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astronomers estimate that there are on
average one point six planets for every
star in our galaxy some stars may harbor
two three four or more planets and
others may have none at all but overall
there are around 160 billion planets in
our Milky Way galaxy exoplanets range in
size from small rocky worlds like ours
to bodies that are more like stars than
planets huge Titanic gas giants many
times the size of our Jupiter finding
these planets is difficult and we have
barely begun our search only recently
have detectors sensitive enough to infer
the presence of these bodies been
developed but we have begun to make
progress
so far we found over 750 planets in
orbit around 632 stars and the Kepler
space telescope has identified another
2300 candidates awaiting confirmation
none of these planets have been seen
directly their presence is inferred from
the effect the planet has on the host
star planets are very small compared to
stars and the blinding light emitted
from them overwhelms our detectors and
precludes direct observation but
astronomers are clever people and have
found ways to see planets in spite of
these obstacles
there are two major techniques that have
led to our discoveries and both are
indirect
the first one is the method used by the
Kepler space telescope which is staring
at over a hundred and fifty thousand
stars in the constellation Cygnus and
looking for minut dips and brightness as
planets past between Kepler and every
star in the field of view known as the
transit method in addition to telling us
that a planet is passing in front it
also gives us a good measure of its
diameter since larger planets block more
light than smaller ones the second
method requires a spectrograph it
measures the pull of an orbiting planet
on the star as the planet goes around
its parent star its gravity pulls two
star around making it wobble as a result
the spectrum of That star gets red or
blue shifted as it moves towards or away
from our detectors we live in a golden
age of astronomy for the first time in
our history using our new detectors and
instrumentation humanity can begin to
answer through science some of the most
important questions about our universe
among them is anybody else out there are
there any planets like ours nearby on
October 16 2012 astronomers using the
harps instrument at the European
Southern Observatory
announced the discovery of the first
earth-sized planet around another
sun-like star
and what's more it's in the Alpha
Centauri system the closest stars to us
specifically the planet is in orbit
around Alpha Centauri be one of three
stars in the system Proxima Centauri is
closest followed by the binary system
Alpha Centauri a and B located only 1.3
parsecs or 4.2 light-years away and the
constellation Centaurus this planet is
in our own backyard Alpha Centauri B is
slightly cooler than the Sun with an
effective temperature of 50 214 Kelvin
and it is slightly smaller having 0.9
times the mass of the Sun because it is
a little smaller and cooler its
habitable zone the theoretical distance
the planet needs to be from the star
that allows liquid water to exist is
closer to Alpha Centauri B than the one
for us here in our solar system further
since this planet was found using the
radial velocity method astronomers were
able to estimate its mass to be about
1.1 3 times the mass of the Earth making
this the smallest exoplanet found so far
but it wasn't easy to find planets the
size of the earth around sun-like stars
are particularly difficult and require
unprecedented precision from the spectra
crafts which harps was able to provide
because planets this size don't have
much of a pull on its parent star
here's why this is the size of the earth
compared to the Sun looking at this it's
easy to see why our planet pulls only
very slightly against a relatively
Titanic star it orbits these effects are
very small when trying to find
earth-size planets and therefore offers
them the most challenges it is because
this effect is so tiny that most of our
first discoveries of earth sized planets
will probably come from the stars
closest to us the close proximity makes
it easier to see the wobbles of the star
as the planet revolves and allows our
instruments to detect smaller variations
this effect is so faint that the wobble
of Alpha Centauri B was measured to be
only 1 point 8 km/h about the speed of a
baby crawling across the floor while
this discovery yielded the closest
exoplanet to us and it is the same size
as Earth the planet in orbit around
Alpha Centauri B is hardly our twin it
orbits only 6 million kilometers away
from the star and has a year of only 3
point 2 days making it much too hot to
support life as we know it
however discoveries from harps as well
as Kepler have shown clearly that the
majority of low-mass planets are found
in systems like this giving us every
reason to hope that there may yet be
even more planets hiding in the closest
star system just inside our celestial
back
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