Milky Way's planet count doubles

COSMIC VIEW: The Kepler space telescope is used to search for planets in other solar systems.
COSMIC VIEW: The Kepler space telescope is used to search for planets in other solar systems.

The Earth's galaxy is looking far more crowded and hospitable. Nasa has confirmed a bonanza of 715 newly discovered planets outside the solar system.

Scientists using the planet-hunting Kepler telescope pushed the number of planets discovered in the galaxy to about 1700. Twenty years ago, astronomers had not found any planets circling stars other than the ones revolving around the sun.

''We almost doubled just today the number of planets known to humanity,'' Nasa planetary scientist Jack Lissauer said in a Wednesday (NZT Thursday) teleconference, calling it ''the big mother lode''.

Astronomers used a new confirmation technique to come up with the largest single announcement of a batch of exoplanets - what planets outside our solar system were called.

While the announcements were about big numbers, they also were about implications for life behind those big numbers.

All the new planets were in systems like ours where multiple planets circle a star. The 715 planets came from looking at just 305 stars. They were nearly all in size closer to Earth than gigantic Jupiter.

And four of those new exoplanets orbited their stars in ''habitable zones'' where it was not too hot or not too cold for liquid water, which was crucial for life to exist.

Douglas Hudgins, Nasa's exoplanet exploration programme scientist, called the discoveries a major step toward Kepler's ultimate goal: ''Finding Earth 2.0.''

It's a big step in not just finding other Earths, but ''the possibility of life elsewhere,'' said Lisa Kaltenegger, a Harvard and Max Planck Institute astronomer who wasn't part of the discovery team.

The four new habitable zone planets were all at least twice as big as Earth so that made them more likely to be gas planets instead of rocky ones like Earth - and less likely to harbour life.

So far Kepler has found nine exoplanets in the habitable zone, Nasa said.

Astronomers expect to find more when they look at all four years of data collected by the now-crippled Kepler; so far they have looked at two years.

Planets in the habitable zone were likely to be farther out from their stars because it was hot close in. And planets farther out take more time orbiting, so Kepler has to wait longer to see it again.

Another of Kepler's latest discoveries indicated that ''small planets are extremely common in our galaxy,'' said MIT astronomer Sara Seagar, who wasn't part of the discovery team.

''Nature wants to make small planets.''

And, in general, smaller planets were more likely to be able to harbour life than big ones, Kaltenegger said.