Planetary Migration

By: Robbie-Lee Kozuska, Jordan Irwin, James Manson, Skylar Tizzard

  Figure 1: https://scitechdaily.com/images/Titan-Might-Pre-Date-Saturn.jpg

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Formation of the Solar System

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Planetary Migration: The Key to Our Solar System

The location of a planet at this moment could be deceiving; the spot it is currently in might not be where it always has been, or where it is going to end up. Planetary Migration is the movement of planets in a solar system. There are many different theories and hypotheses on how our solar system came to be; although, planetary migration is one of the most widely-accepted by scientists. There are multiple different types of migration that help to explain the different ways in which this movement is described. The Nice model is currently the best understanding of how the planets have moved throughout time. This model was proposed by an international collaboration of scientists in 2005 and is used to explain the evolution of the solar system. [12] The Nice 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 sort of migration to occur: gravitational scattering.” [12] Planetary migration is a very large topic with many subtopics. We have explored the main topics and subtopics in depth and will be explaining them below.

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Nebular Hypothesis and the Sun 

There are many hypotheses and models that exist today of solar system formation. Focused on will be the most popular and accepted model which is called the nebular hypothesis. It was first developed in the 18th century and has underwent changes and refinement over the years. The concept starts by explaining that the Solar System formed from the gravitational collapse of a fragment of a giant molecular cloud of gas and dust called a nebula.[1] This collapse triggered a spinning momentum, and as it condensed it spun faster atoms collide more frequently creating heat and creating a protostar.[2] Over millions of years the pressure and heat in the star became so high that hydrogen in it started to fuse leading to the star entering the “main sequence” or main phase of its’ life, which it is still in today.[1]

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Planetary Creation

The planets that exist today are thought to have began as small grains of dust leftover from the nebula collapse that orbited around the protostar. These grains of dust would collide and form together, becoming larger and larger, growing only centimetres per year over the course of millions of years.[3] The inner Solar System was too warm for molecules like water and methane to condense, so the planets such as  Earth, Mercury, Venus, and Mars could only form from metal compounds; which are quite rare in the universe, limiting their possible size. These metal/rocky clumps would become terrestrial planets. The planets that formed farther out, where it was possible for abundant icy compounds to remain frozen grew massive. Giant planets such as Neptune, Uranus, Saturn, and Jupiter.[4] Proof of these conclusions can be found by observing our solar system, firstly “All the planets orbit the Sun in the same direction. Most of their moons also orbit in that direction… (and the Sun) rotate in the same direction. This would be expected if they all formed from a disk of debris around the proto-Sun.”[5] and secondly “The planets also have the right characteristics to have formed from a disk of mainly hydrogen around a young, hot Sun. Those planets near the Sun have very little hydrogen in them as the disk would have been too hot for it to condense when they formed. Planets further out are mostly hydrogen, (since that was what was mostly in the disk), and are much more massive because there was so much more material they could be made from.”.[5] From continuing to observe the planets and materials in our own and other solar systems we can gain better understanding of how they are formed.

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Three Types of Planetary Migration

As stated before, planetary migration “is the decrease or increase in the orbital radius of a planet embedded in a protoplanetary disk due to interactions with the surrounding gas and/or solid material.” [11] Planetary migration is known to occur when there is a change in orbital momentum, such as a planet losing or gaining orbital angular momentum. This can be caused either by friction or by an “imbalance of momentum transfer” [11] between the planet and disk material.

The first type of planetary migration describes a process where a planetary disk pulls or pushes a planet to a new position. This type occurs when a planet is “too small to clear a gap in the protoplanetary disk”. [12]

The second type of migration occurs as a result of gravitational interactions between close-by planets or bodies. Type two only occurs when a large object is able to shatter a smaller one and create an equal force that bounces back on itself and therefor results in planetary movement.

The third type of planetary migration is also due to a gravitational effect, called tidal forces. This type occurs only between the planet and star and almost always results more circular orbits. The third type of planetary migration takes place over a long timescale of billions of years due to the fact that it “occurs through tidal interactions between different celestial bodies”. [12] This type of planetary migration acts over a much longer      period of time than any of the other types.

