Science Digest
April 1982, pp. 78-82; 94


DID A MONSTROUS CATACLYSM
CONVULSE OUR SOLAR SYSTEM AND
SPEED EVOLUTION ON EARTH?

EXPLODING PLANETS


BY THOMAS VAN FLANDERN

It is 4 million years ago. On Earth, the dinosaurs have long been extinct, and the earliest humanlike primates have just appeared on the scene. Elsewhere in the solar system everything is as we know it now except for one significant difference: between
What a sight it must have been
for early man: the sky ablaze
with meteors and comets streaming
among the stars. The earth would
never be the same.
the orbits of Mars and Jupiter a planet, larger than any but the great Jupiter itself, orbits the Sun. Because of its size and closeness to Earth, the tenth planet dominates the night sky with its brilliance and is even bright enough to be seen during daylight.

Then suddenly it explodes. Now its brightness outshines even the Sun's. Like a nova -- a dying star making its last, violent gasp -- the dying planet hurls its mass in the form of solid, liquid, and gaseous debris in all directions at extraordinarily high velocities. In a few months, the leading edge of the blast reaches our planet.

CRUCIAL INGREDIENT

What a sight it must have been for early man: the sky ablaze with meteors, night and day for months, and comets streaming among the stars. Many thousands of years would pass before the heavens would return to a more normal state. But even then the earth would never again be the same. The blast ended a period of very warm
A planet more than a hundred million miles from Mars explodes and douses the Martian surface with water, forming great channels that, though now dry, are still visible. In this artist's conception, the dying eruption illunminates the red planet's landscape.
weather that had continued uninterrupted for 100 million years; the earth would now enter an alternating series of ice ages and mild periods. Was some crucial ingredient in our atmosphere altered by the influx from our exploding neighbor?

In 1766, modern man took his first step toward discovering what happened when the great planet exploded. In that year the astronomer Titius. of Wittenberg noticed a curious fact about the spacing of the six known planets: there is a roughly geometrical progression in their distances from the Sun. There is only one exception: a gap between Mars and Jupiter that is just the right size to hold exactly one additional planet. Although the German astronomer Johann Bode published this observation as a "law" in 1772, most astronomers dismissed Bode's law as a curiosity until 1781. Then, the English astronomer William Herschel found the seventh planet, Uranus, beyond the orbit of Saturn-right where Bode's law predicted it would be.

Herschel's find drew the attention of astronomers back to the mysterious gap between Mars and Jupiter. If Titius and Bode were correct, as it now appeared they might be, there should be a planet between Mars and Jupiter. But where was it? Searches were organized and prizes offered to the first person to find the missing planet. For two decades, however, astronomers were at a loss. Then, on New Year's Day, 1801, quite by accident the Italian astronomer Giuseppe Piazzi spotted what he thought was a new planet, which he called Ceres. Ceres was orbiting the Sun in precise conformation with Bode's law, but something did not quite fit. Ceres, it turned out, was incredibly tiny compared with the other planets, not even big enough to be a good-sized moon. And its orbit was more elliptical and tilted than any of the other planetary orbits.

A little over a year later, the confusion multiplied. What seemed to be another small planet, Pallas, was found circling the Sun. The German astronomer Wilhelm Olbers concluded that the gap must have once been filled by a large planet that had exploded and that Ceres and Pallas were merely fragments of that original planet. Olbers predicted that many more pieces would be found, that they would all have similarly odd-shaped orbits and that they would vary in brightness as they spun because of their irregularly shaped surfaces. Eventually, Olbers's search for more of these fragments led to his discovery of Vesta, apparently another minor planet.

CONFLICTING THEORIES

The French astronomer Lagrange took Olbers's theory a step further when he pointed out that the extremely elongated orbit of a comet could also be a natural byproduct of a planetary explosion. But the ideas of both Lagrange and Olbers were successfully attacked by their better-known colleague, Pierre Laplace, who believed (as did most astronomers of his day) that comets condensed out of clouds of gas at enormous distances from the Sun.

Laplace argued that even if a planetary blast had occurred, the gravitational force exerted by nearby planets would soon distort the orbits of any fragments. And if Olbers had found Vesta where his
The destruction of this planet,
a giant the size of Saturn, released
enormous energy. Could comets be
debris hurled trillions of miles into
space by this event, then later pulled
back by the Sun's gravity?
calculations had predicted, it was just coincidence. (Paradoxically, it turns out that Laplace was right about the planetary fragments' orbital mechanics and Olbers's lucky discovery, but he was wrong about the comets.)

