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Celestia exo-class2.png
Aurorum in natural colour
Orbital characteristics
Aphelion150629177 km (93596631 mi; 1.00689386 AU)
Perihelion145680800 km (90521900 mi; 0.973816 AU)
148154988 km (92059242 mi; 0.99035492 AU)
365.24218985 d (0.9999786170 yr)
Known satellites3
(Candeum, Favillum and Esquarium)
Physical characteristics
Mean radius
72207 km (44867 mi)
6.2654 × 1010 km2
124.4 Esquarium
Volume1.4747 × 1015 km3
1361 Esquarium
Mass1.936 × 1028 kg
3242 Esquarium
1/103 Sun
Mean density
13.13 g/cm3
248.7 m/s2
25.36 g
189.2 km/s
Albedo0.81 (Bond)

Aurorum, sometimes also known as Luna or simply Moon, is the second planet from the Sun. A gas giant with a mass slightly less than a hundredth of the Sun, and more than 40 times of all other known astronomical bodies in the system combined, it is the largest planet in the Solar System. The largest and second-brightest (following Sun) astronomical object in the sky of Esquarium, Aurorum has been known to humanity since antiquity. Its prominence in the sky, combined with its regular cycle of phases, made it one of the most influential astronomical bodies on humanity as a whole, with major presence in language, mythology and calendar since ancient times.

Aurorum is primarily composed of hydrogen and, to a lesser degree, helium. Trace amount of heavier elements exist in both its atmosphere, and is believed to form a rocky core in its centre enveloped by a thick layer of metallic hydrogen and helium due to its tremendous pressure. Like most other planets in the Solar System, Aurorum lacks a well-defined solid surface, with modern definition usually based on altitude with an atmospheric pressure of 1 atm. Despite contributing to less than 0.1% of planetary volume, water vapour dominates the upper cloud layer of the planet, giving it a high albedo and distinctive blue-white hue when viewed from space.


Current model on the evolution of Solar System suggests that Aurorum, along with other planets, were formed in the protoplanetary disk roughly 4.6 billion years ago. Aurorum, like other giant planets, formed beyond the frost line of the Sun (estimated to be around 5 AU during its early stage), where it was far enough for volatile material to remain solid. After its formation, Aurorum is theorised to have migrated inward to its current orbit. The inward migration most likely destabilised most other terrestrial massive planets in the inner System, its massive gravity preventing most of them from being reformed after colliding with each other.

Today, only four terrestrial objects of significant size (enough to be rounded by its own gravitational force) remained in the Solar System: Icarum, Celum, Favillum and Esquarium. Among them, Icarum occupies a closer orbit to the Sun, and is the only known planet inside the orbit of Aurorum - Celum was the only natural satellite of Icarum, while Favillum and Esquarium were both captured by Aurorum as its satellites. Two major circumstellar discs, known as the Inner Belt and Outer Belt based on their orbit relative to Aurorum, are believed to be remains of planetesimals following the disruption caused by Aurorum's migration.

Physical characteristics

Aurorum is the most massive and thus the largest planet in the Solar System, with a diameter of 144,414 km (89,735 mi) at its equator. Despite primarily composed of light elements in gaseous and liquid form, Aurorum is the densest object in the Solar system, with a mean density of 13.13 g/cm3 due to its high gravity compressing itself.

Mass and size

Aurorum is the second most massive object (after the Sun) in the Solar System, more than 40 times of the rest of the System combined. The huge gravitational force of Aurorum resulted in significant compression of its interior, possibly forming a dense rocky core surrounded by a thick layer of metallic hydrogen. The contraction also heats up the interior of Aurorum, causing it to radiate more heat than it received from the Sun. However, theoretical models indicate that it was slightly below the minimum mass (about 1.3 Aurorum mass) where deuterium fusion can occur.

Aurorum is also the biggest non-stellar bodies in the Solar System, with a mean radius of 72207 km (44867 mi). Current model on astrophysics suggest that Aurorum might be among the biggest planets in existence: increase in mass results in stronger gravitational compression cancelling out increase of material, while decrease in mass results in less material along with less significant gravitational compression. It is estimated that the largest planets would have a mass about a third of Aurorum, barring hot gas giants inflated by heat due to extreme proximity to star.


Interaction between magnetosphere of Aurorum and solar wind.

The magnetosphere of Aurorum is the strongest in the Solar system outside sunspots, with its bow shock extending beyond the orbit of Esquarium (78.52 Aurorum radii). The magnetosphere prevents solar wind from directly interacting with its atmosphere or the major satellites, instead trapping the charged particles in the form of a radiation belt. Its shielding effect is believed to play a vital role in the development of life on Esquarium.

