Someday astronomers will likely create a long list of Sun-like stars with Earth-like planets around them. But technology has yet to reveal such worlds, instead allowing the detection only of much larger planets.
Most of the roughly 200 known extrasolar planets are larger than Jupiter. Many complete their orbital years in just a few days. This proximity to their stars creates noticeable wobbles in the stars that make the planets detectable.
But astronomers figure the giants probably formed farther out, in a disk of material swirling around a newborn star, and migrated inward. In doing so they would have destroyed any fledgling habitable worlds.
In recent years, with improving technology, researchers have found a handful of systems that could harbor life-bearing planets, in theory at least. A nearby star called 55 Cancri is one of the leading candidates.
The 55 Cancri system involves three gas giant planets and another world that could be icy or rocky and is about the size of Neptune. The setup is 41 light-years from Earth and about 4.7 billion years old, comparable to our Sun.
Astronomers have said since 2002, when a planet was found at about the same orbital distance from 55 Cancri as Jupiter is from the Sun, that the star had the potential to harbor an Earth-sized world.
A new computer simulation shows that amid the giant worlds orbiting 55 Cancri, a small rocky world could indeed have formed--in theory--and attracted enough water to support life as we know it.
"Our models show a habitable planet, a planet with mass, temperature and water content similar to Earth's, could have formed," said Rory Barnes, a postdoctoral researcher at the University of Arizona.
Barnes and colleagues ran several simulations of varying scenarios around four stars, each known to have at least two giant planets. They put moon-sized planetary embryos into the systems during their youth and allowed them to evolve for 100 million years.
The idea, based on the leading planet-formation theory, is that small objects collect more material and, if they don't collide with another big object, become planets.
Star of the show
Only 55 Cancri consistently yielded a world similar in size and orbital distance to Earth. Our planet sits in what's called a habitable zone, just the right distance from the Sun to allow liquid water.
"Our simulations typically produced one terrestrial planet in the habitable zone of 55 Cancri, with a typical mass of about half an Earth mass," said Sean Raymond, a postdoctoral researcher at the University of Colorado who worked on the project while a doctoral student at the University of Washington. "In many of the simulations, these planets accreted a decent amount of water-rich material from farther out in the disk."
The research, funded by NASA and the National Science Foundation, is described in a recent issue of the Astrophysical Journal.
A computer simulation is of course far from reality. But research like this can guide astronomers to solar systems worthy of further investigation as search technology improves.
"Our assumptions are quite optimistic, but not crazy by any means, and we start our simulations with a decent amount of material for terrestrial planets to form," Raymond told SPACE.com. "If we are wrong about this, then only smaller, perhaps Mars-sized planets could form in the habitable zone."
The best bet
Two other stars yielded little suggestion of habitable worlds. Another star, named HD 38529, is likely to support an asteroid belt and objects up to the size of Mars, the simulations indicate.
"In terms of the systems we looked at, 55 Cancri has the largest zone between giant planets in which terrestrial planets may form and remain on stable orbits," Raymond said. "So, I think the chance of other planets existing in the system is pretty good, but it's certainly not definitive at the moment."
Other modeling by Raymond has shown that only about 5 percent of the known giant-planet systems are likely to have Earth-like planets. But, he and others have said, there may well be many solar systems similar to our own, in which the giant planets are all on the outskirts, that simply can't be detected yet.
Top 10 Star Mysteries
The Wildest Weather in the Galaxy