The search for “Earth 2.0” – another watery planet with life – has come a step closer with the discovery of 715 new planets orbiting stars beyond our own Solar System, Nasa scientists said.
Four of the new planets lie within the habitable "Goldilocks zone" of their own suns, meaning they orbit at a distance that is neither too hot nor too cold for water to exist in liquid form – considered to be an essential precondition for life to exist.
The latest results from Nasa’s programme to find “exoplanets” beyond our Solar System lend further support to the idea that the Milky Way galaxy – which is just one galaxy of many billions – is teeming with planets, many of which are similar to our own.
Scientists identified the new planets using a new statistical method to analyse data gathered by the £360m Kepler telescope, launched in 2009. This has almost overnight boosted the total number of confirmed exoplanets in the Milky Way galaxy to about 1,700 – with another 3,500 candidate planets waiting to be confirmed. The total number of habitable exoplanets has now reached nine.
The very first extra-solar planet was identified about 20 years ago and since then there has been a revolution in the way that astronomers can identify the tiny perturbations in starlight they create as they orbit their own stars.
“We almost doubled just today the number of planets known to humanity,” said planetary scientist Jack Lissauer of Nasa’s Ames Research Centre in Moffett Field California, who led the study of the Kepler data.
“Four years ago, Kepler began a string of announcements of first hundreds, then thousands, of planet candidates – but they were only candidate worlds,” Dr Lissauer said.
“We’ve now developed a process to verify multiple planet candidates in bulk to deliver planets wholesale, and have used it to unveil a veritable bonanza of new worlds,” he said.
“These results are based on the first two years of Kepler observations and with each additional year, we’ll be able to bring in a few hundred more planets,” he added.
The four potentially habitable planets within the new batch are all less than 2.5 times the size of the Earth and orbit at the right distance from their suns to allow water to exist in liquid form – although it is impossible to know yet whether they really do have water, let alone life.
Nasa said that one of these planets, called Kepler-296f, orbits a star that is half the size and five per cent as bright as our own Sun. It is twice the size of Earth, but it may either be a gaseous world with a thick hydrogen-helium envelope, or a water world with a deep ocean.
The 715 new exoplanets were found by analysing the light patterns of 305 stars, meaning that they exist in multi-planet solar systems similar to our own. All the planets were close in size to the Earth than to gas giants like Jupiter where life is considered less likely than on a small, rocky planet.
The Kepler telescope, which is now crippled, gathered vast amounts of data for scientists to analyse before it went defunct. The latest study used a technique called “verification by multiplicity” which gets round the problem of trying to verify planets on a planet-by-planet basis.
Kepler made observations of 150,000 stars before it went out of action and, out of these, scientists identified hundreds of stars with multiple planet candidates, which led them to identify 715 new planets around 305 stars.
Douglas Hudgins, an exploration programme scientist at Nasa, said that the latest study is a significant step towards the ultimate goal of “finding Earth 2.0”, and the possibility of finding another habitable planet in the Milky Way galaxy.
“This is the largest windfall of planets that’s ever been announced at one time. Second, these results establish that planetary systems with multiple planets around one star, like our own Solar System, are in fact common,” Dr Hudgins said.
“Third, we know that small planets – planets ranging from the size of Neptune down to the size of the Earth – make up the majority of the planets in our galaxy,” he said.
Jason Rowe, a research scientist at the Seti Institute in Mountain View, California, said: “From this study we learn planets in these multi-systems are small and their orbits are flat and circular, resembling pancakes… The more we explore the more we find familiar traces of ourselves among the stars that remind us of home.”