What Is the Nice Model, or How Did Our Solar System Form?
Many models of our solar system's birth and growth have been formed and just as quickly disproven. Around 2004 a team of scientists met in Nice, France and developed a new theory as to how the early solar system developed. This new model that they created was an attempt to explain some of the mysteries of the early solar system, including what caused the Late Bombardment Period and what pulled the Kuiper Belt together. Though not a definitive solution, it nonetheless is another stepping stone to the ultimate truth of how the solar system evolved.
Before the Resonance
Initially, in the solar system, all the planets were closer together, in circular orbits, and also nearer to the sun. The terrestrial planets were in the same configuration as they are now, and the asteroid belt was still between Mars and Jupiter, the remnants of destruction through gravity (which plays a central role in this scenario). What was very different about the solar system then was the situation with the gas giants. They were all initially much closer together and therefore closer to the Sun because of gravitational and centripetal forces. Also, Neptune was not the eighth planet nor was Uranus the seventh but were in each other's present positions, switched. Much of the objects that now reside in the Kuiper Belt were closer than they are now but were overall further away from the nearest planet to them than they are now. Also, the belt was much denser and full of icy objects. So what caused this all to change?
Jupiter and Saturn Enter Resonance
A subtle nuance of gravity-bound objects is an effect called resonance. This is when two or more objects complete orbits in a set ratio to one another. A few current examples are Neptune and Plutinos, or objects like Pluto which reside in the Kuiper Belt. These objects exist in a 2:3 resonance, which means that for every three orbits that Neptune completes, the Plutino completes two orbits. Another famous example are the Jovian moons, which are in a 1:2:4 resonance.
Jupiter and Saturn began to enter such a resonance about 500-700 million years after the solar system formed. Slowly but surely, Saturn began to complete one orbit for every two orbits that Jupiter went through. Because of the slightly-elliptical nature of orbital motion and this resonance, Saturn would get extremely close to Jupiter at one end of its orbit and then would get extremely far away at the other end of its orbit. This essentially created a huge tug-of-war with the gravity in the solar system. Saturn and Jupiter would pull on one another, then release much like a spring. The losers in this constant shifting were Neptune and Uranus, for as Saturn was being perturbed it would cause the orbits of the outer two gas giants to grow increasingly unstable. Eventually, the system could not take any more, and chaos ensued (Irion 54).
Resonance Breeds Destruction
Once Saturn got close to the resonance, it began to affect the dynamic between Neptune and Uranus. Its gravity pull would accelerate both planets, increasing their velocities (54). Neptune was kicked out of its orbit and sent farther out into the solar system. Uranus got tugged in the process and was pulled with Neptune. As Neptune moved outward, the closer edge of the Kuiper Belt was tugged by this new planet, and much icy debris was sent flying into the solar system. The Asteroid Belt would have also been kicked up during this. All of this material managed to impact many of the terrestrial planets including Earth and the moon and is known as the Late Bombardment Period (54).
Eventually, though interacting with Uranus on its way outward as well as the inner edge of the Kuiper Belt, Neptune settled down into a new orbit. But now the gas giants were further apart than ever, and the Kuiper Belt now having its closer edge in great proximity to Neptune. The Oort Cloud was possibly formed during this also, with material being shot out of the inner solar system (54). All of the tuggings of the planets pull Saturn out of its resonance with Jupiter, and all traces of the destruction it laid to waste are only visible in certain places in the solar system such as the moon. The planets arrived into their final configuration through this resonance and will remain so...for now...
Large claims require large support, so what if any exists? The Stardust mission after visiting comet Wild 2 returned a sample of comet material. Instead of having carbon and ice (which formed away from the sun), a particular dust speck named Inti (Inca for the god of the sun) had large amounts of rock, tungsten, and titanium nitride (which formed near the sun). Those require a 3000 degree Fahrenheit environment, only possible near the sun. Something had to shake up the order of the solar system, just like what the Nice Model predicts (46).
Pluto was another clue. Way out in the Kuiper Belt, it had an odd orbit that was not in the ecliptic (or plane of the planets) nor was it mostly circular but very elliptical. Its orbit causes it to be as close as 30 AU to the sun and as far away as 50 AU. Finally, as mentioned earlier Pluto and many other Kuiper Belt Objects have a 2:3 resonance with Neptune. They cannot interact with Neptune because of this. The Nice Model shows that as Neptune moved outward, it tugged on the gravity of the Plutinos just enough to cause their orbits to enter resonance (52).
Mercury also provides clues to the likelihood of the Nice Model. Mercury is an oddball, basically a huge ball of iron with a minimal surface. If many objects collided with the planet, it could have had any surface material blasted off. On top of this, the orbit of Mercury is highly eccentric, further hinting at some major interaction(s) to help nudge it out of shape (Redd).
Indirect evidence exists when one examines Kepler systems, specifically the zone that corresponds to the inner zone before Mercury. Those systems have exoplanets in that zone, which is odd considering ours doesn't. Sure, some difference are expected but the more we find, the more likely it is we are an exception. About 10 percent of all exoplanets are located in this zone. Kathryn Volk and Brett Gladman (University of British Columbia) looked at computer models that showed what should end up happening, and sure enough, frequent collisions and planetary ejections would be normal, leaving a zone where roughly 10 percent are left. Turns out, solar system chaos is frequent! (Ibid)
The Nice Model does a better job of explaining the solar system than the traditional solar nebula theory. Simply put, it states that the planets formed in their present spots from all the material that was in their vicinity. Rocky elements are closer to the sun because of gravity and gaseous elements were further away because of the solar wind the sun-generated. But two problems arise with this. First, if this was so, then why was their a Late Heavy Bombardment Period? Everything should have been settled into their orbits or have fallen into other objects so nothing should have been flying around the solar system like we see it did. Secondly, exoplanets seem to counter the solar nebula theory. Giant gas planets orbit very close to their stars which would not be possible unless some gravitational shuffle caused it to fall into a closer orbit. They mainly have highly eccentric orbits also, another sign of not being in their original position but moved there (Irion 52).
Irion, Robert. "It All Began in Chaos." National Geographic July 2013: 46, 52, 54. Print.
Redd, Nola Taylor. "The Solar System's Violent Past." Astronomy Mar. 2017: 24. Print.
© 2014 Leonard Kelley