The hypothetical fifth gas giant is an additional planet added by some theorists to the recent versions of the Nice model. The hypothetical fifth gas giant states that a fifth giant planet was 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 increase the likelihood of their reproducing the current Solar System.
Current theories of planetary formation do now 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.
Colleagues have suggested several names for the hypothetical fifth gas/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 was "Thing 1", from Dr. Seuss' Cat in the Hat children's book.