Earth-Like Planets Common -
Computer Simulation

By Robert Roy Britt
Senior Science Writer

A new computer model designed to explore the range of possibilities for planet formation around other stars had no trouble coming up with worlds similar to Earth.
The simulations generated planets in similar orbits, planets with and without water, and a range of other virtual places that resemble Earth and the other inner, rocky planets.
The effort was designed to determine whether the four inner planets in our solar system, called terrestrials, represent a typical or extreme evolutionary scenario compared to what might develop around a Sun-like star with slightly different dynamics, explained said Sean Raymond, a University of Washington doctoral student in astronomy.
"We found there's a much wider possible range of masses and water content on terrestrial planets," Raymond said in a telephone interview.
"You can have planets that are half the size of Earth and are very dry, like Mars, or you can have planets like Earth, or you can have planets three times bigger than Earth, with perhaps 10 times more water," Raymond said.
Raymond worked with Thomas Quinn, an associate astronomy professor at the university, and Jonathan Lunine, a professor of planetary science and physics at the University of Arizona. Their results, announced today, will be published in the journal Icarus.
Behind the simulations
Astronomers have found more than 100 planets around other stars. All are at least as massive as Saturn and not the sorts of places where intelligent life is likely to flourish. But theorists are using what they've seen as a springboard for imagining what might lurk undiscovered within those systems. A handful are, mathematically, capable of supporting Earth-mass planets in Earth-like orbits.
The new model considered what sorts of rocky planets might form around a star with a known giant planet. The simulations represent the extremes of what is possible, the researchers say, and so it's not known which of them might represent reality.
There is just one giant planet in each of 44 simulations. The model makes an assumption that a giant planet forms quickly, before terrestrials. (Theorists have not determined whether or not that is how things happened in our solar system.) Gravity-based formulas are put in place and time is allowed to evolve. Virtual small rocks collide and stick and eventually form terrestrial planets.
In some cases the initial planet contains the mass of Jupiter, in others it's weightier. Its orbit is like Jupiter's one time, much more elliptical the next.
The validity of the model is suggested by the fact that when the virtual Jupiter takes on characteristics similar to the real Jupiter, a set of inner planets similar to those in our solar system tends to develop.
However, Raymond said, very minor adjustments to the starting conditions fueled wildly different outcomes.
One simulation generated just one terrestrial planet, a whopper up to four times as massive as Earth with up to half again as much water. In another model, five small terrestrials were born, but all were significantly smaller than Earth.
At least one terrestrial planet of some sort was spawned by each scenario.
Key to life: Water
One goal of the study was to determine whether habitable planets might be a common development around other stars. Scientists agree that water is the primary key to life as we know it. Water on the virtual worlds turned out to be dependent on the orbit of the outer, giant planet.
Non-circular routes, called eccentric orbits, are bad news.
"The more eccentric giant planet orbits result in drier terrestrial planets," Raymond said. "Conversely, more circular giant planet orbits mean wetter terrestrial planets."
Here's why: A giant planet in a circular orbit tends to send water-laden asteroids inward, where some of them strike the terrestrial planets and deliver the water. Giant planets orbiting eccentrically tend to kick asteroids outward.
Earth is thought to have been dry when it formed. Water, theorists think, was delivered later by asteroids or comets, which formed farther from the Sun where water could be retained, Raymond said.
In the case of our solar system, Jupiter's orbit is slightly elliptical. The researchers said this middle-of-the-road, real-world scenario could explain why Earth is not a total waterworld nor a complete desert.
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