Essay Sample About Migrating Planets

The paper includes a description of the revolutionary theory in astronomy related to the history of formation of the solar system. Based on the reading in Scientific American, “Migrating Planets” by Renou Malhotra (1999), the author summarizes the essence of the theory, basic supporting arguments and evidence. More recent materials serve as proof that the theory of planet migration is gaining popularity in the scientific world. The conclusion offers the author’s perspective on the given theory and perspectives for its development.

Revolutionary Theory In Astronomy

The contemporary theory of solar system development is based on one basic assumption. It proceeds from the belief that modern planets were born in their orbits. The large planets in the solar system are believed to have formed in a two-step process. In the first stage, the core emerged as a conglomeration of solid planetesimals that later attracted a huge gas disk from the nebula around. This theory of planet formation has existed for decades, underpinning most of astronomical research of the solar system.

However, today scientists begin to question these basic assumptions and advance new theories of planet formation that have the potential to change the way we conceptualize the early years of our planet system. Thus, Renou Malhotra (1999) insists that planets may be now traveling in orbits far removed from those in which they were born. Gravitational interactions with other planets, impact of other bodies in the solar system including asteroids and comets, and formational processes within planets themselves may have altered the original orbits of the planets.

The evidence corroborating this idea is present in the first place in the studies of Pluto, an oddity among others in the solar system. This planet stands out from among others since it “is thousands of times less massive than the four gas-giant outer planets, and its orbit is very different from the well-separated, nearly circular and co-planar orbits of the eight other major planets” (Malhotra, 1999). The planet moves at a varying distance from the sun that ranges from 29.7 to 49.5 astronomical units. Besides, the correlation in movement between Neptune and Pluto is amazing, since the two planets cross each other’s orbits but never collide, with their orbits protected by so-called resonance libration. This means that in the same time period Pluto makes two, and Neptune three revolutions around the sun.

To explain this eccentric orbit, Renou Malhotra proposed that “Pluto was born somewhat beyond Neptune and initially traveled in a nearly circular, low-inclination orbit similar to those of the other planets but that it was transported to its current orbit by resonant gravitational interactions with Neptune” (Malhotra, 1999). The scientist develops a theory that is related to the scattering or accretion of plantesimals by planets in the early stages of their formation. Neptune, after gaining orbital energy due to massive scatterings, migrated outward, and so did Saturn and Uranus. The reverse happened to Jupiter that moved inward losing orbital energy and thus preserving the sum total of orbital energy in the solar system at that point.

Another piece of evidence comes from the orbital distribution of the objects in the Kuiper belt, the area located behind Neptune. The new theory advanced by R.Malhotra allowed the scholar to make predictions about the distribution of bodies in the Kuiper belt, projecting that these bodies will be located at Neptune's two strongest resonances, the 3:2 and the 2:1. Recent observations confirm the presence of a large accumulation of Kuiper belt objects in 3:2 resonant orbits, although the presence of groups of objects in 2:1 orbits has yet to be confirmed by observation.

Peter Goldreich and Scott Tremaine in their 1980s works describe the formation of the planets and the interactions between a protoplanet and the surrounding gaseous envelope that can explain sudden changes in the orbits of planets. They believe that “the gravitational forces between a protoplanet and the surrounding disk of gas, as well as the energy losses caused by viscous forces in a gaseous medium, could lead to very large exchanges of energy and angular momentum between the protoplanet and the disk” (Malhotra, 1999). Such large-scale exchanges could exert significant impact on the movement of the planet and possibly change its orbit. Apart from interactions between the planet and the gaseous disk, another factor that can explain the migration of planets from original orbits can be the scattering of the nearby planets that happened to be too close to the planet in question. Both planets that happened to be too close could be forced to migrate to different areas and orbits by gravitational forces.

Planet migration caused by these factors can explain why large planets can be found in close orbits to the sun. According to the widely accepted theory, such planets cannot accumulate much gas in its envelope since it transgresses a smaller amount of gas than other planets and makes its way through colder gas masses than most other planets. However, if one accepts the idea that planets in the solar system were born in one place and then moved to a different one, it is possible that larger planets in fact had formed in different places and then moved to a position close to the Sun.

Several other scientists corroborate the view held by Renou Malhotra. Thus, Hal Levison of the Southwest Research Institute in Boulder, Colo., and a group of his colleagues also agree that originally Neptune, for example, was moving in an orbit that was half of its current one. The international team of scholars prepared a computer simulation of the solar system formation in which “the four biggest planets start out bunched together but then break into a planetary version of bowling that violently rearranges the structure of the outer solar system” (Cowen, 2005). The researchers claim that their computer model reproduces the process described by other theorists with greater accuracy than ever before. It may, therefore, unveil new phenomena, such as explanation of the Heavy Bombardment that left traces on the earth’s moon and inner planets.

N. Murray, B. Hansen, M. Holman, and S. Tremaine in their article entitled “Migrating Planets” in Science (1998) refer to planet migration as a way to explain the phenomenon of Jupiter-mass objects circulating in small orbits around neighboring stars. They explain planet migration through interaction between the planet and the surrounding disk of planetesimals that result in the collision between the planet and the planetesimals and increase in their eccentricities. These interactions may also result in changes in the planetary orbit. Explanation of formation and migration of Jupiter-mass planets is an interesting problem for scientists. Now they are also confronted with Neptune-mass objects travelling around other stars (Britt 2004). The study of extrasolar planets is extremely important for the understanding of the solar system. Indeed, if they underwent orbit changes resulting from “an exchange of angular momentum with material surrounding the planets at their formation”, the same can be true for the solar system (Jewitt 2004).


The issue of planet migration has important repercussions for the study of our planet system as well as extrasolar bodies. Indeed, if planets formed in places outside their current orbits and then moved toward them, the solar system in particular and the universe may be a much more complex world than mankind used to think. The concept of planet migration will have far-reaching consequences for the study of the solar system, its early years and forecasts of its future. If migration happened billions of years ago, it may be at work today, if not in the solar system, then in other planet systems observable from the Earth. Today, most scientists today accept the idea of planet migration as a viable hypothesis about the history of our system. More in-depth studies of the solar planets, extrasolar bodies, accumulations of objects like the Kiuper belt will help to confirm or disprove the planet migration theory.


Britt, R.R. (2004, 31 August). Two Neptune-Mass Planets Found, Earth-Size Worlds Next.  Retrieved on September 18, 2005 from at

Cowen, R. (2005, May 28). Roaming Giants: Did migrating planets shape the solar system? Science News Online, 167 (22), 340. Retrieved on September 18, 2005 from

Jewitt, D. (2004, September). Moving the Orbits of Planets. Retrieved on September 18, 2005 from

Malhotra, R. (1999). Migrating Planets. Scientific American, 281(3), 56-63

Murray, N., Hansen, B., Holman, M., & S. Tremaine. (1998, 2 January). Migrating Planets.  Science, 279 (5347), 69-72.