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 still 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. The fact that we still do not definitively know the answer means that discovering it could revolutionize our understanding of planetary and possibly even solar system formation. It will likely also help us to understand 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 (most notably hydrogen) and dust, called a molecular cloud. This cloud underwent gravitational collapse, which precipitated a swirling movement; the cloud began to spin. Most of the material was concentrated in the center of the cloud (due to gravity) 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. Particles 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). The material continued to accrete to form planetesimals, or pre-planetary bodies. Planetesimals obtained sufficient mass to gravitationally alter the movement of other bodies, which made collisions more common and accelerated the process of accretion. The planetesimals grew into “planetary embryos” which gained enough mass to eventually 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 of ice.
Already part of the picture appears very clear and simple. Metal and rock existed close to the forming Sun and accreted to form the inner planets. The ice 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 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 due to their larger masses they were actually able to gravitationally attract some of the material being ejected by the Sun. This explains their gaseous content and large size, as the more matter they accumulated the larger gravitational pull they could exert to attract additional material. The result is large bodies composed primarily of gas and ice.
Considering all of the above information, we have a reasonable explanation for the differences we see between the inner and outer planets. The inner four planets have an average mass of 2.95x1024 kg, whereas the outer four have an average of well over 200 times that value--663.70x1024 (Williams 2015). The size disparity is a result of a combination of factors, but primarily the planets’ distances from the Sun. Farther away, planets could accumulate ice which increased mass and therefore gravitational pull for additional material. This increased gravitational pull allowed the outer planets to attract gas that was ejected in tremendous quantities by the newly forming Sun.
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). The differences in both mass and overall size for the inner and outer planets seem pretty easily explainable.
But why do the inner planets have so few moons and no rings when all of the outer planets have rings and many moons? Both their moons and rings were likely gained as result of gravity. The outer planets have sufficient gravity to capture asteroids that come streaking by in their near neighborhood. Sometimes instead of passing by a sufficiently massive planet, an asteroid will be drawn in and locked in orbit about the planet—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 (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 shatter to form rings).
In addition, the gas (ejected when the Sun began nuclear fusion) which the outer planets attracted was not necessarily just smashing into the planet and accreting that way, but likely also getting trapped in orbit. It would be similar to captured asteroids, except on a much smaller scale. Then the moons could have formed around these planets in a similar way to how the planets formed around the Sun—by process of accretion whilst orbiting. Some of these moons may have been forming to ultimately be crushed under the gravity of their parent planet to form rings.
Still more inconsistencies remain. 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 ground to cover in order to make a complete revolution around the Sun. 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 data is difficult and especially expensive to obtain, as 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).
We also do not currently understand why Jupiter and Saturn should be gas giants while Uranus and Neptune are ice giants—the two pairs so clearly different from each other in this way as well as others. 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.
It is essential to attempt to further understand how our solar system and planetary bodies formed, as well as how the differences between our inner and outer planets arose. In order to understand these processes, we will be forced to totally transform our current understanding; what we have now simply is not sufficient to explain everything that we are seeing and discovering.
So the dissimilarities between the inner and outer planets seem to 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, 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.