Earthlike Planets May Be Common In The Universe

Either we are not alone or there are some prime planets waiting to be colonized.

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.

Suppose there are a lot of planets which are similar to Earth in size and in the amount of radiation they receive from their own suns. Even if some of them do not contain sentient lifeforms they still may have native life forms and those life forms may be incompatible with human life. Imagine pathogens that human immune systems couldn't even recognize let alone effectively fight. Or all native plant matter might be poisonous not only to humans but to any plants humans would bring to grow on such a planet.

It is incredibly common in science fiction movies and television shows for humans to mate and reproduce with aliens and to find edible food on distant planets. But if there is life on other planets both of these possibiliities are very unlikely. Other lifeforms will probably use different combinations of compounds for genetic encoding and for building tissues. Species on other planets may use amino acids to build proteins but probably not the exact same set of amino acids humans use. Ditto for sugars and other biological compounds.

The real tragedy is that even if humans and sentient species from other planets could get along and even if other sentient species lived under similar levels of gravity and atmospheric pressure and also were oxygen breathing it would probably be necessary to never have direct physical contact due to fears that pathogens would jump from one species to another with deadly results.