Of the trillions of comets that inhabit the Solar System, a new study suggests that as many as five percent could have originated in other stellar systems. A computer simulation of the Sun's movements within the Milky Way galaxy has revealed that a great many comets might have been captured by the star's gravitational pull in the four billion years it has been wheeling through space.
According to astronomer Stephen Levine of Lowell Observatory in Flagstaff, Ariz., and undergraduate student Catherine Gosmeyer of Indiana University, their simulation of the Sun's swing around the galactic center indicates that over time, the star would have passed other stellar objects. In so doing, it very well could have robbed a weaker star of some of its retinue.
Of course, our Solar System could have lost a few comets as well.
And it could have occurred more often than one might guess, Levine told Space.com.
Most comets in the Solar System reside in a vast population outside the orbit of Pluto called the Oort Cloud. With stars passing relatively close by every one or two million years, the give and take of material from the stars' gravitational sphere of influence could have occurred up to as many as 50,000 times. The computer simulation suggested that five percent was most likely the least amount of comets in the Oort Cloud that potentially once inhabited another star's gravity field. In short, there could be much, much more.
"I might not be able to say we have increased the size of the Oort cloud," Levine noted, "but we have probably exchanged material at least, so that some fraction of what's in our cloud probably did come from something else."
One problem with the hypothesis is that it could be difficult to prove. There is no guarantee that other stellar systems include formations like the Solar System's Oort Cloud. And even if star's had such a feature, they could be of varying sizes and populations. There is also the problem of being certain that any given comet actually originated in another star system. Unless the comets of Sun have a certain chemical signature that would mark them as exclusive to the Solar System, it would be nearly impossible to ascertain whether or not ice fragments were indigenous or originated elsewhere.
Still, the captured comets hypothesis has promise. The researchers predicated their ideas on there being a larger Oort Cloud than had been predicted given the Sun's mass, suggestive that all of the comets therein were not part of the Solar System's original formation.
The Oort Cloud, which extends to the furthest limit of the Solar System (estimated to be 1.5 to 3.2 light years distant from the Sun), is where most comets, called "long-period comets," are believed to originate. Although some are believed to come from the Kuiper Belt ("short-term comets"), which is located outside Neptune's orbit (but borders the interior limit of the Oort Cloud), this enormous band of comets, asteroids, and small planetoids (including the dwarf planet Pluto) is believed to be much too stable in composition. There is also a third and fourth classification of comet, the "single-apparition comet" (with extreme parabolic and hyperbolic orbits, some hypothesized to take the comets out of the Solar System) and the "main-belt comets" (which maintain fixed orbits in the asteroid belt).
It is the study of comets that have led to several theories about the nature of the Solar System, such as the effect Jupiter has on comets and small asteroids, making its position ideal for life to have a chance to exist on Earth. Called the Rare Earth theory, it maintains -- among many other factors and circumstances -- that without the existence of Jupiter where it orbits within the Solar System, the Earth would be under constant bombardment from celestial objects and unable to support life for any great amount of time. In short, conditions exist just so in our Solar System for the formation and evolution of multicellular life, making life a rare commodity in the universe.
And comets may have been factors in the composition of the Earth itself. Just last year, scientists at the Max Planck Institute for Solar System Research in Katlenburg-Lindau, Germany, posited that icy comets slamming into the Earth in its formative years may have been the originators of the planet's oceans.
(photo credit: NASA, Wikimedia Commons)