--- title: Sol summary: The Sun and its planetary system, the one star system in Pax Abyssi built entirely from measurement, with nine planets including Pluto and 28 moons on orbits fitted to JPL ephemerides. science_status: [observed, sim] categories: [Planetary systems, Solar System, The Galaxy] aliases: [Solar System, The Solar System, Sol system] infobox: type: planetary_system image: File:Sol_system_planet_sizes.jpg star: "the Sun (Sol)" spectral_type: G2V star_mass_kg: 1.9884e30 star_radius_km: 695700 star_luminosity_w: 3.828e26 star_effective_temperature_k: 5772 age_gyr: 4.567 distance_from_galactic_centre_kpc: "8.2 to 8.3" galactic_orbit_myr: "about 220 to 230" planets: 8 dwarf_planets_iau: "5 (Ceres, Pluto, Haumea, Makemake, Eris)" known_moons_as_of_2026_09: {Earth: 1, Mars: 2, Jupiter: 115, Saturn: 293, Uranus: 29, Neptune: 16, Pluto: 5} habitable_zone_au: "about 0.95 to 1.7 (conservative); about 0.75 to 1.77 (optimistic)" water_frost_line_au: "about 2.7 (classic solar nebula model)" main_asteroid_belt_au: "2.06 to 3.27" main_asteroid_belt_mass_earth: 4.0e-4 kuiper_belt_au: "39.4 to 47.8" kuiper_belt_mass_earth: 0.0197 heliopause_au: "about 120 (Voyager 1 crossed at about 122 AU in 2012)" sim_bodies: "37: the nine planets including Pluto, and 28 moons" sim_epoch: "positions solved for 2538-01-01, then propagated" sim_analogue_archetype: "frost_clustered (the generator's Sol-like architecture)" sim: entity: system.Sol refs: - id: nasafactsheet type: webpage author: [{family: Williams, given: David R.}] title: "Planetary Fact Sheet - Metric" container-title: NASA Space Science Data Coordinated Archive issued: 2025 URL: https://nssdc.gsfc.nasa.gov/planetary/factsheet/ - id: nasasunfact type: webpage author: [{family: Williams, given: David R.}] title: "Sun Fact Sheet" container-title: NASA Space Science Data Coordinated Archive issued: 2024 URL: https://nssdc.gsfc.nasa.gov/planetary/factsheet/sunfact.html - id: nasasunfacts type: webpage author: [{literal: "NASA Science"}] title: "Sun: Facts" container-title: science.nasa.gov URL: https://science.nasa.gov/sun/facts/ - id: prsa2016 type: article-journal author: [{family: Prša, given: Andrej}, {literal: "et al."}] title: "Nominal Values for Selected Solar and Planetary Quantities: IAU 2015 Resolution B3" container-title: The Astronomical Journal issued: 2016 volume: 152 page: 41 DOI: 10.3847/0004-6256/152/2/41 - id: connelly2012 type: article-journal author: [{family: Connelly, given: James N.}, {literal: "et al."}] title: "The Absolute Chronology and Thermal Processing of Solids in the Solar Protoplanetary Disk" container-title: Science issued: 2012 volume: 338 page: 651-655 DOI: 10.1126/science.1226919 - id: nasajupitermoons type: webpage author: [{literal: "NASA Science"}] title: "Jupiter Moons" container-title: science.nasa.gov issued: 2026 URL: https://science.nasa.gov/jupiter/moons/ note: "115 moons recognised by the IAU; page updated 21 September 2026" - id: nasasaturnmoons type: webpage author: [{literal: "NASA Science"}] title: "Saturn Moons" container-title: science.nasa.gov issued: 2026 URL: https://science.nasa.gov/saturn/moons/ note: "293 confirmed moons as of August 2026; page updated 22 September 2026" - id: nasauranusmoons type: webpage author: [{literal: "NASA Science"}] title: "Uranus Moons" container-title: science.nasa.gov issued: 2026 URL: https://science.nasa.gov/uranus/moons/ note: "29 officially recognised moons as of August 2026" - id: nasaneptunemoons type: webpage author: [{literal: "NASA Science"}] title: "Neptune Moons" container-title: science.nasa.gov issued: 2026 URL: https://science.nasa.gov/neptune/moons/ note: "16 officially recognised moons; page updated 21 September 2026" - id: nasadwarf type: webpage author: [{literal: "NASA Science"}] title: "Dwarf Planets" container-title: science.nasa.gov URL: https://science.nasa.gov/dwarf-planets/ - id: pitjeva2018 type: article-journal author: [{family: Pitjeva, given: E. V.}, {family: Pitjev, given: N. P.