Much more massive than Esquarium's own magnetosphere, interaction with charged particles resulted in a faint, permanent aurora at its pole; by contrast, aurora on Esquarium only occur when fluctuation of solar wind and magnetosphere cause its atmosphere to be bombarded by charged particles from either the Sun or Aurorum.

Orbit and rotation

Due to its high mass, the barycentre of Sun-Aurorum system lies at 2.074 solar radii from the centre of the Sun, the only planet in Solar System known to have a barycentre outside the Sun. As astronomical unit (AU) is defined as the average distance between the Sun and Aurorum (148,154,988 km), the semi-major axis of Aurorum around the system barycentre is 148154988 km (92059242 mi; 0.99035492 AU), despite common belief of it being 1 AU by definition.


Due to its deep gravity well, Aurorum is orbited by numerous astronomical bodies and, since the advent of space exploration, artificial satellites. Among them, the vast majority of satellites are smaller than a kilometre in diameter. Only three natural satellites of Aurorum are large enough to achieve hydrostatic equilibrium and thus considered major satellites by modern astronomical definition.

Major satellites

The three major satellites of Aurorum - Candeum, Favillum and Esquarium - are the biggest known satellites within the Solar System, with Esquarium being the largest. Because of their relatively high mass and close orbits, their orbits are in a stable 1:2:4 orbital resonance; for every orbit Esquarium makes around Aurorum, Favillum makes exactly two and Candeum exactly four. This phenomenon significantly increase the mutual gravitational influence between the satellites, most well-known of which being a major contributor of tide on Esquarium, but is also believed to drive tectonic activity of both Esquarium and Favillum, as well as chronic cracking of the icy surface of Candeum. A combination of orbital resonance and tidal force of Aurorum results in tidal circularization and stabilisation of their orbits.

The outermost satellite, Esquarium, is the only object in the Universe known to harbour life, and is home to humanity. The other two major satellites are among the brightest objects in the sky of Esquarium after Sun, Aurorum and Icarum-Celum. Visible to the naked eyes and known since ancient times, both satellites feature prominently in many human culture and mythology. Closest major astronomical bodies to Esquarium, the regular orbits of Candeum and Favillum make them some of the most explored worlds outside Esquarium in modern times, and are considered prime candidates for manned human missions due to relatively hospitable environment.

Planetary rings

Image of the main ring of Aurorum obtained from probe in forward-scattered light.

Aurorum has a faint ring system primarily made of cosmic dust. Believed to have formed by ejecta or remnants of minor body collisions, the rings are invisible to naked eyes on Esquarium, and can only be observed by the largest telescopes or in infrared in back-scattered light from the Sun.

The entirety of the ring system has a radius of 96,757 km (60,122 mi)* to 176185 km (109476 mi), and is divided into two major components: torus ring and halo ring at the radius of 121485 km (75487 mi). The inner torus ring is significantly thicker than the outer halo ring, with a maximum width of about 10,000 km, giving it the namesake torus shape compared with the flat halo ring, which has a thickness beneath a few hundred kilometre. The total mass of ring system of Aurorum is estimated to be within the range of 1017 kg (Esquarium has a mass about 6 × 1024 kg).

Despite dust being constantly removed from the ring system through gravitational and electromagnetic force of the system, the mass and distribution of dust particle in the ring system remains constant since its first discovery. It is believed that dust in the ring system is regularly replenished by collision of minor astronomical bodies orbiting Aurorum at a close range, with sizes between 1 cm and 500 m. Comets and asteroids impacting Aurorum also supply the ring system with particles, including temporary increase in albedo due to highly reflective ice particles.

Cultural influence

Sun and Aurorum with face. Some cultures perceive Aurorum or its personification to have a blue or otherwise pale appearance due to its slight white-blue hue.

As the most prominent astronomical body in the sky, Aurorum has a significant impact on many cultures in Esquarium. In many polytheistic religions, Aurorum is personified as a lunar deity, occasionally being related to solar deity of the pantheon, being personification of the Sun. Many pre-Christian or pagan religions worshipped Aurorum and the Sun as their main deities, particularly Slavic pagan religions in Luziyca, Graznia, Baisel and Ceresnia.

The regular change of phase of the planet makes it one of the most obvious timepieces in nature, giving rise to some of the oldest calendars in Esquarium. On average, the phase of Aurorum repeats every 29.53 days, resulting in a lunar month being defined as having either 29 or 30 days in all lunar and lunisolar calendars by definition, or otherwise serve as the basis of length of month in many solar calendars.