What is planetary migration?
Planetary migration is when a planet or a stellar satellite interacts with a gas disk or planetesimals (minute planets), altering the satellite's orbit.
How is it related to the solar system?
The four planets in the star system Kepler-223 appeared to have a little in common with the planets in our system. The Kepler’s planets orbit their star in the same configuration as Jupiter, Saturn, Uranus, and Neptune may have had in the early history of our solar system.
It all started with our gas giant, Jupiter. It traveled up all the way to Mars' orbit, 1.5 AU(Astronomical Units, one Unit is 93 million miles) is from the sun. Then it went back to the location where it sits now, nearly four times the distance from the sun. Lucky for Mars, this happened before any of the terrestrial planets were born and there were only the gas giants.
Jupiter was drawn to the Sun by the first type of planetary migration, gas driven, effects depending on the mass of the planet. For lower mass planets, like Earth, the mechanism occurs when the planet’s orbit perturbs the gas around or planetesimal disk sending spiral density waves into it. An imbalance can occur between the strength of the interaction with the spirals inside and outside the planet’s orbit, making the planet to gain or lose angular momentum. If angular momentum is lost the planet migrates inwards, and if it's gained it travels outwards. This is known as Type I migration.
In the case of higher mass planets, like Jupiter, their strong gravitational pull makes a sizable gap in the disk which ends Type I migration, and allows Type II to take over. Here, the material enters the gap and moves the planet and gap inwards over the gradually increasing timescale of the disk. This migration mechanism is thought to explain why hot Jupiters (huge gas planets that orbit really close to their star, closer than Mercury is to the Sun) are found in such close proximity to their stars in other planetary systems. The third type of gas driven migration is sometimes referred to as runaway migration, where large-scale vortexes in the disk rapidly draw the planet in towards the star in many orbits.
The three types of planetary migration. Credit to Frédéric Masse.
The best understanding of how the planets have moved in throughout our system’s evolution rose from the Nice Model, proposed by an international collaboration of scientists in 2005. This model suggests that at the inner edge of the icy disk, some 35 AU from the Sun, the outermost planet began interacting with icy planetesimals, influencing the second type of migration to occur: gravitational scattering. Comets were shot from one planet to the next, which gradually caused Uranus, Neptune, Saturn and the belt to migrate outwards. Jupiter’s powerful gravity flung the icy objects that reached it into highly elliptical orbits or out of our solar system entirely, which in order to conserve angular momentum, further propelled its journey inwards.
An addition to this theory is the 'Grand Tack model', which is named after the unusual course of Jupiter’s migration towards the Sun before stopping and migrating outwards again, like a sailboat tacking about a buoy. At the distance that Mars would later form, material had been swept away due to Jupiter’s presence. This resulted in the stunted growth of Mars and a material abundant region from which the Earth and Venus formed, explaining their respective sizes. The gas giant’s travels also stopped the rocky material in the asteroid belt from turning into larger bodies due to its strong gravitational influence. Although Jupiter switched positions with the asteroid belt twice, the movements were so slow that collisions were minimal, resulting in more of a gentle displacement.
Why did Jupiter’s migration to the Sun cease? It was Saturn. As the two planets moved further away from each other, it was thought they became temporarily locked in a 2:1 orbital resonance. That meant that for every orbit of the Sun Saturn made, Jupiter made two. The Nice Model showed that the planetary coupling increased their orbital eccentricities and rapidly destabilized the entire system. Jupiter forced Saturn outwards, pushing Neptune and Uranus into extremely elliptical orbits where they gravitationally scattered the dense icy disk far into the inner and outer Solar System. This disruption, in turn scattered almost the entire primordial disk. Some models also show Neptune to have been pushed past Uranus to become the farthest planet from the Sun. Over time, the orbits of the outermost planets settled back into the near circular paths we observe today.
The Nice Model explains the present day absence of a dense trans-Neptunian population and the positions of the Kuiper belt and Oort cloud. It also accounts for the combination of icy and rocky objects in the asteroid belt, like water-rich dwarf planet, Ceres, which likely originated from the icy belt. The rapid scattering of icy objects, around 4 billion years ago, dates with the onset of the late heavy bombardment period, which is, for the most part, recorded from the Moon’s well-preserved surface.
However, there are problems with the original Nice Model, where some simulations found that the gradual 2:1 repeating coupling of Jupiter and Saturn would have resulted in an extremely unstable inner Solar System from which Mars would have been ejected. Later research has resulted in the 'Nice 2 Model', which suggests that the gradual scattering of planetesimals made the two gas giants to fall into a 3:2 orbital resonance (not the originally proposed 2:1), allowing for the Nice Model to work with a stable inner Solar System.
The final mechanism for planetary migration occurs through tidal interactions between different bodies. Unlike gas driven migration and gravitational scattering, tidal forces act over a much longer timescale, billions of years. The process begins due to the Kozai mechanism, which is suggested to pump orbit from circularity into a planet’s orbit. As the tidal forces correct this effect by circularizing its orbit, the planet moves closer in. While the orbits of the terrestrial planets are thought to have stayed pretty stable throughout the evolution of the Solar System, this gradual process is likely to have slightly altered their paths and will remain to do so.
The knowledge of how our own planetary system evolved has helped answer many questions about unusual exoplanet orbits, but there is still a lot left to uncover. One such question asks, why we observe so many hot Jupiters unfathomably close to their star, as without another large body’s influence, should it not eventually be swallowed up? Perhaps planet-disk interactions decouple at such close proximities to the star and tidal forces prevail, or perhaps we are capturing a snapshot in time right before the planet meets its destiny. For now, only time, further observations and more exoplanet discoveries will tell!
Savage, Katy. "The Role of Planetary Migration in the Evolution of the Solar System." Planet Hunters. ?, 09 May 2014. Web. 14 Feb. 2017.