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?
Though astronomers have found many planetary systems around neighboringstars, 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.
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 Saturnand 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 2006-2007survey 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.
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-Earthmass worlds. Uranus and Neptune must have formed closer into the sun and then migrated outward.
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