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There could
be many billions of planets not much bigger than Earth circling faint stars in
our galaxy, says an international team of astronomers.
The
estimate for the number of "super-Earths" is based on detections
already made and then extrapolated to include the Milky Way's population of
so-called red dwarf stars.
The team
works with the high-precision Harps instrument.
This is
fitted to the 3.6m telescope at the Silla Observatory in Chile.
Harps
employs an indirect method of detection that infers the existence of orbiting
planets from the way their gravity makes a parent star appear to twitch in its
motion across the sky.
"Our
new observations with Harps mean that about 40% of all red dwarf stars have a
super-Earth orbiting in the habitable zone where liquid water can exist on the
surface of the planet," said team leader Xavier Bonfils from the
Observatoire des Sciences de l'Univers de Grenoble, France.
"Because
red dwarfs are so common - there are about 160 billion of them in the Milky Way
- this leads us to the astonishing result that there are tens of billions of
these planets in our galaxy alone."
The Harps
team came up with its numbers after surveying 102 carefully chosen red dwarfs,
which are dimmer and cooler than our Sun.
The group
found a total of nine super-Earths (which are defined as planets with one to 10
times the mass of the Earth), with two judged to be orbiting inside their
stars' habitable zones.
Putting all
its data together, including observations of stars that did not have planets,
the team was able to produce an estimate for how common different sorts of
planets are around red dwarfs.
This
assessment suggests super-Earths in the habitable zone occur in 41% of cases,
with a range from 28% to 95%.
Given how
many red dwarf stars there are in close proximity to the Sun, it means there
could be perhaps 100 super-Earth planets in the habitable zones of stars that
are less than about 30 light-years distant.
"The
habitable zone around a red dwarf, where the temperature is suitable for liquid
water to exist on the surface, is much closer to the star than the Earth is to
the Sun," commented co-researcher Stephane Udry from the Geneva
Observatory.
"But
red dwarfs are known to be subject to stellar eruptions or flares, which may
bathe the planet in X-rays or ultraviolet radiation, and which may make life
there less likely."
The latest
Harps research will appear in the journal Astronomy & Astrophysics [PDF].
Jonathan.Amos-INTERNET@bbc.co.uk
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