8 January 2013 Last updated at 07:16 GMT
Kepler telescope: Earth-size planets ‘number 17bn’
By Jason Palmer Science and technology reporter, BBC News, California
This artist’s rendering shows the different types of planets in the Milky Way detected by Nasa’s Kepler spacecraft
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Astronomers say that one
in six stars hosts an Earth-sized planet in a close orbit – suggesting a
total of 17 billion such planets in the galaxy.
The result comes from an analysis of planet candidates gathered by the Kepler telescope.
The Kepler team also announced 461 new planet candidates, bringing the satellites’ total haul to 2,740.
The findings were announced at the 221st meeting of the American Astronomical Society in California.
TransitSince its launch into orbit in
2009, Kepler has stared at a fixed part of the sky, peering at more
than 150,000 stars in its field of view.
It detects the minute dip in light coming from a star if a planet passes in front of it, in what is called a transit.
But it is a tricky measurement to make, with the total light changing
just tiny fractions of a percent, and not every dip in light is due to a
So Francois Fressin of the Harvard-Smithsonian Center for Astrophysics – who discovered the first Earth-sized planets
set about trying to find out not only which Kepler candidates might not
be planets, but also what planets might not have been visible to
Kepler Space Telescope
- Stares fixedly at a patch corresponding to 1/400th of the sky
- Looks at more than 150,000 stars
- Has so far found 2,740 candidate planets
- Among them are 461 Earth-sized planets, at least 10 of which are in the “habitable zone” where liquid water can exist
“We have to correct for two things – first [the Kepler candidate list] is incomplete,” he told BBC News.
“We only see the planets that are transiting their host stars, stars
that happen to have a planet that is well-aligned for us to see it, and
[for each of those] there are dozens that do not.”
“The second major correction is in the list of candidates – there are
some that are not true planets transiting their host star, they are
other astrophysical configurations.”
These might include for example binary stars, where one star orbits
another, blocking some of the light as the stars “transit” each other.
“We simulated all the possible configurations we could think of – and
we found out that they could only account for 9.5% of Kepler planets,
and all the rest are bona fide planets,” Dr Fressin explained.
The results suggest that 17% of stars host a planet up to 1.25 times
the size of the Earth, in close orbits lasting just 85 days or fewer –
much like the planet Mercury.
That means our galaxy hosts at least 17 billion Earth-sized planets.
In the zoneEven as Dr Fressin
reported an analysis of the most recent Kepler catalogue, it was
increased substantially by results reported by Christopher Burke of the
Dr Burke announced 461 new candidate planets, a substantial fraction
of which were Earth-sized or not much larger – planets that have until
now been particularly difficult to detect.
“What’s particularly interesting is four new planets – less than
twice the size of Earth – that are potentially in the habitable zone,
the location around a star where it could potentially have liquid water
to sustain life,” Dr Burke told BBC News.
One of the four, dubbed KOI 172.02, is a mere 1.5 times the size of
the Earth and around a star like our own Sun – perhaps as near as the
current data allow to finding an “Earth 2.0″.
“It’s very exciting because we’re really starting to pick up the
sensitivity to these things in the habitable zone – we’re just really
getting to the frontier of potentially life-bearing planets.”
William Borucki, the driving force behind and principal investigator
on the Kepler mission, said he was “delighted” by the fresh batch of
“The most important thing is the statistics – not to find one Earth
but to find 100 Earths, that’s what we’ll be seeing as the years go on
with the Kepler mission – because it was designed to find many Earths.”
ttBBC News – Kepler telescope: Earth-size planets ‘number 17bn’ .
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