Why Are the Inner and Outer Planets so Different?
People have always marveled over the heavens and all that they hold, particularly now that technology allows us to view deep space. However, right in our own cosmic neighborhood some fascinating oddities exist—things that just don't seem to make sense. One such oddity is the disparity between the outer and inner planets. The inner planets are small and rocky; low on moons and entirely lacking in ring systems. Yet the outer planets are huge, icy and gaseous, with ring systems and many moons. What could cause such strange, vast inconsistencies? Why are the inner and outer planets of our solar system so dissimilar?
The implications of this answer are far reaching. Through models and simulations, scientists are confident that we now grasp at least the gist of how our planets formed. Excitingly, we may be able to apply what we learn about our own solar system to exoplanetary formation, which could lead us to understand more about where life might be most likely to exist. Once we understand the formation of our own solar system’s planets, we could be a step closer to discovering life elsewhere.
We do understand some of the factors that come into play for planetary formation, and seem to create a rather complete understanding. Our solar system began as a massive cloud of gas (mainly hydrogen) and dust, called a molecular cloud. This cloud underwent gravitational collapse, probably as a result of a nearby supernova explosion that rippled through the galaxy and caused a churning of the molecular cloud that led to an overall swirling movement: the cloud began to spin. Most of the material became concentrated in the center of the cloud (due to gravity), which sped up the spinning (due to the conservation of angular momentum) and began to form our proto-Sun. Meanwhile the rest of the material continued to swirl around it, in a disk referred to as the solar nebula.
Within the solar nebula, the slow process of accretion began. It was first led by electrostatic forces, which caused tiny bits of matter to cling together. Eventually they grew into bodies of sufficient masses to gravitationally attract one another. This is when things were really set into motion.
When electrostatic forces ran the show, the particles were traveling in the same direction and at close to the same speed. Their orbits were pretty stable, even as they were being gently drawn toward one another. As they built up and gravity became an increasingly stronger participant, everything grew more chaotic. Things started slamming into each other, which altered the bodies' orbits and made them more likely to experience further collisions.
These bodies collided with one another to build up larger and larger pieces of material, similar to using a piece of Play Doh to pick up other pieces (creating a larger and larger mass all the while--though sometimes the collisions resulted in fragmentation, instead of accretion). The material continued to accrete to form planetesimals, or pre-planetary bodies. They eventually gained enough mass to clear out their orbits of most of the remaining debris.
The matter closer to the proto-Sun—where it was warmer—was composed primarily of metal and rock (particularly silicates), whereas the material farther away consisted of some rock and metal but predominantly ice. The metal and rock could form both near the Sun and far from it, but ice obviously couldn't exist too close to the Sun because it would vaporize.
So the metal and rock that existed close to the forming Sun accreted to form the inner planets. The ice and other materials found farther away accreted to form the outer planets. This does explain part of the compositional differences between the inner and outer planets, but some dissimilarities still remain unexplained. Why are the outer planets so large and gaseous?
To understand this, we must first understand what is often called the “frost line” of our solar system. This is the imaginary line which divides the solar system between where it is warm enough to harbor liquid volatiles (such as water) and cold enough for them to freeze; it is the point away from the Sun beyond which volatiles cannot remain in their liquid state, and could be thought of as the dividing line between the inner and outer planets (Ingersoll 2015). The planets beyond the frost line were perfectly capable of harboring rock and metal, but they also could sustain ice.
The Sun ultimately amassed enough material and reached a sufficient temperature to begin the process of nuclear fusion, fusing atoms of hydrogen into helium. The onset of this process spurred a massive ejection of violent gusts of solar wind, which stripped the inner planets of much of their atmospheres and volatiles (Earth’s atmosphere and volatiles were delivered afterward and/or contained underground and later released to the surface and atmosphere--for more, check out this article!). This solar wind still flows outward from the Sun now, however it is lower in intensity and our magnetic field acts as a shield for us. Farther from the Sun the planets were not as strongly affected, however they were actually able to gravitationally attract some of the material being ejected by the Sun.
Why were they larger? As mentioned above, the matter in the outer solar system consisted of rock and metal just like it did closer to the Sun, however it also contained vast amounts of ice (which couldn't condense in the inner solar system because it was too hot). The solar nebula that our solar system formed from contained far more of the lighter elements (hydrogen, helium) than rock and metal, so the presence of those materials in the outer solar system created a huge difference. This explains their gaseous content and large size; they were already larger than the inner planets because of the lack of ice close to the Sun. When the young Sun was experiencing those violent ejections of solar wind, the outer planets were massive enough to gravitationally attract a lot more of that material (and were in a colder region of the solar system, so they could retain them more easily).
In addition, ice and gas are also far less dense than the rock and metal that make up the inner planets. The density of materials results in a wide size gap, with the less dense outer planets being much larger. The average diameter of the outer planets is 91,041.5 km, vs 9,132.75 km for the inner planets--the inner planets are almost exactly 10 times as dense as the outer planets (Williams 2015).
But why do the inner planets have so few moons and no rings when all of the outer planets have rings and many moons? Recall how the planets accreted from material that was swirling around the young, forming Sun. For the most part, moons formed in much the same way. The accreting outer planets were pulling in huge quantities of gas and ice particles, which often fell into orbit about the planet. These particles accreted in the same way their parent planets did, gradually growing in size to form moons.
The outer planets also achieved sufficient gravity to capture asteroids that went streaking by in their near neighborhood. Sometimes instead of passing by a sufficiently massive planet, an asteroid would be drawn in and locked in orbit—becoming a moon.