}] title: "Masses of the Main Asteroid Belt and the Kuiper Belt from the Motions of Planets and Spacecraft" container-title: Astronomy Letters issued: 2018 volume: 44 page: 554-566 DOI: 10.1134/S1063773718090050 - id: hayashi1981 type: article-journal author: [{family: Hayashi, given: Chushiro}] title: "Structure of the Solar Nebula, Growth and Decay of Magnetic Fields and Effects of Magnetic and Turbulent Viscosities on the Nebula" container-title: Progress of Theoretical Physics Supplement issued: 1981 volume: 70 page: 35-53 DOI: 10.1143/PTPS.70.35 - id: kopparapu2013 type: article-journal author: [{family: Kopparapu, given: Ravi Kumar}, {literal: "et al."}] title: "Habitable Zones around Main-sequence Stars: New Estimates" container-title: The Astrophysical Journal issued: 2013 volume: 765 page: 131 DOI: 10.1088/0004-637X/765/2/131 - id: kopparapu2014 type: article-journal author: [{family: Kopparapu, given: Ravi Kumar}, {literal: "et al."}] title: "Habitable Zones around Main-sequence Stars: Dependence on Planetary Mass" container-title: The Astrophysical Journal Letters issued: 2014 volume: 787 page: L29 DOI: 10.1088/2041-8205/787/2/L29 - id: nasavoyager type: webpage author: [{literal: "NASA Science"}] title: "Voyager: The Interstellar Mission" container-title: science.nasa.gov URL: https://science.nasa.gov/mission/voyager/interstellar-mission/ - id: irwin2024 type: article-journal author: [{family: Irwin, given: Patrick G. J.}, {literal: "et al."}] title: "Modelling the seasonal cycle of Uranus's colour and magnitude, and comparison with Neptune" container-title: Monthly Notices of the Royal Astronomical Society issued: 2024 volume: 527 page: 11521-11538 DOI: 10.1093/mnras/stad3761 - id: zhu2021 type: article-journal author: [{family: Zhu, given: Wei}, {family: Dong, given: Subo}] title: "Exoplanet Statistics and Theoretical Implications" container-title: Annual Review of Astronomy and Astrophysics issued: 2021 volume: 59 page: 291-336 DOI: 10.1146/annurev-astro-112420-020055 images_wanted: - file: File:Sol_system_planet_sizes.jpg subject: "The eight planets lined up at their true relative sizes (distances not to scale)" source: nasa source_page: https://science.nasa.gov/resource/solar-system-sizes/ credit: "NASA/Lunar and Planetary Institute" licence: "NASA, public domain with credit; no endorsement implied" - file: File:Sol_sun_sim_render.png subject: "The Sun from five solar radii in the game's test stage: a textured orange disc with a bright limb and long rays" source: sim source_ref: "QA:sun_sol_distance_series_200_to_1p3_radii/sol_5r.png" note: "prefix legend in content/wiki/_notes/writer-c.md; test-stage capture, no level in frame; the orange tint is a render choice, caption says so" - file: File:Sol_orrery_sim.png subject: "The in-game orrery of Sol with habitable zone, frost lines and an Earth to Mars Hohmann transfer drawn" source: sim note: "shot list: no approved orrery capture exists yet" --- **Sol** is the Sun and everything bound to it: eight planets, five recognised dwarf planets, more than 450 known moons, two great belts of small bodies and a wind of charged particles that reaches beyond 120 astronomical units (AU; 1 AU is Earth's mean distance from the Sun, about 150 million km). It is the only planetary system known in full detail and the yardstick against which every other is measured. In [[Pax Abyssi]] it is also the one system built entirely from measurement, where every other system is generated. ::figure{src="File:Sol_system_planet_sizes.jpg" size=wide alt="The eight planets in a row from Mercury to Neptune at their correct relative sizes, the giants dwarfing the four small rocky worlds." caption="Diagram. The planets at their true relative sizes. The distances between them are not to scale: Neptune orbits more than 30,000 Jupiter diameters from the Sun."