Earthlike Planets Likely

Astrobiologists disagree about whether advanced life is common or rare in our universe. But new research suggests that one thing is pretty certain – if an Earthlike world with significant water is needed for advanced life to evolve, there could be many candidates.

In 44 computer simulations of planet formation near a sun, astronomers found that each simulation produced one to four Earthlike planets, including 11 so-called "habitable" planets about the same distance from their stars as Earth is from our sun.

"Our simulations show a tremendous variety of planets. 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," said Sean Raymond, a University of Washington doctoral student in astronomy.

Raymond is the lead author of a paper detailing the simulation results that has been accepted for publication in Icarus, the journal of the American Astronomical Society's Division for Planetary Sciences. Co-authors are Thomas R. Quinn, a UW associate astronomy professor, and Jonathan Lunine, a professor of planetary science and physics at the University of Arizona.

The simulations show that the amount of water on terrestrial, or Earthlike, planets could be greatly influenced by outer gas giant planets like Jupiter.

"The more eccentric giant planet orbits result in drier terrestrial planets," Raymond said. "Conversely, more circular giant planet orbits mean wetter terrestrial planets."

In the case of our solar system, Jupiter's orbit is slightly elliptical, which could explain why Earth is 80 percent covered by oceans rather than being bone dry or completely covered in water miles deep.

The findings are significant because of the discovery in recent years of a large number of giant planets such as Jupiter and Saturn orbiting other suns. The presence, and orbits, of those planets can be inferred from their gravitational interaction with their parent stars and their affect on light from those stars as seen from Earth.

It currently is impossible to detect Earthlike planets around other stars. However, if results from the models are correct, there could be planets such as ours around a number of other suns relatively close to our solar system. A significant number of those planets are likely to be in the "habitable zone," the distance from a star at which the planet's temperature will maintain liquid water on the surface. Liquid water is thought to be a requirement for life, so planets in a star's habitable zone are ideal candidates for life. It is unclear, however, whether those planets could harbor more than simple microbial life.

The researchers note that their models represent the extremes of what is possible in forming Earthlike planets rather than what is typical of planets observed in our galaxy. For now, they said, it is unclear which approach is more realistic.

Their goal is to understand what a system's terrestrial planets will look like if the characteristics of a system's giant planets are known, Raymond said.

Quinn noted that all of the giant planets detected so far have orbits that carry them very close to their parent stars, so their orbits are completed in a relatively short time and it is easier to observe them. The giant planets observed close to their parent stars likely formed farther away and then, because of gravitational forces, migrated closer.

But Quinn expects that giant planets will begin to be discovered farther away from their suns as astronomers have more time to watch and are able to observe gravitational effects during their longer orbits. He doubts such planets will be found before they have completed whatever migration they make toward their suns, because their orbits would be too irregular to observe with any confidence.

"These simulations occur after their migration is over, after the orbits of the gas giants have stabilized," he said..

Source: University of Washington

"The context for life is much broader than just what we see on Earth. Organic material is falling from the sky all the time, and we're learning that what happens out there is very important down here. Who knows? Maybe life on Earth came from Mars billions of years ago, when it had liquid water on its surface."
-- John Rummel
director, NASA astrobiology program

Centauri Dreams
Imagining and Planning Interstellar Exploration 
by Paul Gilster

At 4.3 light years distant, Alpha Centauri is the closest star in the heavens and the most likely detination for mankind's first instellar exploration. Getting there at speeds of even one-tenth the speed of light (well beyond our current capabilities) will require a generation of years... in each direction.

"Interstellar flight is in our future, although we cannot know when," writes Paul Gilster in this informative survey of the propulsion and communications challenges that must be met. "The challenges are immense but achievable, if only by our descendants."

Gilster reviews the surprisingly large body of scientific literature on the subject of interstellar flight and introduces researchers at NASA, the Jet Propulsion Laboratory and elsewhere who are seriously laboring on the issues.

"A single insight from any one of a number of running experiments in different areas could change everything," he writes. "The field is top heavy with ideas, alive with the loopy challenge of pushing a spacecraft to a sizeable percentage of the speed of light."

Science, Society, And the Search for Life in the Universe
Science, Society, And the Search for Life in the Universe

Protostars and Planets V
Protostars and Planets V