How to shuffle planets around? - Understanding Planetary migration in the solar system

In 1995, astronomers found the first-ever planet orbiting another main-sequence star. It was 51 Pegasi, and it was nothing like astronomers had ever envisioned. It was almost the mass of Jupiter but orbited its star every four days. Even Mercury, the smallest planet in our Solar System, needs 88 days to around the Sun. But four days? It seemed almost impossible.

As astronomers continued their hunt, more and more of these massive planets orbiting so close to their stars kept popping up, and they named them Hot Jupiters. We now know of hundreds of these hot Jupiters. And it turns out that about 1.2% of stars have hot Jupiters orbiting around them. It seems absurd, and we have two problems with these types of planets. One, they shouldn't exist. Two, they shouldn't exist at the places we see them!

51 Pegasi b infographic by NASA/JPL-Caltech
51 Pegasi b infographic by NASA/JPL-Caltech

What is planetary migration?

Our old planetary formation models say that you are supposed to see rocky worlds of terrestrial planets close to the star, gas planets in between, and ice worlds farther out. Gas giants like Jupiter can't form close to their star; the heat would make all the volatile gasses evaporate. The radiational eruptions from a newly forming star should have wiped out any excess matter from the internal orbits, stopping planets' formation.

The only way out of this mess is if the planets developed farther out and moved to their current positions, resulting from planetary migration.

Planetary migration happens when a planet interacts with a disk of gas or planetesimals or any other body in the star system, resulting in modifying its orbit around the star. The most popular theory about planetary migration is the Nice model. An international group of scientists drafted a series of papers explaining the Solar System's arrangement as we see it today.

According to the paper, billions of years ago, only the giant planets - Jupiter, Saturn, Uranus, and Neptune clustered together into a much more compact formation around the Sun in perfectly circular orbits.

Presently the planets don't follow circular orbits; they're more elliptical, orbiting with a much more significant gap between them.

Uranus seems to be rotating sideways. Neptune's Moon that orbits in the reverse direction to every other large Moon in the

Solar System. It's complete chaos! What could have possibly happened to create such a dramatic change? Scientists account for these changes through Planetary migration.

simulation by By AstroMark - Own work

Simulation showing the outer planets and the Kuiper belt: a) Before JupiterSaturn 2:1 resonance. b) Scattering of Kuiper belt objects into the Solar System after the orbital shift of Neptune. c) After ejection of Kuiper belt bodies by Jupiter. Planets are shown: Jupiter (green circle), Saturn (orange circle), Uranus (light blue circle), and Neptune (dark blue circle). Simulation created using data from the Nice Model

Types of planetary migration:

There are three kinds of migration that can occur in a planetary system. Type I is gas-driven. In the early stages of any planetary system, the primordial planets orbit the star along with a disk of gas and dust. As they travel around, the gas stacks up into higher density regions; in these regions' gravity causes a planet to gain or lose angular momentum, pushing it closer or farther from its star. If the planet loses angular momentum, it gets drawn outwards, and if it gains angular momentum, it is drawn inwards.

Larger planets like Jupiter clear out these regions pretty quickly, which leads us to Type II, which happens after a planet has wiped out the disk. Now the mass of the planet itself pulls the gas around it into the open gap in the disk.

The pushing and pulling effects cause the planet to lose momentum, making it migrate inward; thus, planets like hot Jupiters can get close to their stars. This process gets out of control, leading to Type III migration, where other material like gases enters the gap, causing an inward migration. In just a few orbits, the planet gets pulled significantly inward. Our Solar System chose the path of violence when it comes to migration. Planets and planetesimals smashed into each other, threw things out of orbit, and planets switched positions. Yet, planetary migration played a pivotal role in forming the solar systems we see today.

The Grand tack hypothesis:

The grand tack hypothesis is an extension of the Nice model that explains the size of Mars and the asteroid belt. According to this hypothesis, Jupiter migrated inwards up to the current position of Mars and later reversed its location due to Saturn's gravitational pull, ultimately ending up near its current orbit. Jupiter's planetary migration's reversal is almost like a sailboat changing directions (tacking) as it travels against the wind.

The planetesimal disk got trimmed by Jupiter's movement, limiting the raw material available to form Mars. Jupiter double-crosses the asteroid belt, scattering asteroids both outward and inward. As a result, the asteroid belt ends up with a reduced mass, disorders, inclinations, irregularities, and materials originating from both inside and outside Jupiter's orbit. The collisions were so intense; it may have pushed an early generation of planets into the Sun.

How did everything happen?

As we read in the previous article about planets' formation, only the giant planets - Jupiter, Saturn, Uranus, and Neptune formed first. They weren't at their current positions but were grouped in compact, circular orbits formation around the Sun. Small planetesimals surrounded them like an envelope. The innermost planet Jupiter was almost at the current position of Mars, closely followed by the other planets.

Planetesimals at the inner edge interacted with the outermost giant planet, which changes these small objects' orbits. As a result, the planets scatter the small icy bodies and push them inwards. These icy bodies' inward movement causes the outermost planet to tug outwards, preserving the angular momentum. This event continues till these planetesimals scatter off the next planet they encounter, successively moving the orbits of Uranus, Neptune, and Saturn outwards. A minute movement can change the orbits of the planets by significant amounts.

The process continues until the planetesimals interact with the most massive planet, Jupiter, whose enormous gravity throws them into highly elliptical orbits or even expels them entirely from the Solar System, which in turn causes Jupiter to move slightly inward.

After several millions of slow, gradual migration, Jupiter and Saturn attain their 1:2 mean-motion resonance. 1:2 mean motion resonance means for every rotation Jupiter completes, Saturn completes two. It's the perfect gravitational dance. This resonance amplifies their orbital irregularities, destabilizing the whole planetary system. The order of the giant planets changes suddenly, and Jupiter moves Saturn to its current position. This position shift among the gas giants causes gravitational interferences between two ice giants, driving Neptune and Uranus into eccentric orbits.

This time it's the ice giants' turn to perpetuate the chaos. Neptune and Uranus then break into the planetesimal disk, entirely scattering it and eliminating 99% of its mass. Some of the planetesimals get thrown into the inner Solar System by these giants like fun confetti, creating unexpected havoc on the terrestrial planets, called the Late Heavy Bombardment.

The planets mangling among themselves is not a new thing. They get caught in each other's gravity or tidal forces from the Sun all the time. But all these events shape the star system we see today, and it's the only one we know that bears life. The late heavy bombardment, planetary migration, formation of satellites and ring systems around planets shaped us. They gave way to life on this planet. Stay tuned to learn more about the events that shaped the solar system.


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