Hypothetical fifth gas giant
The hypothetical fifth gas giant is an additional planet added by some theorists to recent versions of the Nice model. The fifth giant planet is ejected from the Solar System following gravitational encounters with Saturn and Jupiter. The inclusion of five giant planets in numerical models of the early Solar System has been shown to increases the likelihood of their reproducing the current Solar System.
Current theories of planetary formation do not allow for the accretion of Uranus and Neptune in their present positions. The protoplanetary disk was too diffuse and the time scales too long for them to form before the gas disk dissipated and numerical models indicate that later accretion would be halted once Pluto-sized planetesimals formed.
It is now widely accepted that the Solar System was initially more compact and that the outer planets migrated outward to their current positions. The planetesimal-driven migration of the outer planets was first described by Fernandez and Ip. This process is driven by the exchange of angular momentum between the planets and planetesimals originating from an outer disk. Early dynamical models assumed that this migration was smooth. In addition to reproducing the current positions of the outer planets, these models offered explanations for: the populations of resonant objects,  the eccentricity of Pluto's orbit, the inclinations of the hot classical objects and the retention of a scattered disk, and the mass depletion of and the location the outer edge of the Kuiper belt near the 2:1 resonance with Neptune. However, these models failed to reproduce the eccentricities of the outer planets, leaving them with very small eccentricities at the end of the migration.
The original Nice model resolved this problem by beginning with the Jupiter and Saturn inside their 2:1 resonance. Jupiter's and Saturn's eccentricities are excited when, after a period of slow divergent migration, they cross the 2:1 resonance. This destabilizes the outer Solar System and a series of gravitational encounters ensues during which Uranus and Neptune are scattered outward into the planetesimal disk. There they scatter a great number of planetesimals inward accelerating the migration of the planets. The scattering of planetesimals and the sweeping of resonances through the asteroid belt produce a bombardment of the inner planets. In addition to reproducing the positions and eccentricities of the outer planets, the original Nice model provided for the origin of: the Jupiter and Neptune Trojans; the irregular satellites of Saturn, Uranus, and Neptune; the various populations of trans-Neptunian objects; the magnitude of, and with the right initial conditions, the timing of the Late Heavy Bombardment.
The original Nice model was not without its own problems, however. During Jupiter's and Saturn's divergent migration secular resonances sweep through the inner Solar System. As the ν5 secular resonance sweeps through the terrestrial planet region it excites eccentricities beyond their current values potentially destabilizing the inner Solar System. Jupiter's and Saturn's slow approach to the 2:1 resonance is particularly problematic as in numerical simulations Mars's orbit intersects those of the other planets resulting in collisions between planets or in Mars's ejection from the Solar System. The orbits of the asteroids are also significantly altered as the ν6 secular resonance excites eccentricities and the ν16 secular resonance excites inclinations as they sweep across the asteroid belt. As a result the surviving asteroid belt is left with a larger fraction of high inclination objects than is currently observed.
Maintaining the low eccentricities of the terrestrial planets and reproducing the eccentricities and inclinations of the asteroid belt requires a giant planet migration more rapid than that produced in models of planetesimal-driven migration. As a solution to this problem theorists propose that the divergent migration of Jupiter and Saturn was dominated by planet-planet scattering. Specifically, one of the ice giants was scattered inward onto a Jupiter-crossing orbit by an gravitational encounter with Saturn after which it was scattered outward by a gravitational encounter with Jupiter. Jupiter's and Saturn's orbits rapidly diverge as a result. This evolution of the giant planets orbits, similar to processes described by exoplanet researchers, is referred to as the jumping-Jupiter scenario.
Five giant planet early Solar System
Researchers have found that the jumping-Jupiter scenario makes reproducing the current outer Solar System unlikely when numerical simulations are begun with four giant planets. The inner ice giant is often ejected following its encounter with Jupiter when planetesimal belt masses typical of the Nice model are used. While increasing the mass of the planetesimal belt was found to increase the likelihood of retaining the ice giant it typically resulted in the excessive separation of Jupiter and Saturn. This led David Nesvorný of the Southwest Research Institute to propose that the Solar System began with five giant planets with an additional Neptune-mass planet between Saturn and Uranus. Using 1000's of simulations with a variety of initial conditions he found that the simulations beginning with five giant planets were ten times more likely to reproduce the current Solar System. More extensive investigations using a wider variety of initial conditions including systems beginning with six planets produced similar results. These investigations also revealed that Jupiter's eccentricity was the most difficult aspect of the current Solar System to reproduce. Simulations in which Neptune migrated outward into the planetesimal disk before the gravitation encounters between the ice giant and Jupiter began were found to yield the best results. Other researchers using different criteria to identify successful simulations found that the four and five planet systems had a similar likelihood of reproducing the outer Solar System and that preserving a primordial cold classical belt required that the additional planet be ejected in 10,000 years.
The whereabouts of the hypothetical fifth gas giant are currently unknown, although according to Takahiro Sumi of Osaka University, other observable rogue planets exist in interstellar space away from other stars.
According to Nesvorny, colleagues have suggested several names for the hypothetical fifth ice giant- Hades, after the Greek god of the underworld: Liber, the Roman god of wine and a cognate of Dionysus and Bacchus: and Mephitis, the Roman goddess of toxic gases. Another suggestion is "Thing 1" from Dr. Seuss' Cat in the Hat children's book.
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