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The Solar System: A Family Affair
Columnist: Dr. James Green

The Solar System: A Family Affair
1 January 2011

It's mind-blowing to realize that at one time the Earth and its siblings were nothing but cosmic dust. About 4.5 billion years ago, the sun was a mere protostar drawing in the gas and matter swirling around it. Our solar family members were yet to be born from this vast, spinning cloud known as the solar nebula.

Our sun finally became a "normal looking" star, but in no way can we say that what was created from this collapsing cloud was a set of "normal looking" planets. And there were certainly no identical twins.

The story of the planets' birth and evolution is a "family affair" as fraught with drama as any holiday gathering of kinfolk. The more we learn of that story the more it enthralls us as we begin to realize how special this solar system really is. After all, it's the only one we know of that supports life. And over the last 10 years, we've discovered more and more reasons why it does so.

By studying Earth's relatives -- the planets, dwarf planets, asteroids, and comets orbiting the sun -- we're learning about the origins of the solar system, our planet and life itself.

The outer giant planets loom large in the story. For example, Jupiter was a bit of a glutton, leaving little material for the other planets forming around it. Jupiter's presence thus had a profound influence on the masses and compositions of its siblings, and its influence continues today.

And we've recently found a whole line of evidence indicating that the planets didn't form where they are today, but have instead sashayed since their birth in a cosmic family line dance. Billions of years ago, in fact, Jupiter moved inward, sparking some critical family dynamics. The big gasser wreaked as much havoc as Cousin Eddie at the Griswold's over Christmas vacation.

The main asteroid belt, a huge expanse of rocky bodies between the orbits of Mars and Jupiter, reveals clues to what transpired. The belt's contents are fundamental planetary pieces -- chunks, rocks and dust that tried for millions of years to form a planet, only to be foiled time and again. And the distribution of those pieces tells us it was the Godfather Jupiter that foiled them.

Certain areas or slots in the belt are entirely devoid of asteroids and other debris. The intriguing thing is that these slots, known as Kirkwood gaps, are in various resonances with Jupiter -- sure evidence that, through time, the gas giant caused changes in the orbits of objects they contained. When two objects are in orbital resonance, they apply gravitational influence on each other at regular intervals and thus affect each other's orbits. Over millions of years the tugs and flicks Jupiter exerted on the slot objects accumulated until, finally, some of the objects were flung outward while others were flung inward.

Scientists surmise that Jupiter exerted similar effects on sister Saturn. And when Jupiter gave her a push outward about 3.8 billion years ago, as some simulations suggest, it perturbed family relations, instigating a sort of "family feud." Saturn "turned around" and pushed both Uranus and Neptune outward, closer to the Kuiper Belt.

What happened next led to key events in the feud. But first let's take a look at what Earth had been up to while all the pushing and shoving was going on.

Our planet was immersed in its own drama, busily sweeping up material, when it may have collided with a Mars-sized body. This enormous crash broke the Earth apart, forming two bodies -- Earth and its moon. The smashup profoundly changed Earth's character. Its volatiles evaporated, leaving behind a barren place -- a lonely, hot cinder floating in a sort of space "time-out."

Meanwhile, back to the outer solar system. When Saturn, Uranus and Neptune were pushed toward the Kuiper belt, they cinched in the Kuiper belt gravitationally, positioning it to pelt the inner solar system with a torrent of small bodies and comet-like material in an event now known as the Late Heavy Bombardment.

This event had profound implications for our planet. The onslaught of water-bearing material delivered not only a rich supply of water, but also ammonia, gases, ices, and even amino acids to our Earth, freeing her from time-out and perhaps seeding life.

So, it's probably no coincidence that the oldest life on Earth also dates back 3.8 billion years. And we owe it all to a "family affair."


The Kuiper Belt is a disc-shaped region beyond Neptune, extending from about 30 to 55 AU. It is probably populated with hundreds of thousands of icy bodies larger than 100 km (62 miles) across and an estimated trillion or more comets.

More evidence supporting the migration of planets in our solar system: Since 1995, scientists have discovered many new planets and planetary systems. Most of the gas giants discovered in other systems are orbiting very close to their host stars. If the standard model of planetary formation is correct in predicting that giant planets form far from their stars, some of these planets must have migrated inward.

For more information on "The Curious Case of the Missing Asteroids," see .

Near Earth Objects are examples of asteroids that have been pulled inward. Such objects have even caused extinction of species on Earth. See and for more information. On the other hand, Jupiter helped stem the tide of cometary bombardments from the Oort cloud. Jupiter's gravity pulls many comets in so they don't hit us.

We have to look to Earth's "kinfolk" for evidence of the Late Heavy Bombardment, erased here by the workings of Earth's atmosphere, life and geological activity. Earth's daughter moon still bears the 3.8 billion year old scars of the late heavy bombardment wrought by Earth's outer siblings. The evidence of this event is inscribed forever in lunar craters, crags and rocks because it has no atmosphere, life or geological activity, elements that long ago erased the story on our own planet.

Read More by Dr. James Green

About: Dr. James Green
Photo of Dr. James Green
Dr. Green is the Director of Planetary Science at NASA Headquarters. He views his job as the keeper of NASA's planetary program and the top advocate for that program to flourish and grow.
Read More by Dr. James Green
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Last Updated: 16 Feb 2011