For the next 160 years the controversy smoldered, flaring only occasionally. By the 1960s, most professional astronomers believed that the thousands of minor planets then known to be orbiting the Sun in the asteroid belt between Mars and Jupiter were the remains of a planet, but that it was a planet that had never fully formed rather than one that had broken up.

The pendulum started to swing back to the exploding-planet theory in 1972, when the Canadian astronomer Michael Ovenden, of the University of British Columbia, developed a law stating that major planets will seek the orbit of greatest stability. According to Ovenden's calculations, the solar system was unstable without a planet in the Mars-Jupiter gap. Because of this instability, Ovenden predicted that the planets would seek a new, more stable configuration if the solar system were subjected to a severe nongravitational force such as might be caused if it passed through an interstellar dust cloud.

Ovenden also predicted that the missing planet must have been a giant the size of Saturn and much larger than all of the minor planets put together. This important possibility, which had never been previously considered, meant that the event that destroyed the planet must have involved enormous energy.

Suddenly, other parts of the puzzle began to drop into place. Scientists calculated that in 100,000 years or so, the exploded debris except for the pieces between Mars and Jupiter that have never been close to a major planet-would have been destroyed by collisions with Jupiter and other planets or have picked up enough velocity to go
Enclosed Story:

BREAKAWAY MOON
flying out of the solar system. The only other remaining debris that we have any chance of seeing today would be objects hurled to great distances from the Sun but eventually pulled back by its gravity.

Only one kind of celestial object matches that description perfectly-the comet. Comets come from incredible distances, trillions of miles from the Sun. A great many are known to be approaching the planets for only the first or second time since their birth. Most amazing of all, those that are visiting us for the first time since their birth are traveling in orbits that took them around the Sun in just 4 million years. We therefore know that they were born just that many years ago, which tells us when the planetary explosion took place.

A great deal of other evidence also supports the "recent" explosion hypothesis. By applying the laws of gravitation to comets we can trace their orbits back in time. When we do, we find that comets all seem to have originated from a common point between Mars and Jupiter 4 million years ago.
PLANET DISTANCE
in AUs		 BODE'S
NUMBER
Mercury 0.4 0.4
Venus 0.7 0.7
Earth 1.0 1.0
Mars 1.6 1.6
? ? 2.8
Asteroid Belt 2.1-3.3
Jupiter 5.2 5.2
Saturn 9.5 10.0
Uranus 19.2 19.6 According to Bode's law, which establishes a geometrical progression in the distances of the planets from the Sun, a planet is missing in the area between Mars and Jupiter.
Neptune 30.1 38.8
Pluto 39.5 77.2

DISTORTED ORBITS

While we cannot watch planets explode today, scientists have observed what happens when man-made satellites explode; the effect is similar to what has been hypothesized for the lost planet. For the first few orbits after a satellite explodes, the fragments intersect at the blast point. In a short time, however, gravitational influences from other, larger bodies, e.g., the earth and moon, distort these orbits. Similarities in orbital distortion tend to suggest that asteroids originated in an explosion.

Other evidence comes from meteorites that reach the ground without burning up and are recovered for study. Here one can see firsthand what sort of material the missing planet was made of. From the tracks of cosmic rays in these meteorites, we learn that the fragments have been traveling in space only a few million years, a small fraction of the age of the solar system. Some of them show evidence of rapid melting a long time ago, as if they were subjected to sudden, immense heat. A few show evidence of shock; others are badly charred. Some meteorites have tiny diamonds embedded in them, evidence that they formed in a high-temperature or high-pressure environment, such as the interior of a large planet.

The surface of Mars, a close neighbor to the exploding planet, has numerous features that suggest it felt the full force of the blast: extensive recent cratering and the mysterious channels that seem to have once held vast flowing rivers. Could huge amounts of water have been dumped on Mars just after the explosion? From studies of comets, believed to be composed mostly of water and ice, we can infer that water must have been abundant on the missing planet. Some of that water
What caused the planet to explode? Was it uniquely
unstable or could our own planet share its fate someday?
may even have reached our moon, which has many surface features that appear to be water-carved. Debris from an exploding planet could also explain the mysterious presence of magnetism in lunar rocks and of radioactivity in lunar soil.

It is also a good guess that a very special type of meteorite called a tektite may be among the remains of the initial blast wave that reached Earth. (Whether tektites are in fact meteorites is one of the most hotly debated topics in astronomy today. The naysayers' chief objection to their being meteorites is lack of a suitable origin other than Earth.) These glassy objects are found scattered in huge fields covering millions of square miles.

