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article imageWas a giant planet ejected from our solar system to save Earth?

By JohnThomas Didymus     Nov 11, 2011 in Science
An article recently published in The Astrophysical Journal Letters says our solar system might, in the past, have had an extra giant planet which was ejected to allow the inner terrestrial planets, including our Earth, to settle into a stable order.
The four giant planets in our solar system are Jupiter, Saturn, Neptune and Uranus. Recent computer models of the early evolution of our solar system suggest there might have been a fifth planet.
According to Science Daily , Dr. David Nesvorny of the Southwest Research Institute, San Antonio, Texas, says, "We have all sorts of clues about the early evolution of the solar system. They come from the analysis of the trans-Neptunian population of small bodies known as the Kuiper Belt, and from the lunar cratering record."
The body of evidence, according to Dr. Nesvorny, suggests that dynamic instability in our solar system, when it was about 600 million years old, affected the orbits of the giant planets and resulted in gravitational interactions that scattered the giant planets and smaller bodies. The "scattering" of the small bodies led to the formation of the Kuiper Belt, though some of the small bodies moved into the center of the solar system, in the direction of the terrestrial planets, and collided with them. The giant planets also moved inward, scattering small bodies away from the center of the solar system.
According to Dr. Nesvorny, the inward movement of the giant planets and their interaction with small bodies could have had a destabilizing effect on the inner terrestrial planets and could have led to Earth colliding with Mars or Venus.
Nesvorny explains that his colleagues, however, came up with a suggestion which altered the picture:
"They proposed that Jupiter's orbit quickly changed when Jupiter scattered off Uranus or Neptune during the dynamical instability in the outer solar system. The 'jumping-Jupiter' theory, as it is known, is less harmful to the inner solar system, because the orbital coupling between the terrestrial planets and Jupiter is weak if Jupiter jumps."
What this explanation implies is that the gravitational interaction mechanism by which Jupiter "scattered off" Uranus and Neptune re-directed the trajectory of Jupiter and spared the inner solar system and our Earth the destabilizing effect of a giant planet's intrusion.
The "jumping-Jupiter" theory, as experts call it, gave surprising results when computer simulations were implemented. The simulations seemed to confirm the "jumping-Jupiter" theory, but there was a problem. The mechanism of "scattering off" should have bounced Neptune and Uranus out of our solar system. Nesvorny concludes: "Something was clearly wrong."
But it soon occurred to Nesvorny that Jupiter might not have scattered from Neptune and Uranus, but from a fifth giant planet which our solar system is now missing. Nesvorny then ran new simulations, now inserting into his model of our solar system, a fifth planet about the mass of Uranus and Neptune. The picture that emerged was eye-opening. The fifth planet was knocked out of our solar system and Jupiter's trajectory "jumped," sparing the inner terrestrial planets the disruptive intrusion of a giant wanderer.
Nesvorny comments on the implication of the simulation models:
"The possibility that the solar system had more than four giant planets initially, and ejected some, appears to be conceivable in view of the recent discovery of a large number of free-floating planets in interstellar space, indicating the planet ejection process could be a common occurrence."
Further evidence suggesting there might have been a fifth giant planet comes from observation that models of our solar system based on the assumption that it had four giant planets from the beginning yield unstable systems very unlikely to settle into our system as we know it today. Models of the early evolution of our solar system which begin with five planets match the present state of our solar system better.
Nesvorny spoke to Space.com on the implication of the theory that there was a fifth giant planet:
"The work raises interesting questions about the early history of the outer solar system. For example, traditionally, most research was focused on the giant planets, their satellites, Kuiper belt objects, and their interaction — that's what we have in the outer solar system now. But how about Mars to super-Earth-size bodies? Have such objects formed on the outer solar system and were eliminated later? If not, then why?"
The fifth planet hypothesis has intrigued astronomers because it is believed to explain why our planetary system is unique among other planetary systems that have been found so far. Our solar system in unusually neatly ordered compared to other systems we have found. Discovery.com notes that even though planetary systems formed around stars have been shown to be common, most of the systems have erratic and unstable orbital orders. Discovery.com explains:
"...planetary systems around other stars found so far present sort of a Wild West of planets. Their orbits can be steeply inclined to one another (our eight major planets are coplanar). There are many giant planets that have migrated precariously close to their stars. Other planets are in roller-coaster highly elliptical orbits that alternatively freeze and cook them."
The theory that a fifth giant planet was ejected from our solar system is being held to explain why our system settled into its present neatly ordered orbits, and some astronomers are already saying the fifth giant planet was sacrificed for our Earth.
More about fifth planet, giant planet, Solar system, Earth
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