Fifth Giant

This object is hypothetical.

This object is hypothetical, and was proposed to explain a phenomenon or occurrence. Note that such object has not been discovered or observed.

This page is about the Fifth Giant. You may be looking for Nibiru, Tyche, or Planet Nine.

Fifth Giant
Discovery
Discovered By	None (Hypothesized)
Discovery Data	Theorized (in 2011)
Orbital Characteristics
Semi-major Axis	Varied up until it was Ejected
Orbital Period (sidereal)	Varied up until it was ejected
Satellites	???
Physical Characteristics
Mean Radius	~37,407 km
Mass	211.14 (1024 kg) 35.35 M⊕
Mean Density	963 kg/m3 0.17 ρ⊕
Atmospheric Characteristics
Composition by Volume	??? (Presumably Hydrogen, Helium, Methane, and Ammonia)
Observation Data
Mean Distance from Earth	Likely incredibly Distant

The Fifth Giant is a planet proposed by the Five-planet Nice model and the Jumping-Jupiter scenario, theorized to have orbited between Saturn and Uranus during the early formation of the Solar System. Over time, Neptune migrated outward, disturbing the Fifth Giant’s orbit. Eventually, gravitational interactions with Saturn and Jupiter caused the Fifth Giant to be ejected from the Solar System. It is believed to have been an ice giant, similar to Uranus and Neptune. Some researchers speculate that the Fifth Giant could also be linked to Planet Nine, another hypothetical ice giant thought to have been ejected during the same migratory events.

Formation

The Fifth Giant likely formed between Saturn and Uranus around 4.5 billion years ago, during the early stages of the Solar System's development. Like other planets, its formation began with the accumulation of a dense clump of rocky and icy material, which served as its core. The immense gravity of this core attracted additional gas and particles, allowing the Fifth Giant to grow into a Jovian ice giant. This growth mirrored the development of Uranus and Neptune, making the Fifth Giant potentially similar in composition and structure to these planets.

Atmosphere

Since the Fifth Giant has not been directly observed or confirmed, its atmospheric composition remains theoretical. However, based on its classification as an ice giant, scientists hypothesize that it would share similarities with Uranus and Neptune. Its atmosphere was likely composed of hydrogen and helium, along with trace amounts of methane, ammonia, water, and other ices. Methane absorption would have given it a bluish hue, much like Uranus and Neptune.

Ejection (The Five-planet Nice model)

The ejection of the Fifth Giant is a central element of the Five-planet Nice model. This event occurred around 4.49–4.47 billion years ago during a chaotic period known as the Migration of Neptune. During this time, the outer planets shifted significantly from their original positions. The Fifth Giant, which lacked a stable orbit, was pulled into a chaotic trajectory by the gravitational forces of Jupiter and Saturn. These interactions eventually flung the planet out of the Solar System entirely.

Theories about the Fifth Giant's fate include:

• Capture by a Nearby Star System: The Fifth Giant may have become an exoplanet in another star system, potentially already discovered but not yet identified as the missing ice giant.
• Transformation into a Rogue Planet: It might now wander the galaxy as a rogue planet, untethered to any star.
• Survival as Planet Nine: Some researchers argue that the Fifth Giant was not fully ejected but instead ended up in a distant, highly elliptical orbit, making it synonymous with the elusive Planet Nine.

Name

The Fifth Giant has not been given a formal name, as its existence remains unconfirmed. Several names have been proposed, notably by planetary scientist David Nesvorný. Among these suggestions are Hades, Liber, and Mephitis, which are drawn from Roman and Greek mythology. A more whimsical proposal, Thing 1 (from Dr. Seuss's Cat in the Hat), was also suggested but was dismissed by Nesvorný as unsuitable.

If the Fifth Giant is ever discovered and confirmed, the International Astronomical Union (IAU) will certify an official name. Priority for naming would go to its discoverers, with the likely inspiration coming from mythology, as is traditional for celestial bodies.

Trivia

• While the Fifth Giant and Planet Nine share some similarities, they are distinct. The Fifth Giant is believed to have been fully ejected from the Solar System, while Planet Nine is hypothesized to have a highly elliptical orbit far beyond Neptune.
  • It should also be worth noting that Planet Nine's mass is way too low for it to be an ice giant and would have to be a super-earth, in contrast to the Fifth Giant.
  • Another theory posits that Planet Nine was captured from another star system, unlike the Fifth Giant, which formed within the Solar System.
• The name Hades, proposed for the Fifth Giant by Nesvorný, was also used in 1902 by Theodor Grigull for a different hypothetical planet.

Gallery

Artist’s impression of the Fifth Giant, slowly drifting away in interstellar space.

Algol's version of the Fifth Giant.

A random user(s) version of the Fifth Giant.

Billtran2015’s Purposed Fifth Giant Design and edited for it look like this photo was made in 1995.

WhoInvitedThisGuy?!'s version of the Fifth Giant.

Crafter1234's interpretation of the Fifth Giant.