 Artist's impression | |
| Orbital characteristics | |
|---|---|
| 9 AU (hypothesized) | |
| 60-70 Earth years (hypothesized) | |
| Star | Sun (formerly) |
| Physical characteristics | |
| 74,814 km (hypothesized)[citation needed] | |
| 37,407 km (hypothesized)[citation needed] | |
| Mass | 10–20M🜨 (hypothesized) |
Meandensity | 1.27 g/cm³ (hypothesized)[citation needed] |
TheFifth Giant is a hypotheticalice giant proposed as part of theFive-planet Nice model, an extension of theNice model ofsolar system evolution. This hypothesis suggests that the earlySolar System once contained a fifth giant planet in addition to the four currently known giant planets:Jupiter,Saturn,Uranus, andNeptune.[1] The Fifth Giant is theorized to have been ejected from the Solar System due to gravitational interactions during the chaotic phase ofplanetary migration, approximately 4 billion years ago.[2]
TheNice model, developed in the early 2000s, describes thedynamical evolution of the Solar System following the dissipation of theprotoplanetary disk. It posits that thegiant planets initially formed in a more compact configuration and subsequently migrated to their current orbits due to interactions with a massive disk ofplanetesimals.[1] These interactions are believed to have triggered a period of orbital instability, resulting in the dispersal of the planetesimal disk and the capture ofirregular moons.[2]
The addition of a fifth giant planet to this model arose as researchers attempted to resolve discrepancies between the Nice Model's predictions and observational data, particularly regarding the current orbital distribution of theouter planets and theKuiper belt.[1][2]
The Fifth Giant is hypothesized to have been anice giant, similar in composition toUranus andNeptune. It likely had a mass between 10 and 20Earth masses and an orbit initially located between those ofSaturn and Uranus.[1] Computer simulations indicate that such aplanet could have influenced thedynamical evolution of the Solar System, shaping the orbits of theouter planets and accounting for the observed gaps in theKuiper belt.[1][2]
The ejection of the Fifth Giant is believed to have occurred during theearly Solar System's period of instability, when gravitational interactions between thegiant planets became chaotic.[3] The planet likely encountered a series of close gravitational encounters withJupiter orSaturn, resulting in its eventual expulsion from theSolar System.[1][3] Such an event would have minimized the disruption to the orbits of the remaining planets while aligning with constraints derived from their current orbital architecture.[4]
The ejection process may have also played a role in scatteringplanetesimals to form theOort cloud or altering the trajectories ofcomets andasteroids.[1]
Direct evidence for the Fifth Giant's existence is lacking, as the planet would have been ejected intointerstellar space and is no longer gravitationally bound to theSun. However, indirect evidence has been cited to support the hypothesis:
The concept of an additional giant planet is distinct from the search forPlanet Nine, ahypothetical planet proposed to explain the clustering of certaintrans-Neptunian objects.[5] While both hypotheses suggest the presence of a missing planet, the Fifth Giant would have been ejected billions of years ago,[3] whereas Planet Nine is theorized to remain within theSolar System.[6] However, it is possible that if Planet Nine exists, it could very well be the Fifth Giant as stated byMichael E. Brown during a Twitter inquiry.[7]
