Dynamical history of the Solar System

6 problems

• When did the giant-planet orbital instability happen?

  Contested

  Models suggest Jupiter, Saturn, Uranus and Neptune once went through a violent rearrangement of their orbits. Whether this happened within the first 100 million years or about 700 million years later is still debated.

• How did Neptune's migration shape the structure of the Kuiper belt?

  Open

  The Kuiper belt, the ring of icy bodies beyond Neptune, contains a calm population on nearly circular orbits, a stirred-up population and bodies locked in step with Neptune. Which migration history of Neptune produced this mix is not settled.

• Was there a late heavy bombardment spike about 3.9 billion years ago?

  Contested

  Moon rocks brought back by Apollo showed many impact melts about 3.9 billion years old, suggesting a sudden burst of impacts. It may instead be the tail of a steadily declining impact rate, sampled unevenly.

• Why does Mercury have such an outsized iron core?

  Open

  About 70 percent of Mercury's mass is in its metal core, far more than for Earth or Mars. It may have lost its rocky outer layers in a giant impact, or formed from material enriched in iron near the Sun.

• Is there an undiscovered planet beyond Neptune?

  Contested

  Some very distant icy bodies have orbits that seem lined up in a way that a hidden planet several times Earth's mass could explain. Others argue the alignment is an accident of where telescopes looked.

• Why is Mars only about one tenth of Earth's mass?

  Contested

  Standard models of planet formation build a Mars almost as big as Earth, yet the real Mars is about ten times lighter. Something must have removed or never supplied material near Mars's orbit.