Planetary migration is believed to be responsible for the position of giant extrasolar planets that have been discovered “orbiting at very small orbital radii” [12] and actually may be an important part in the evolution of protoplanetary bodies. It is also thought to have influenced the architecture of the Solar System.

 

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Results of Planetary Migration

Planetary migration is responsible for many phenomena that we observe in our night sky. It can be attributed to most of what we study in Astronomy, including the formation of planets and the elliptical orbits that our planets take. The solar system is under constant transformation which is a direct cause of planetary migration. The formation of planet cores is unique to planetary migration the debris within the disks are what eventually clumps together amassing for the core of a developing planet the core then moves inward or outward in the disk and this subsequently propagates what the planet will be made of typically of a hard rocky outer crust or else enveloped in gaseous materials.

Core accretion is the first theory on how planets came to be, but does not seem to be a great enough explanation for the development of the gas giants; although it seems convincing enough for the creation of terrestrial planets. Disk instability is a relatively new discovery that helps explains what core accretion cannot in terms of an overall explanation of how the planets came to be and provides an explanation as to why there is Jovian planets and Terrestrial planets. The core accretion model states that the rocky cores of the planets were formed first, then gathering the lighter elements to form the planets outer layers; this being an explanation for the terrestrial planets. [9] To back up the theory of core accretion is the observation of one such exoplanet gave credibility to the theory. A giant planet orbiting a star like sun called HD 149026 was observed and confirmed. Core accretion sees that a core needs to accumulate critical mass before it can accrete gas, thus, giving credit to the third type of gas-driven migration; which uses a vortex of winds to accumulate gases.[9] It is important to note that the core is dependant upon planetesimal accretion, which is the gathering of planetesimal debris to help expand the mass of the core. For the gas giants, this [G3] [G4] [G5] seems untimely; the ring of gas that will orbit the sun will only last 4-5 million years. It is either gathered by the planets or it simply evaporates.

The Disk instability theory suggests that over time, clumps will compact into a planet. These planets can form as quickly as a thousand years, trapping the disappearing gases inside. It is also suggested that these masses quickly stabilize themselves so they do not orbit into the sun. With the lighter materials being trapped inside, the heavier denser materials eventually sink to the core. Planetary migration is of course not just exclusively responsible for the formation of the planets, but the eventual orbit pattern that is observed today, along with any changes that have happened over time. [7] Planetary migration interacts with disks that are made of either gas or planetesimals, typically resulting in the alteration of the orbital parameters. The grand tack hypothesis can conceptualize this by suggesting that when Jupiter formed at 3.5 AU (Astronomical Units) from the sun (AU being the measurement of how far the coinciding planet is from the sun.) [8] It is thought that Jupiter migrated closer to about 1.5 AU away from the sun until it reversed its orbit to move outwards during the event of acquiring Saturn in its orbital resonance. This stopped when it reached its current distance at 5.2 AU. [8]

Migrating inward is due to loss of angular momentum. When a planet migrates outward, it is because of a gain in angular momentum until a planet equilibrates and stabilizes. This is also key in explaining the orbital parameters because the planets all have specific movement, which is also the ecliptic pattern that Kepler proved in his first law when researching the orbit of Mars. Of course, this attributed to the different size of the orbit, but all move relative to the ellipse pattern. When a planet orbits, it reaches the parhelion (which means the planet is the closest it will be to the sun on the opposite end of the spectrum). It will be at a point where the planet will be furthest from the sun, called the aphelion (which means that planets orbit slower when at the aphelion opposed to the increased speed in orbit when at their parhelion which attributes to the length of our years). Planetary migration has contributed to all of these theories and hypothesis and has allowed us to gain a better understanding of the processes that formed the planets and their orbital paths.

 

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Jupiter’s Role in our Solar System’s Evolution

The Solar System has gone through billions of years of planetary migration and evolution to achieve the current positions of the planets in our Solar System. It is common for other Solar Systems to be tightly packed with planets ranging in sizes around that of Earths, most having circular orbits. Another common trait of planetary development found in solar systems across space is that they contain short orbital periods, usually as short as a few days to months. Planets in our solar system have unique traits that differ from those of common solar systems found throughout space. The role Jupiter played started in the early stages of development when the formation of the solar system was in its infancy. Astronomers believe our solar system’s planet migration has been affected by the early evolution of Jupiter’s inward and outward migration.

