Data from Kepler space observatory suggests planets capable of supporting life are far more common than previously thought.
Our galaxy probably contains at least two billion planets
that, like Earth, have liquid water on their surfaces and orbit around
their parent stars in the "habitable zone" for life. The nearest,
according to astronomers, could be a mere 12 light years away.
new study, published on Monday in the Proceedings of the National
Academy of Sciences, suggests that Earth-like planets capable of
supporting life are far more common than previously thought.
measurements from Nasa's Kepler space observatory, scientists led by
Erik Petigura at the University of California, Berkeley, estimated that
22% of our galaxy's sun-like stars have rocky planets circling them in
the zone where they get roughly the same amount of light energy as Earth
receives from the sun.
There are around 100bn stars in our galaxy, of which 10% are like the sun.
far Kepler has studied more than 150,000 stars and identified more than
3,000 candidate planets, but many of these are "gas giants", similar to
Jupiter, that orbit close to their parent stars.
If there is life
out there, it is far more likely to have evolved on rocky planets with
liquid water on their surfaces, similar to Earth.
To get their results, Petigura's team looked for planets in Kepler data that had a radius up to double that of Earth.
searched for planets that orbited far enough from their star that
liquid water would not evaporate, but not so far that the water would
Subhanjoy Mohanty, an astrophysicist at Imperial
College London who was not involved with the study, said: "This is the
first estimate of the frequency of Earth-like planets around sun-like
stars, in orbits large enough to lie in the habitable zone of their
The finding that roughly one in five sun-like stars may
host such planets is an incredibly important one, probably exceeding the
expectations of most cautious astronomers."
He added that the latest analysis increased the chances that there might be life somewhere among the stars.
analyses of Kepler data had shown that red dwarfs – the most common
type of star in the galaxy, making up about 80% of the stellar
population – very frequently harbour Earth-size planets, including in
their habitable zones.
This new study shows that the same is true around stars more like our own sun.
is certainly an added impetus for planned future missions which will
study the atmospheres of these potentially habitable planets, enabling
us to investigate whether they are in fact habitable or not, and also
whether their atmospheres show actual biosignatures of existing life."
also announced on Monday that the Kepler probe would be given a new
lease of life, following fears that it would have to end its mission
after only four years in space.
In May 2013, scientists discovered
that one of the gyroscopic wheels – known as "reaction wheels" – that
kept the probe pointing in the right direction had stopped working and,
try as they might, Nasa engineers could not restart it.
point itself at the stars with any accuracy, the probe could no longer
be used to collect data about the position of new exoplanets.
it looks as though there could be a solution that involves reorienting
the probe to look along the plane of the galaxy, which will allow it to
remain stable with only two of its reaction wheels working.
"The old saying 'necessity is the mother of invention'
has rung true here, with engineers and scientists from Nasa and the
spacecraft manufacturers having figured out this way to – we hope –
recover much of the performance we thought we had lost. We are very
excited," said Bill Chaplin, an astrophysicist at the University of
Birmingham in the UK.
If all goes well, the new Kepler mission –
dubbed "K2" – will look for planets around smaller stars than the sun,
and will also study the stars themselves.
"There are a wealth of
fantastically interesting targets for astrophysics that can be observed
in the ecliptic plane, which were not accessible in the original Kepler
field, notably brighter clusters of stars – where the common origins and
distances to these stars make the clusters excellent laboratories for
testing our understanding of stars – and young, star-forming regions,"