Posted on December 4, 2012 by Laura
December 03, 2012
“Tens of Billions of Earth-like Rocky Planets Orbit Milky-Way Red Dwarf Stars”
Data released early this year from the European Space Agency’s (ESO)
HARPS planet finder shows that rocky planets not much bigger than Earth
are very common in the habitable zones around faint red stars. The
international team estimates that there are tens of billions of such
planets in the Milky Way galaxy alone, and probably about one hundred in
the Sun’s immediate neighbourhood. This was the first direct
measurement of the frequency of super-Earths around red dwarfs, which
account for 80% of the stars in the Milky Way.
This first direct estimate of the number of light planets around red
dwarf stars was announced early this year by an international team using
observations with the HARPS spectrograph on the 3.6-metre telescope at
ESO’s La Silla Observatory in Chile. A prior announcement, showing that
planets are ubiquitous in our galaxy used a different method that was
not sensitive to this important class of exoplanets.
The HARPS team has been searching for exoplanets orbiting the most
common kind of star in the Milky Way — red dwarf stars (also known as M
dwarfs). These stars are faint and cool compared to the Sun, but very
common and long-lived, and therefore account for 80% of all the stars in
the Milky Way.
“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,” says Xavier Bonfils
(IPAG, Observatoire des Sciences de l’Univers de Grenoble, France), the
leader of the team.”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
The HARPS team surveyed a carefully chosen sample of 102 red dwarf
stars in the southern skies over a six-year period. A total of nine
super-Earths (planets with masses between one and ten times that of
Earth) were found, including two inside the habitable zones of Gliese
581 and Gliese 667 C respectively. The astronomers could estimate how
heavy the planets were and how far from their stars they orbited.
By combining all the data, including observations of stars that did
not have planets, and looking at the fraction of existing planets that
could be discovered, the team has been able to work out how common
different sorts of planets are around red dwarfs. They find that the
frequency of occurrence of super-Earths in the habitable zone is 41%
with a range from 28% to 95%.
On the other hand, more massive planets, similar to Jupiter and
Saturn in our Solar System, are found to be rare around red dwarfs. Less
than 12% of red dwarfs are expected to have giant planets (with masses
between 100 and 1000 times that of the Earth).
As there are many red dwarf stars close to the Sun the new estimate
means that there are probably about one hundred super-Earth planets in
the habitable zones around stars in the neighbourhood of the Sun at
distances less than about 30 light-years.
“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,” says Stephane Udry (Geneva
Observatory and member of the team). “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
One of the planets discovered in the HARPS survey of red dwarfs is
Gliese 667 Cc. This is the second planet in this triple star system and
seems to be situated close to the centre of the habitable zone. Although
this planet is more than four times heavier than the Earth it is the
closest twin to Earth found so far and almost certainly has the right
conditions for the existence of liquid water on its surface.
This is the second super-Earth planet inside the habitable zone of a
red dwarf discovered during this HARPS survey, after Gliese 581d was
announced in 2007 and confirmed in 2009.
“Now that we know that there are many super-Earths around nearby red
dwarfs we need to identify more of them using both HARPS and future
instruments. Some of these planets are expected to pass in front of
their parent star as they orbit — this will open up the exciting
possibility of studying the planet’s atmosphere and searching for signs
of life,” concludes Xavier Delfosse, another member of the team.
The image at the top of the page is an artist’s conception of the
planetary system around the M dwarf Gliese 876. Currently, 3 planets are
known to be orbiting this low mass M star, which is only 15.3
The Daily Galaxy via ESO
Image credit: http://physics.rice.edu/Content.aspx?id=363 and The Astrophysical Journal
“Tens of Billions of Earth-like Rocky Planets Orbit Milky-Way Red Dwarf Stars”.
Thanks to: http://2012indyinfo.com