Was a Giant Planet Ejected From Our Solar System?

A new set of computer simulations suggests that there was once a fifth giant planet in addition to Jupiter, Saturn, Uranus, and Neptune. (NASA)

The Fifth Planet sounds like a great name for a sci-fi movie -- perhaps a sequel to the 1997 movie "The Fifth Element." But the fifth planet may be real; a hypothesized giant world that was flung out of our solar system four billion years ago. It would have drifted tens of thousands of light years away by now.

Why even suspect that such a planet existed?

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Though astronomers have found many planetary systems around neighboring stars, our own solar system looks like it's the exception rather than the rule. We're discovering that our system is an uncommonly orderly place where the planets behave themselves in wide orbits that are nearly circular.

But the 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.

Even more befuddling, it's hard for theoreticians to build a planet formation model that winds up looking like our solar system.

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A new set of computer simulations by David Nesvorny of the Southwest Research Institute in Boulder, Colorado, shows that this will work if there was once a fifth giant planet in addition to Jupiter, Saturn, Uranus, and Neptune.

Nesvorny models place a fifth hefty planet, several dozen times the mass of the Earth at various possible locations in the outer solar system: midway between Saturn and Uranus, and just beyond Neptune. In this game of orbital musical chairs, the fifth planet was ejected after a tussle with Jupiter –- sort of a celestial King Kong vs. Godzilla.

This may sound extraordinary but there is plenty of evidence for orphaned free-floating planets wandering our galaxy. A 2006-2007 survey of the Milky Way used gravitational lensing to find 10 dark objects drifting in front of distant background stars. Statistically, this means there could be as many as hundreds of billions of castoff planets plying inside our galaxy.

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Looking for evidence of a fifth giant planet calls for solar system forensics. God didn't leave behind any file footage of the solar system's formative years, after all.

First, it is known that Uranus and Neptune are too far from the sun for them to have formed in their present locations. There simply has not been enough time and materials for them to agglomerated into 15-Earth mass worlds. Uranus and Neptune must have formed closer into the sun and then migrated outward.

This implies that the early solar system was very chaotic. Smaller bodies, the planetesimals, were gravitationally kicked around and the exchange of momentum widened the orbits of the outer planets. Our moon bears the scars of this rough and tumble period called the Late Heavy Bombardment, of about 4 billion years ago.

The planetesimal debris was then snowplowed outward to form the Kupier belt, where Pluto dwells. Kuiper belt objects are not spread out uniformly in but are clustered into three distinct populations. This means that the belt was; extensively sculpted by the gravitational influence of the giant planets.

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The ejected planet would not necessarily be lifeless even though it is sailing through the numbing cold of interstellar space. It presumably would have moons. Gravitational tidal forces could heat them so that they would remain warm in the absence of a star. The moons Io (orbiting Jupiter) or Enceladus (orbiting Saturn) are the archetype of what would be plausible.

Microbes could live happily without the need for a sun in the sky. Among the billions of flung off worlds in our galaxy, this one would be isotopically stamped: Made By Sol.