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Planetary Science: Birth of New Worlds
Columnist: Dr. James Green

Planetary Science: Birth of New Worlds
1 November 2010

NASA's Year of the Solar System is underway -- the start of a tremendous revolution in planetary science. Whole new worlds are opening to us!

As we orbit, flyby, land on, and rove the variety of worlds that revolve in the glow of our Sun, they will tell us the fabulous story of our solar system's formation and evolution. We will learn about the births of its planets, begin to understand more about how Earth evolved, and discover something about our own place in the solar system and in the Universe.

I'm delighted to take you along for the ride! The action begins as our veteran EPOXI spacecraft approaches comet Hartley 2 for a November 4th rendezvous.

Hartley 2 is not even a mile wide, but its coma is already about 8,000 km (almost 5,000 miles) in radius. That's larger than the radius of the Earth! And Hartley 2's tail is a spectacular 1.8 million km (almost 1.2 million miles) long.

Despite comets' dazzling beauty, almost all past cultures have viewed them with trepidation, as harbingers of doom. Even as recently as 1910, when the Earth passed through comet Halley's tail, there was a lingering fear that the Earth would be bathed in noxious gases. Now we are so eager to study comets that we fly out to meet them.

Why are we so excited about comets? It's because they are key chapters in the grand story of our very early solar system.

Here's how the story goes.

Nearly 5 billion years ago, the solar nebula collapsed gravitationally, forming a slowly rotating blob of gas and dust. The blob began to rotate faster and faster, gradually flattening into a disk of dust with a central core. This core became our Sun. The dust disk formed the planets, along with their satellites, and the asteroids and comets.

The comets coalesced in the outer, cooler regions of the nebula. Because they spend so much time in the cold depths of the solar system, comets are little changed from when they formed. They are very fundamental leftovers of the early solar system's building blocks -- leftovers we can study to discover where we came from.

During the Earth's early days, comets often hit us. And we think they could have contributed some of the molecules, such as amino acids, from which life eventually arose. So maybe comets can give us clues about how our planet was seeded with life's elements.

We haven't seen many comets up close, and the more we visit the more we will learn about our solar system -- maybe even about our Universe.

Scientists know that our Sun was born amid a host of neighboring stars that formed in the same gas cloud. Now we are beginning to think those stars may have swapped comets -- capturing the icy bodies from one another as gas giants slung them out of their solar systems. Our own Sun may have captured a big cloud of comets -- a significant part of the Oort cloud -- in this way!

So maybe we have shared spit -- through comets like Hartley 2 -- with a whole series of other stars! And by visiting these little worlds, we can learn more about our Universe, and even about the origin of life itself.

Welcome to a golden age of exploration -- an age of new worlds and new discoveries!

(1) For more information on the EPOXI mission, see and

(2) Hal Levison and his colleagues at the Southwest Institute of Boulder theorize that all those stars formed comets out of the same kinds of materials from within the disk of gas and dust surrounding them. They also say those comets were thrown out of their home planetary systems because large planets in those systems interacted with them gravitationally, casting them out. The comets were then rendered homeless, until another star drew them in. Hal Levison calls the process sharing spit.

(3) The number of comets in the Oort Cloud exceeds the theoretical limit -- what the models predict it should contain -- and there has to be some reason for this.

Also this month: See me at TEDxNASA on November 4th on Look for the O/OREOS launch on November 19th, from Kodiak, Alaska.

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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