If you want to know where other Earth-like planets could survive, one way to find out is to simply toss an Earth into an existing system where planets orbit a star and see if it hangs in there or is tossed out.
That's what Barrie Jones and Nick Sleep did.
The researchers sorted through the handful of stars now known to harbor nearly 100 planets altogether. Most of the planets are huge and very close to their host stars, making it nearly impossible for Earth-sized planets to exist in potentially habitable, Earth-like orbits. They would simply be swallowed or tossed out of the system.
Jones and Sleep, of the Open University in England, identified five stars they were interested in. They figured two of the systems would not support other Earths, and computer simulations proved that to be the case.
"We had no particular expectations of the other three," Jones said in an e-mail interview. "But in fact in all of them, Earth-mass planets could be in stable orbits in at least some of each habitable zone."
Why does any of this matter? Because hunting for planets the size of Earth around other stars will be very difficult. Expensive next-generation telescopes will barely be able to pull the task off. And there are lots of stars out there, so knowing where to look could save a lot of time and money.
"Our work should identify the exosytems where one is more likely to find 'Earths,' and so our work should help in target-selection," Jones said. "If no Earths are found in habitable zones where our work indicates they could exist, then that should be of interest to people trying to model the formation of exosystems."
The researchers will discuss their findings today the annual UK National Astronomy Meeting. A paper on the early results of their work appeared previously in the journal Astronomy & Astrophysics, and they are preparing a new paper on their full results.
The ultimate goal in searching for other Earths is, of course, to then look for signs of life. So far, the other solar systems that have been found look very much unlike our own. But most researchers agree that's probably because technology only allows for the detection of large planets very near stars. Over the next decade, most experts expect solar systems more similar to our own are likely to be discovered.
But no one knows if those systems will support rocky planets with stable atmospheres and moderate temperatures -- places like Earth.
And many other factors might play a role in supporting life.
As Jones points out, some researchers figure that shifting crustal plates -- the things that cause earthquakes -- are needed to recycle material and make life possible. A moon might be needed to stabilize a planet's rotation and help avoid large changes in climate. "I'm not convinced that this is essential," Jones said.
Others argue large outer planets, like Jupiter, are needed to shield an inner planet from excessive comet bombardment. "But this is conjectural," Jones maintains.
Regardless of the exact ingredients needed to make a habitable planet, the new study does not promise a whole lot. Importantly, it is based on a computer simulation, not on observations. Several assumptions were made in regards to whether other solar systems can even form in the same manner as our own.
"Our biggest assumption is that Earth-mass planets can form in the exosystems," Jones said. "Studies by others are beginning to show that this is possible, at least in some of the exosystems."
The system most like our own that Jones and Sleep looked at is called 47 Ursae Majoris. The star is sun-like, though a bit older. It is hotter and brighter, so its habitable zone is a little further out than the region around our Sun that's considered able to support life, as we know it.
Two giant planets orbit the star 47 Ursae Majoris. The inner one, a bit farther from the star than Mars is from our Sun, is about 2.5 times as massive as Jupiter. The outer one is probably about the size of Jupiter. Other planets may well exist in the system, but they can't be found with current technology.
"It's certainly a system worth exploring for an earthlike planet and for life," Jones said.
The computer simulations may prove more useful as more and more planetary systems are found, and astronomers develop a greater need to winnow down the best candidates for further study.
"I think we will shortly discover systems with the giants further out, more like where Jupiter is in the solar system," Jones said. "We will then see whether Earth-mass planets could exist in their habitable zones."
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