mostly consists of objects in high orbital eccentricities (above and below the "ecliptic," the plane the planets mostly orbit on) 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; there are also "cubewanos," or "classical Kuiper Belt objects" that are not in resonance and follow the ecliptic fairly closely. 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. An exception was made for Eris due to its brief stint as the "tenth planet".

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 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, and you could fit every planet in the Solar System, plus Pluto, in between the Earth and the Moon, and have a bit of room to spare.

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), which is (conveniently, for those who like round numbers) about 100 AU out from the Sun. 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. In fact, the Sun's influence doesn't "end". Rather, it merges with its closest neighbors, including Alpha Centauri. In other words, you're not out of the Solar System til you're in another one.

Past and future

The Solar System was born roughly 4½ billion years ago from the collapse of a large gas cloud, that as typical per the process of star formation fragmented into many smaller onesnote . One of said fragments, as it collapsed, began spin faster flattening and taking a flying saucer-like shape. Its center, as it collapsed, became hotter and denser until hydrogen fusion began at its very center. The Sun was bornnote 

While that happened in the center, things were considerably more active in the surrounding disk. While the countless grains of matter that formed it were in full Fusion Dance forming larger ones that would fuse to form even larger ones and so on, the heat of the contracting core vaporized low-melting point compounds as water or methane and the like of those coalescing bodies too close, that were able to condense only further out, meaning the inner Solar System would be composed of low-mass (proto)planets formed of metal and rocks. In the outside, with those volatile compounds safe from vaporization, things happened at a considerably faster pace and much larger core bodies formed that began to accrete gas from the surrounding in a runaway process that stopped both by the disk running out of matter and the Sun's strong winds blowing away the remaining gas and dust, leaving behind the major planets of the Solar Systemnote 

It was by then when the real planetary dance began. While those protoplanets of the inner Solar System were gravitationally interacting until they began to smash among themselvesnote , both gravitational interactions between the large planets and the many remaining planet-building blocks caused them to migrate from their original orbits, closer to the Sun than now. While Saturn, Neptune, and Uranus moved outwardsnote  Jupiter moved inwards, messing so much with the protoplanets existing in what is now the asteroid belt that, where a large planet could have been formed, remained only what would amount comparatively speaking to detritus, either expelling it to the cold outer reaches or the Solar System, forming the Kupiter belt and the Oort Cloud or even ejecting outside itnote , and would have kept approaching until it reached a very close orbit around the Sun had not gravitational interactions with Saturn caused it to move outwards to its present positionnote . Uranus and Neptune would also have played havoc with the remaining protoplanets of the Solar System's outer regions where they orbited scattering it even further out.

The revenge of those protoplanets and other debris came four billion years ago in the form of a large number of them ending up in the inner Solar System, where they pummeled to death the more or less formed terrestrial planets, scarring them with countless craters and causing quite a lot of damage to themnote . This hellish rain may have lasted a few hundred million years, and was the last event of significance in the Solar Systemnote . From there until now, little has happened in the Solar System in a global scale besides the occasional impact here and there of a stray asteroid or comet, the star passing across the Oort cloudnote  sending a shower of comets to the inner Solar System, and a Sun that as it ages becomes more luminous.note 

As for the future, things will not change very much for the Solar System as a whole during the next five billion yearsnote . Sure, the orbits of some moons such as Mars' Phobos or Neptune's Triton will decay and the latter will produce both a hell of a meteor shower and a splendid ring system around its planet, the occasional asteroid/impact here and there will happen, the occasional star passing too close and shaking the Oort Cloud sending comets to the inner Solar System, and the Earth becoming unhabitable because of a Sun that is brightening with time as described above but if we could see the Solar System by then it is expected we'd see something very similar to the current onenote . Five billion years was not chosen at random for the fate of our Solar System is tightly tied to the one of its most massive body: the Sun. By that epoch it will have run out of hydrogen at its center and things will become interesting, so let's fast forward to the year 7,590,000,000 ADnote . In that year our familiar Sun will be in full red giant-mode: a bloated and distorted star with a surface temperature having dropped to half of what it has in our epoch, thousands of times more luminous and more than two hundred times larger than our daystarnote , and so big that has lost more than a quarter of its mass carried away by strong solar winds, something that has caused the planet's orbits to wide, but not enough to avoid searing hot temperatures or worsenote . It will also be the time when the Sun will suffer a dramatic transformation: the dense, inert helium core will ignite and will produce for a few seconds as much energy as an entire galaxy in what astronomers know as the "helium flash". While this looks like a supernova, that energy will actually just be used to re-expand the core and nothing of that will be seen at its surface — in fact, with the Sun's innards expanding to fuse helium stably it will release less energy and will collapse back to a much smaller and less luminous star a bit more hot than its red giant past (somewhat more than four thousand degrees Celsius), but still quite luminous (forty-fifty times more) and large (ten times larger) compared with its past long-lived incarnation requiring several thousand years to become thatnote . Unfortunately for the Sun, helium, fusing to produce carbon and oxygen, is a worse fuel than hydrogen, and even if helium burning is supported by some hydrogen fusing around the core, our star will run out of it in just a hundred million years and then the Sun will face a similar crisis to the one it faced when it ran out of hydrogen there. Only this time said crisis will be fatal. As happened with hydrogen before, helium will begin to fuse around an inert, contracting, core of carbon and oxygennote . Further out, hydrogen will keep fusing to helium and as the core contracts it will "squeeze" those two burning shells causing them to fuse with more force. The result is that the Sun will become again — in just twenty million years compared to the considerably longer time (more than two billion years, see above) it needed to expand for the first time — a huge and luminous red giant suffering even stronger mass loss but, as helium burning under those extreme conditions is highly sensitive to the temperaturenote , an unstable one. Each hundred thousand years, the Sun will convulse suffering a pulse caused by a runaway helium shell burning ignition that will increase its luminosity as well as it radius just to contract when it stops and having things starting again. Those violent pulses will increase even more the already heavy mass loss as mentioned before, and by the fourth one all that will remain of the Sun will be the extremely hot — one hundred thousand degrees Celsius — and dense, as it has contracted to the size of the Earth, carbon-oxygen core half as massive as the Sun is now: a white dwarf. If the Sun is luminous enough, its ultraviolet radiation will cause the matter that ejected before to fluoresce as a beautiful planetary nebula, that will however be short-lived as a few thousand years later both the gases will be rarefied and far away enough and the white dwarf's ultraviolet radiation will fade away as it begins to cool to stop shining. All that will remain to the dead white dwarf Sun is to keep cooling over many billions of years until it will fade into oblivion as a black dwarf — or, if you prefer it, as a big and dense diamond as all those carbon will crystalize during said cooling.note