} ## The Sun The Sun is a G2V star, a [[Yellow dwarf|yellow dwarf]] on the main sequence, fusing hydrogen into helium in its core. Its nominal values, fixed by the International Astronomical Union in 2015 so that every paper uses the same conversion constants, are a radius of 695,700 km, a luminosity of $3.828 \times 10^{26}$ W and an effective temperature of 5,772 K. :cite[prsa2016] Its mass is $1.988 \times 10^{30}$ kg, about 333,000 times Earth's. :cite[nasasunfact] Those three numbers are tied together by the Stefan-Boltzmann law, which gives the power radiated by a sphere of radius $R$ and surface temperature $T$: $$ L = 4 \pi R^2 \sigma T^4 $$ With $\sigma = 5.670 \times 10^{-8}$ W m$^{-2}$ K$^{-4}$, $R = 6.957 \times 10^{8}$ m and $T = 5{,}772$ K, the formula returns $3.83 \times 10^{26}$ W, the Sun's measured luminosity. The oldest solids in meteorites, calcium- and aluminium-rich inclusions, formed 4.567 billion years ago, and that date is taken as the birth of the Solar System. :cite[connelly2012] The Sun is a little less than halfway through its life. In about 5 billion years it will swell into a red giant large enough to engulf Mercury and Venus, then shed its outer layers and end as a [[White dwarf|white dwarf]]. :cite[nasasunfacts] It carries its planets around the centre of the [[Milky Way]] about 8.2 kiloparsecs away, once every 220 to 230 million years. ## The planets | Planet | Mean distance (AU) | Orbital period | Diameter (km) | Mass (Earths) | Density (g/cm³) | Moons (Sept 2026) | |---|---|---|---|---|---|---| | Mercury | 0.387 | 88.0 days | 4,879 | 0.055 | 5.43 | 0 | | Venus | 0.723 | 224.7 days | 12,104 | 0.815 | 5.24 | 0 | | Earth | 1.000 | 365.2 days | 12,756 | 1 | 5.51 | 1 | | Mars | 1.524 | 1.88 years | 6,792 | 0.107 | 3.93 | 2 | | Jupiter | 5.20 | 11.9 years | 142,984 | 318 | 1.33 | 115 | | Saturn | 9.57 | 29.4 years | 120,536 | 95.2 | 0.69 | 293 | | Uranus | 19.2 | 83.7 years | 51,118 | 14.5 | 1.27 | 29 | | Neptune | 30.2 | 164 years | 49,528 | 17.1 | 1.64 | 16 | Orbital and physical values are from NASA's planetary fact sheet. :cite[nasafactsheet] Moon counts are those recognised by September 2026 and change often. :cite[nasajupitermoons] :cite[nasasaturnmoons] :cite[nasauranusmoons] :cite[nasaneptunemoons] The periods and distances obey Kepler's third law: the square of a planet's period in years equals the cube of its distance in AU, $P^2 = a^3$. Jupiter at 5.20 AU gives $P = \sqrt{5.20^3} = 11.9$ years, as observed. See [[Orbit]] for where the law comes from. The system splits cleanly in two. The four inner planets are small, dense and rocky: [[Barren rock world|Mercury]], [[Greenhouse world|Venus]], Earth and [[Arid world|Mars]]. The four outer planets are giants: [[Gas giant|Jupiter and Saturn]], made mostly of hydrogen and helium, and the [[Ice giant|ice giants]] Uranus and Neptune, rich in water, ammonia and methane. Neptune is often shown deep blue, but careful recalibration of Voyager and telescope images shows it is a pale greenish-blue only slightly bluer than Uranus. :cite[irwin2024] The dividing line is the **frost line** (or snow line), the distance in the young solar nebula beyond which water condensed as ice and gave growing planets far more solid material to build from. The classic model of the solar nebula puts it at about 2.7 AU, between Mars and Jupiter. :cite[hayashi1981] Jupiter formed just outside it. Many other Sun-like stars have planets larger than Earth and smaller than Neptune orbiting closer in than Mercury, a kind of planet the Solar System does not have at all. :cite[zhu2021] Sol is one arrangement among many; see [[Planet occurrence]] and [[Planetary system archetypes]]. ## Moons, belts and dwarf planets Most of Sol's moons are small, irregular bodies a few kilometres across, captured on distant, tilted and often backward orbits around the giants. Jupiter's 115 include only eight regular moons that formed with it: four small inner moons and the four large Galilean moons. :cite[nasajupitermoons] The large moons are worlds in their own right: Titan has a nitrogen atmosphere thicker than Earth's, and Europa and Enceladus hide oceans under their ice. See [[Natural satellite]]. The **main asteroid belt** fills the zone from 2.06 to 3.27 AU, where Jupiter's gravity stopped a planet from forming. Its total mass, measured from its pull on the planets and on spacecraft, is $(4.008 \pm 0.029) \times 10^{-4}$ Earth masses, about 3 percent of the Moon. :cite[pitjeva2018] The **Kuiper belt** of icy bodies lies mostly between Neptune's 3:2 and 2:1 resonances, 