So far, skywatchers have identified 105 planetary systems around other stars in the Milky Way, thanks to a "wobble" in the star's motion that happens when a large planet passes by, exerting a gravitational pull.

One of the big factors in determining whether any of these systems have Earth-type planets is whether it has a big Jupiter-sized planet, and the distance of that planet from the star.

This is because, in order to bear life, an Earth-style planet has to be in the so-called Goldilocks zone: not too hot, not too cold, just right.

In other words, it would have to be close enough to the star to have liquid water -- yet not so close that its oceans would boil away -- and not so far that its oceans would freeze.

The presence and location of a Jupiter is a key to that.

Our own Jupiter plays the role of gatekeeper, taking the hit from asteroids and comets that could otherwise rain down on Earth and destroy life there.

But it is also sufficiently distant from the Earth to enable our planet to orbit the Sun peacefully, without feeling the effect of its giant gravitational tug.

Just how many of these exoplanets might contain habitable Earth equivalents is one of the great unknowns about this new branch of astronomy.

But astronomers at Britain's Open University have taken a stab at it, and believe that as many as half of the known systems may have another Earth.

The team drew up a computer model of nine known exoplanet systems and injected an "Earth", with masses of between 0.1 and 10 times that of our home, in an orbit in the Goldilocks zone.

In some planetary systems, the proximity of one or more Jupiter-like planets caused the putative "Earth" to be kicked out of the habitable zone, so that it froze or roasted.

But in others, the planet was moved to a "safe haven" in the habitable zone, or was not significantly troubled.

"The analysis shows that about half of the known exoplanetary systems could have an 'Earth' which is currently orbiting in at least part of the habitable zone, and which has been in this zone for at least one billion years," the Royal Astronomical Society said in a press release.

A billion years is believed to be the minimum time for life to arise and establish itself on Earth.

"Furthermore, the models show that life could develop at some time in about two-thirds of the systems, since the habitable zone moves outwards as the central star ages and becomes more active."

The research was to be presented at the RAS' National Astronomy Meeting in Milton Keynes, England, on Thursday. The study was led by Barrie Jones, a professor at the Open University.

If a second Earth is ever detected, its typical expected distance would be 20,000 light years -- way too far to think of establishing any contact or of travelling there with the present rocket technology.

SPACE.WIRE