Other hints about the cataclysm are found throughout the solar system. For example, Saturn's outer satellite, Iapetus, is the only body in the outer solar system that is known to rotate more slowly than once a month. Iapetus requires nearly 80 days to complete one spin. The significance of this is that, at such distances from the explosion, several weeks would have elapsed between the arrival times of different parts of the blast wave. Because of its slow spin, Iapetus is the only object that would have encountered the blast on only one side. And indeed, true to that prediction, Voyager photos show Iapetus today to be heavily blackened, but only on one side.

It is unsettling to conclude that planets can explode. After all, we live on a planet ourselves and are totally dependent upon it for survival. Unfortunately we have almost no evidence at all as to what caused the explosion. Consequently, it is impossible right now to know whether the missing planet was uniquely unstable or whether our own planet could one day share its fate.
Iapetus, one of Saturn's 12 moons, has puzzled astronomers with its strange coloration: black on one side, white on the other. A colossal planetary upheaval (background) 4 million years ago could have charred half of this small moon (foreground).
Tektites, glassy meteorites, may have rained down on Earth as the blast from a tremendous explosion moved through the solar system. These pebble-size objects, once molten, reached our planet as solids.

GALAXY BLOWUPS

We do know that stars explode when they have exhausted their nuclear fuel. Recent evidence presented by the Dutch astronomer Jan Oort indicates that entire galaxies, composed of hundreds of billions of stars, also explode. Indeed, the Universe itself seems to have originated in what must have been the granddaddy of all explosions, the Big Bang.

But some scientists maintain that no known process can explain how planets could get enough energy to explode. Still, we have only recently learned that thermonuclear processes similar to those in a star may now be taking place in the core of the planet Jupiter; our sample of planets is just too small to generalize about what goes on in their cores. Moreover, it would be terribly presumptuous of us to assume that we already know all of the important energy-generating processes in the Universe while the largest of them all, those operating in the distant and mysterious quasars and radio galaxies, remain mysterious to us.

We are only now beginning to see the Universe as a place of constant change with a rich and varied history and as the site of a great many unique events that continue to this day. Mercury, for example, may once have been a satellite of Venus that, in the not-so-distant past, escaped. Whether or not this happened, it is clear that some event caused Venus to spin in a different direction and at a slower speed (243 Earth days per rotation) than any other planet. Both satellites of Mars and most of Jupiter's did not originate with those planets but also seem to have been captured relatively recently. Jupiter's Great Red Spot, which seems to be a gigantic permanent storm, may actually be just an eddy caused by the presence of an Earth-size "pebble" recently tossed into the turbulent Jovian atmosphere, where it floats, disrupting the flow of gases around it.

Saturn's rings could be yet another example of the solar system's dynamism. Were they once a satellite that broke apart? And what of Uranus' recently discovered rings, entirely different from Saturn's in appearance and behavior? What event tilted Uranus on its side? What disrupted the existing satellites of Neptune? Could the planet Pluto be an escaped satellite of Neptune? If so, how did Pluto get its own satellite? Where did the newly discovered minor planet Chiron, whose unstable orbit crosses those of Saturn and Uranus, come from?

Perhaps the breakup of a giant planet 4 million years ago is merely the most recent and most readily reconstructed of a long series of remarkable events.

Finally, we cannot help but look and wonder at one puzzling juxtaposition of astronomical and human events: is it only a coincidence that humanity felt its early stirrings at about the time of the great explosion? While we must of necessity depart here from the sound scientific reasoning we have so far been using and venture into the realms of speculation and even guesswork, our venture seems worth the risks. For it is always stimulating to speculate about the roots of our species.

The connection between man and the explosion may have been passive; extra cosmic radiation reaching Earth may have speeded up evolution, or some natural enemy of the primates may have been killed off.
We even have evidence that water and oxygen,
substances necessary for life, existed on the missing planet.
Perhaps the connection was a more active one-life may have come from the missing planet. From comets, we know that water was present in abundance. Also, tektites, which are so abundant on Earth but may have originated elsewhere, consist of melted glassy condensations of sedimentary Earth-like material. With an internal heat source and the proper type of atmosphere, temperatures could have been very mild on the planet. And, based on other properties of meteorites, we know for sure that oxygen was abundant in the planet's atmosphere. Most significantly, cells that might have biological significance have been found in certain meteorites that presumably were once part of the planet.

As science progresses we will eventually unravel the mystery of our origins, and the solution will come sooner if our minds are prepared to accept the truth when it is found, however fantastic it may be. If we are guided by our reason and our scientific method, if we let the Universe describe its wonders to us, rather than telling it how it ought to be, then we will soon come to the answers we seek, perhaps even within our own lifetimes.