By Helen Briggs Science reporter, BBC News, Boston

Scientists say there may be many more worlds in our galaxy

Rocky planets, possibly with conditions suitable for life, may be more common than previously thought in our galaxy, a study has found.

New evidence suggests more than half the Sun-like stars in the Milky Way could have similar planetary systems.

There may also be hundreds of undiscovered worlds in outer parts of our Solar System, astronomers believe.

Future studies of such worlds will radically alter our understanding of how planets are formed, they say.

New findings about planets were presented at the American Association for the Advancement of Science (AAAS) in Boston.

Nasa telescope

Michael Meyer, an astronomer from the University of Arizona, said he believed Earth-like planets were probably very common around Sun-like stars.

I expect that we will find a very large number of planets Alan Stern, Nasa

"Our observations suggest that between 20% and 60% of Sun-like stars have evidence for the formation of rocky planets not unlike the processes we think led to planet Earth," he said. "That is very exciting."

Mr Meyer's team used the US space agency's Spitzer space telescope to look at groups of stars with masses similar to the Sun.

They detected discs of cosmic dust around stars in some of the youngest groups surveyed.

The dust is believed to be a by-product of rocky debris colliding and merging to form planets.

Nasa's Kepler mission to search for Earth-sized and smaller planets, due to be launched next year, is expected to reveal more clues about these distant undiscovered worlds.

Frozen worlds

Some astronomers believe there may be hundreds of small rocky bodies in the outer edges of our own Solar System, and perhaps even a handful of frozen Earth-sized worlds.

We have to find the right mass planet and it has to be at the right distance from the star Debra Fischer, San Francisco State University

Speaking at the AAAS meeting, Nasa's Alan Stern said he thought only the tip of the iceberg had been found in terms of planets within our own Solar System.

More than a thousand objects had already been discovered in the Kuiper belt alone, he said, many rivalling the planet Pluto in size.

"Our old view, that the Solar System had nine planets will be supplanted by a view that there are hundreds if not thousands of planets in our Solar System," he told BBC News.

He said many of these planets would be icy, some would be rocky, and there might even be objects with the same mass as Earth.

"It could be that there are objects of Earth-mass in the Oort cloud (a band of debris surrounding our planetary system) but they would be frozen at these distances," Dr Stern added.

"They would look like a frozen Earth."

Goldilocks zone

Excitement about finding other Earth-like planets is driven by the idea that some might contain life or perhaps, centuries from now, allow human colonies to be set up on them.

The key to this search, said Debra Fischer of San Francisco State University, California, was the "Goldilocks zone".

This refers to an area of space in which a planet is "just the right distance" from its parent star so that its surface is not-too-hot or not-too-cold to support liquid water.

"To my mind there are two things we have to go after: we have to find the right mass planet and it has to be at the right distance from the star," she said.