By John von Radowitz
Life on Earth-like planets in other solar systems could be unpredictable to say the least, according to a new study.
Many such worlds are likely to have roller-coaster conditions which switch rapidly between habitable and uninhabitable.
As their orbits alter shape, surface water could appear and boil away in as little as 1,000 years, research suggests.
And some planets could suffer big freeze cycles of global glaciation.
The Earth is spared such catastrophic events because it exists in a relatively stable system.
The orbits of planets circling the Sun are roughly circular. But if a gas giant such as Jupiter had a highly elliptical orbit, its gravity could pull the Earth in and out of the "habitable zone" around the Sun where conditions are just right to support life.
Most exoplanets discovered beyond the Solar System have elliptical rather than circular orbits.
Scientists used computer simulations to test what would happen to Earth-like planets in "habitable zones" if they had eccentric giant neighbours.
They found that the smaller planets' orbits would elongate and become more circular again within as little as 1,000 years.
"For part of the time liquid water could exist on the surface, but at others it would boil off," said study leader Dr Rory Barnes from the University of Washington in Seattle, US.
"The bigger issue here is that the habitable zone is very complicated. Earth's climate is affected slightly over tens of thousands of years by the orbits of other planets in the Solar System, but it is possible that in many exoplanetary systems the layout of the planets is very important to habitability."
The "habitable zone" - also called the "Goldilocks zone" - is the narrow orbital belt just far enough away from the parent star to allow the existence of liquid water. Its position depends on the size and brightness of the star.
The situation becomes more complex for potentially habitable planets orbiting small stars with about one-third the mass of the Sun, said Dr Barnes.
In those systems the habitable zone is close to the star, near enough for tidal forces from the star's gravity to exert a big effect quite apart from the influence of other planets.
"There could be planets out there that have their geological properties change over very long timescales," said Dr Barnes. "You can imagine planets that cycle in and out of intense volcanism and earthquake stages."
He added: "There is this crazy zoo of planets out there that are probably habitable, but their properties are very different from Earth and they're different from Earth because of their eccentric neighbours."
The findings will be presented this week at an American Astronomical Society meeting in Miami, Florida.