        When the solar system was in its early formative period, Jupiter migrated inward from 5 AU to a 1.5 astronomical units (AU) before moving back to its position of 5.2 AU where it now resides.[6] This inward migration shows us why the overall mass in many of the Solar Systems terrestrial planets to be low compared to those in other Solar Systems. It is also believed that during the early stages, Jupiter in its inner orbit around the Sun destroyed other planets that eventually formed together into small moons. [10] The debris from the destruction of the planets caused the asteroid belt that resides roughly between Mars and Jupiter. Overall Jupiter caused massive changes to the solar systems structure and can be attributed to Earth’s existence.

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Conclusion

In conclusion, most of what is outlined is backed by scientists around the world and are considered the basis of the beginning of our solar system. Just as anything with due diligence and the ability to create and learn on top of original ideas, comes a well rounded and well-balanced understanding that will allow for a clear solar system identity moving forward. The details of these models are most likely subject to change, but the consensus accommodates more models and is only an expansion of each other to consolidate the ideas into a well working undeniable model that everyone can agree upon. Just like the application of the disk instability model to facilitate the creation of gas giants this model needed to be adopted based on the improbability of the gas giant creation process stated in the core accretion model. The core accretion model gives a great explanation for the formation of terrestrial planets but an inept explanation for the formation of the Jovian planets. Furthermore, the Nebular Hypothesis which is recognized as the process by which our Solar System began, is joined and is the aggregate of all localized planetesimal phenomenon and their beginnings; which is all in synchronous to the models that explain how planets found their orbital parameters. Planetary migration is arguably one of the most significant events to ever happen due to the fact that without it, Earth most likely would seize to exist, consciousness would be nothing more than a mere enigma, and the planet, with all of its functions, would only be a paradox. That is how important planetary migration is.

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Sources:

[1] Thierry Montmerle, Jean-Charles Augereau, Marc Chaussidon, Solar System Formation and Early Evolution: The First 100 Million Years. (Springer,2006)  

[2]  Jane S. Greaves, Disks Around Stars and the Growth of Planetary Systems. (Science,2005)

[3]  P. Goldreich, W. R. Ward, The Formation of Planetesimals, (Astrophysical Journal, 1973)

[4] Ann Zabludoff, The Nebular Theory of the origin of the Solar System, (University of Arizona, 2003)

[5] Karen Masters, What is the evidence supporting the nebula theory of Solar System formation? (Ask an Astronomer ,2015)

[6] Konstantin Batygin; Greg Laughlin, Jupiter’s decisive role in the inner Solar System’s early evolution, (Proceedings of the National Academy of Sciences)

[7] Redd, N. T. (2016, October 31). How Was Earth Formed? Retrieved March 14, 2018, from https://www.space.com/19175-how-was-earth-formed.html

[8] Sanders, R. (2015, December 24). How Did Jupiter Shape Our Solar System? Retrieved March 18, 2018, from https://www.universetoday.com/88374/how-did-jupiter-shape-our-solar-system/

[9] Savage, K. (2014, January 14). What Do We Really Understand About Planetary Formation? Retrieved March 14, 2018, from https://blog.planethunters.org/tag/disk-instability/

[10] Batygin, Konstantin; Greg Laughlin. (2015, April). Jupiters decisive role in the inner Solar System’s early evolution. Retrieved March 7, 2018, from http://www.pnas.org/content/112/14/4214

[11] Mandell, A. M. Planetary Migration. SpringerLink (1970). Available at: https://link.springer.com/referenceworkentry/10.1007/978-3-642-27833-4_1220-4. (Accessed: 4th March 2018)

[12] The Role of Planetary Migration in the Evolution of the Solar System. Planet Hunters (2014). Available at: https://blog.planethunters.org/2014/05/09/the-role-of-planetary-migration-in-the-evolution-of-the-solar-system/. (Accessed: 6th March 2018)