Milky Way 'Has 17 Billion Earth-Like Planets'

The Milky Way is home to at least 17 billion planets of a similar size to Earth, according to a new estimate by scientists.

The galaxy containing our solar system is known to host about 100 billion stars, meaning that around one in six has an Earth-sized planet around it.

However, Dr Francois Fressin of the Harvard-Smithsonian Centre for Astrophysics, said it was "simply too early to call" how many were located in areas which could make them habitable.

To support life, a planet must not only be the right size but also in the so-called Goldilocks zone where it is not too hot or too cold for water to be in liquid form on the surface.

Dr Fressin and his team came up with their figure by conducting a fresh analysis of data collected by Nasa's Kepler telescope, which was launched in 2009 to track down other Earths.

"If you look up on a starry night, each star you're looking at - almost each one of them - has a planetary system," he said.

They found that 17% of stars have a planet 0.8 to 1.25 times the size of Earth in an orbit of 85 days or less.

Kepler-22b is believed to be the most Earth-like planet ever discovered

About a quarter of stars have a super Earth (1.25 to twice the size of Earth) in an orbit of 150 days or less, with a same fraction having a mini Neptune (two to four times the size of Earth) in orbits up to 250 days long.

Larger planets are a much rarer occurrence. Only about 3% of stars have a large Neptune (four to six times the size of Earth) and only 5% have a gas giant (six to 22 times the size of Earth) in an orbit of 400 days or less.

Separately, Nasa's Kepler mission announced it had discovered 461 new possible planets, bringing the total to 2,740 potential planets.

Four of them are less than twice the size of Earth and orbit their sun's "habitable zone," where liquid water might exist on the planet's surface.

"You need very specific conditions to have liquid water," explained Christopher Burke, a scientist with the SETI Institute in Mountain View, California.

"You can't have your planet too close to your star where it's too hot. You can't have it too far away for the planet conditions to be too cold.

"We're trying to find these planets in this very specific habitable zone."

The Kepler telescope works by tracking slight decreases in the amount of light coming from 160,000 target stars caused by a planet or planets passing by, or transiting, relative to the telescope's point of view.

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