Hypothetical fifth gas giant
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The fifth gas giant hypothesis is an attempt to explain apparent inconsistencies in the formation of the Solar System. Apart from Jupiter, Saturn, Uranus and Neptune, theorists argue that there was once a fifth gas giant, which was expelled from the Solar System in its formative period.
Current theories of planetary formation do not allow for the coalescence and development of Uranus and Neptune in their present positions; it is contended that the gaseous primordial disc of dust and gas that formed the early Solar System would have been too diffuse and unable to account for the bulk of these ice giant planets. It is therefore theorized that the early Solar System was more compact than at present, and that they migrated to their current positions as free interstellar gas and dust involved in their formation was incorporated into them.
Hypothetical additional trans-Saturnian ice giant
However, computer simulations indicate that the process of migration should have displaced either Uranus or Neptune. According to David Nesvorny of the Southwest Research Institute in Boulder, Colorado, there was originally a third ice giant between the orbits of Saturn and Uranus that was expelled from the Solar System after a close encounter with Jupiter slung it into interstellar space. The hypothesis is explored in Nesvorney's paper  for Astrophysical Journal Letters.
In 2007, Eric B. Ford (Harvard-Smithsonian Center for Astrophysics) and Eustace Chiang (Center for Integrative Planetary Science) University of California, Berkeley) presented a similar paper within the journal, arguing for the presence of such a hypothetical object as an explanatory mechanism for prior difficulties in existing theories of planetary formation.
There may have been more than one ice giant involved in the process described above, although Nesvorny's reconstruction of a fifth ice giant presence seems to offer the best prospects for the ultimate emergence of a Solar System configured much like our own.
Fifth gas giant and formation of terrestrial planets
Nesvorny argues that his hypothesis also accounts for the survival of the inner terrestrial planets of the Solar System—Mercury, Venus, Earth and Mars—as well as possible additional protoplanets that were lost in the early period of planetary formation due to collisions or accretion with other bodies. In this framework, Jupiter lost angular momentum when it flung the fifth gas giant out of the Solar System, leading Jupiter to recede from the Sun's vicinity, ensuring the stability and survival of the inner planets and causing turbulence within the Kuiper belt and Oort cloud en route out of the Solar System. This may have led to intensive and numerous cometary and asteroid impacts in the inner Solar System, resulting in intensive cratering. This period is known as the Late Heavy Bombardment and occurred 3.9 billion years ago.
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.
- Lisa Grossman: "Lost planet explains solar system puzzle" New Scientist: 01.10.2011: 14-15
- D. Nesvorney, Young Solar System's Fifth Giant Planet? preprint
- Eric B. Ford and Eustace Chiang: "The Formation of Ice Giants in a Packed Oligarchy: Instability and Aftermath" 2007 Astrophysics Journal 661.602doi.10.1086/513598: http://iopscience.iop.org/0004-637X/661/1/602?fromSearchpage=true
- Grossman, 2011: 15
- A New Name for an Old Planet: New Scientist: 01.10.2011: 15