39.4 to 47.8 AU, with about $1.97 \times 10^{-2}$ Earth masses, some 50 times the main belt. :cite[pitjeva2018] See [[Asteroid belt]]. The IAU recognises five **dwarf planets**, bodies massive enough to be round that have not cleared their orbits: Ceres in the main belt, and Pluto, Haumea, Makemake and Eris beyond Neptune. :cite[nasadwarf] Pluto has five known moons. :cite[nasafactsheet] ## Zones and boundaries The **habitable zone** is the range of distances where an Earth-like planet with a carbon dioxide and water atmosphere could keep liquid water on its surface. For the Sun its conservative limits run from about 0.95 AU, where an Earth-mass planet would tip into a runaway greenhouse, to about 1.7 AU, where even a thick carbon dioxide atmosphere could no longer keep it warm. Optimistic limits, based on the evidence that Venus once had water and Mars once had rivers, stretch from about 0.75 to 1.77 AU. :cite[kopparapu2013] :cite[kopparapu2014] Earth sits near the inner edge. The details are on the [[Habitable zone]] page. The **heliopause** is where the solar wind meets the gas between the stars. Voyager 1 crossed it on 25 August 2012 at about 122 AU, the first spacecraft to reach interstellar space, and Voyager 2 followed on 5 November 2018. :cite[nasavoyager] ## How we know Planetary distances are known to metres from radar ranging and the tracking of spacecraft, which together feed the numerical ephemerides that predict every body's position. Planet masses come from the orbits of their moons and from the paths of spacecraft that fly past. The belts are too faint and scattered to weigh by counting, so their masses are inferred from the small tugs they give the planets and spacecraft. :cite[pitjeva2018] The Sun's age comes from radioactive clocks in meteorites. :cite[connelly2012] ::orrery{system=sol} :::callout{type=sim title="In Pax Abyssi"} **BUILT.** Sol is the one system in the game that is not generated. It holds 37 bodies around the Sun: the nine planets, with Pluto kept alongside the eight, and 28 real moons (the Moon; Phobos and Deimos; six of Jupiter's; nine of Saturn's; five of Uranus's; four of Neptune's; and Charon). Their orbital elements were fitted to NASA JPL's Horizons ephemerides. Positions are solved for 1 January 2538, the game's date, and every body then moves on its orbit each tick, at true scale in double precision. The game's orbit solver matches the original simulation's to 0.8 mm across the nine planets. The orrery charts the system with its habitable zone, frost lines, Hill spheres and Lagrange points, and plans a live Hohmann transfer: Earth to Mars takes burns of 2.94 and 2.65 km/s and 259 days, with a launch window every 780 days. **Departures from reality.** Sunlight does not dim with distance in the game, by design, so Neptune is lit as brightly as Earth, in the way spacecraft images are exposed to show detail. The moons move on fixed orbital elements. Checked against a year of JPL positions the fit never saw, most of the large moons land within about two degrees of their true places, while resonance-driven Mimas and Hyperion and the distant irregular moons are off by 11 to 25 degrees, because fixed elements cannot carry the forces that move them. ::: ::figure{src="File:Sol_sun_sim_render.png" size=wide alt="An orange disc with a mottled surface and a bright rim, with long rays reaching out into a black star field." caption="Sim render. The Sun at five solar radii in the game's test stage. The orange tint is an artistic choice of the render; seen from space the Sun is close to white."} :::callout{type=lore title="Sol in 2538"} In the fiction of Pax Abyssi, Sol was abandoned about a century before the game begins, under raids by the Velkarai, and Old Earth is lost and quarantined. Its planets and moons still circle on their real orbits. ::: ## See also - [[Yellow dwarf]], the Sun's class of star - [[Milky Way]], the Sun's place in the Galaxy - [[Star catalogue]] - [[Orbit]] and [[Habitable zone]] - [[Natural satellite]] and [[Asteroid belt]] - [[Planetary system archetypes]] - [[Star system generation]], how every other system is built