Rings form when a planet's moons collide or are crushed under the gravitational pull of the parent planet, due to tidal stresses (The Outer Planets: How Planets Form 2007). The resulting debris becomes locked in orbit forming the beautiful rings we see. The likelihood of a ring system forming around a planet increases with the number of moons it has, so it makes sense that the outer planets would have ring systems while the inner planets do not.
This phenomenon of moons creating ring systems is not limited to the outer planets. Scientists at NASA have believed for years that the Martian moon Phobos might be headed toward a similar fate. On November 10, 2015, NASA officials stated that there are indicators which strongly support this theory—particularly some of the grooves featured on the moon’s surface, which may indicate tidal stress (You know how tides on Earth cause a rise and fall of water? On some bodies, tides can be strong enough to cause solids to be similarly affected). (Zubritsky 2015). In less than 50 million years, Mars too may have a ring system (at least for a while, before all of the particles rain down on the planet's surface). The fact the outer planets currently have rings while the inner planets do not is primarily due to the fact that the outer planets have so many more moons (and therefore more opportunities for them to collide/shatter to form rings).
Next question: Why do the outer planets spin much faster and orbit more slowly than the inner planets do? The latter is primarily a result of their distance from the Sun. Newton’s law of gravitation explains that gravitational force is affected by both the mass of the bodies involved and also the distance between them. The Sun’s gravitational pull on the outer planets is lessened due to their increased distance. They also obviously have much more distance to cover in order to make a complete revolution around the Sun, but their lower gravitational pull from the Sun leads them to travel more slowly as they cover that distance. As for their rotational periods, scientists are actually not completely certain why the outer planets rotate as quickly as they do. Some, such as planetary scientist Alan Boss, believe that the gas ejected by the Sun when nuclear fusion began likely created angular momentum when it fell on the outer planets. This angular momentum would cause the planets to rotate more and more rapidly as the process continued (Boss 2015).
Most of the remaining differences seem rather straightforward. The outer planets are much colder, of course, due to their great distances from the Sun. Orbital velocity decreases with distance from the Sun (due to Newton’s law of gravitation, as previously stated). We cannot compare the surface pressures since these values have not yet been measured for the outer planets. The outer planets have atmospheres composed almost entirely of hydrogen and helium—the same gases that were ejected by the early Sun, and which continue to be ejected today in lower concentrations.
Some other differences exist between the inner and outer planets; however, we still lack a lot of the data necessary to really be able to analyze them. This information is difficult and especially expensive to obtain, since the outer planets are so far away from us. The more data about the outer planets we can acquire, the more accurately we will likely be able to understand just how our solar system and planets formed.
The problem with what we believe we currently understand is that it is either not accurate or at least incomplete. Holes in theories seem to keep popping up, and many assumptions have to be made in order for theories to hold. For example, why was our molecular cloud spinning in the first place? What caused the initiation of gravitational collapse? It has been suggested that a shockwave caused by a supernova could have facilitated the molecular cloud’s gravitational collapse, however the studies that have been used to support this assume the molecular cloud was already spinning (Boss 2015). So...why was it spinning?
Scientists have also discovered ice giant exoplanets found much closer to their parent stars than should be possible, according to our current understanding. In order to accommodate these inconsistencies that we are seeing between our own solar system and those around other stars, many wild guesses are being proposed. For instance, perhaps Neptune and Uranus formed closer to the Sun, but somehow migrated farther away over time. How and why such a thing would occur of course remain mysteries.
While there are certainly some gaps in our knowledge, we have a pretty good explanation for many of the discrepancies between the inner and outer planets. The dissimilarities primarily come down to location. The outer planets lie beyond the frost line and could therefore harbor volatiles while forming, as well as rock and metal. This increase in mass accounts for many other disparities; their large size (exaggerated by their ability to attract and retain solar wind that was ejected by the young Sun), higher escape velocity, composition, moons, and ring systems. However, observations we have made of exoplanets cause us to question if our current understanding is truly sufficient. Even so, there are many assumptions made within our current explanations that are not entirely evidence based. Our understanding is incomplete, and there is no way to measure the extent of the effects of our lack of knowledge on this topic. Perhaps we have more to learn than we realize! The effects of obtaining this missing understanding could be extensive. Once we understand how our own solar system and planets formed, we will be a step closer to understanding how other solar systems and exoplanets form. Perhaps one day we will be able to accurately predict where life is likely to exist.
Boss, A. P., and S. A. Keiser. 2015. Triggering Collapse of the Presolar Dense Cloud Core and Injecting Short-Lived Radioisotopes with a Shock Wave. IV. Effects of Rotational Axis Orientation. The Astrophysical Journal. 809 (1): 103
Ingersoll, A. P., H. B. Hammel, T. R. Spilker, and R. E. Young. “Outer Planets: The Ice Giants.” Accessed November 17, 2015. http://www.lpi.usra.edu/opag/outer_planets.pdf
“The Outer Planets: How Planets Form.” Solar System Formation. August 1, 2007. Accessed November 17, 2015. http://lasp.colorado.edu/education/outerplanets/solsys_planets.php.
Williams, David. "Planetary Fact Sheet." Planetary Fact Sheet. November 18, 2015. Accessed December 10, 2015. http://nssdc.gsfc.nasa.gov/planetary/factsheet/.
Zubritsky, Elizabeth. "Mars’ Moon Phobos Is Slowly Falling Apart." NASA Multimedia. November 10, 2015. Accessed December 13, 2015. http://www.nasa.gov/multimedia/imagegallery/image_feature_1199.html.
© 2015 Ashley Balzer