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Artist's visualization of a "water world" and a creature living it its vast, planet-covering ocean. Planets like these could form as part of a "second generation" after a gas giant migrates through a star's habitable zone. (Image Credit: Nahks Tr'Enhl) |
(PLANETQUEST) -- In order to find habitable planets orbiting other stars, it's helpful for scientists to be able to understand how planets like ours are created. Sean Raymond of the University of Colorado and his team are not only finding clues to how this mysterious process works, they're also discovering evidence that Earthlike planets may be much more common than was originally thought.
Raymond's area of expertise is in creating computer models of how terrestrial planets form from the disks of gas and dust that surround young stars. "We can simulate the orbits of small bodies in disks around young stars, and the collisions of those small bodies that cause them to grow and eventually become planets," he says. "It's quite fun -- kind of like getting to play God." Collaborating with him on this groundbreaking research are Avi Mandell of NASA's Goddard Space Flight Center, Tom Quinn of the University of Washington, Jonathan Lunine of the University of Arizona, and Steinn Sigurdsson of Penn State.
According to Raymond, the properties of a star's disk are the ingredients that determine what kind of planetary system will form. "The most important factor is what the disk looks like. That tells you how material is distributed and what planets you're going to get," he says.
The first planets to arise are typically gas giants, like Jupiter and Saturn in our own solar system. "These planets take about two million years to form -- much faster than the Earth, which took about one-hundred million years," Raymond says. We know that they come together so quickly, he says, because planet-forming disks around other stars lose their gas in that time. The predominant theory is that gas giants form "bottom-up" -- the gravity of their rocky cores quickly attracts a thick blanket of gas from the debris disk.
The small, rocky planets, like Earth and Venus, form from rocks and dust that collide and clump together closer to the star. Raymond points out that the asteroid belt in our own solar system is probably a remainder of the material that helped create Earth and the other inner planets.
Once the rocky core of an Earth-like planet starts to come together, it's time to add water. It is commonly thought that icy comets smashing into the Earth brought the water that today fills our lakes and oceans, but Raymond says that this isn't likely to have been the case. "Comets are really small and have very elongated orbits with very small collision probabilities with Earth," explains Raymond. "What probably happened is that icy asteroids collided with the Earth and brought the water."
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One of Raymond's simulations of how planets form from scattered material in an protoplanetary disk over time. Blue rings indicate the presence of water, red drier material. In this simulation, an Earthlike planet (the second black dot to the left) has formed in about the same location as our own.
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Raymond's team has also found that gas giant planets, like Jupiter in our own solar system, can have a powerful effect of the formation of Earth-like planets. "If Jupiter's orbit around the sun was just a bit more eccentric (oblong), it would have scattered a lot of the material that delivered water to the Earth, kicking it out of the solar system instead," he says. "The result would have been an Earth that had only 10 percent of the water it does now."
But perhaps most surprising are Raymond's simulations concerning solar systems with "hot Jupiters," gas giant planets that form far away from their stars, then spiral inward, often to scorchingly close orbits.
Scientists initially figured that the presence of these planets indicated a barren solar system, with no chance of harboring life. Any young Earth-like planet, they thought, would be obliterated by the gas giant as it marched inwards. "They migrate right through where terrestrial planets would be forming," Raymond says. "When we tried simulating the formation of Earths in these systems, we didn't think it would work."
Instead, Raymond's team found that hot Jupiters help create a "second generation" of terrestrial planets. As they migrate inward, gas giants corral some disk material closer to the star, and scatter the rest of it outwards. The material closer to the star, Raymond explains, forms hot Super-Earths, sweltering planets just a few times bigger than our own.
The rocky material that gets tossed outward, on the other hand, "can settle down into stable orbits and come together to create a new set of terrestrial planets. In our models, these planets have lots of water -- up to 50 times more than what's on the Earth," says Raymond. "They'd be covered in oceans." Once thought to be the sign of a dead solar system, the presence of hot Jupiters may instead indicate the presence of exotic water worlds.
Raymond's research will go a long way to helping planet hunters "narrow down the field" as they pick which extrasolar systems might be good candidates for harboring an Earth-like planet. These planets might be good targets for follow-up study by future planet-finders, like NASA's planned SIM Lite mission.
It's also a sign that Earth-like planets elsewhere in the galaxy may be anything but rare. "One third of the exoplanet systems we've found are pretty good candidates for a terrestrial planet," Raymond says. "There might be Earths all over the place...we just have to wait and see."
This image compares our own solar system (top) with a system in which a gas giant has migrated inward, helping to create a "water world" farther out from the star. |
Written by Joshua Rodriguez/PlanetQuest
It's quite fun -- kind of like getting to play God. 
