Exoplanets

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Exoplanets
Or extra-solar planets have recently been
discovered. There are important to find to help
fill in the Drake Equation that determines the
probability of life existing elsewhere in the
universe. What is a 'planet'? A planet is an
object that has a mass between that of Pluto and
the Deutrium-burning limit (0.015 solar masses)
that forms in orbit around a star.
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The formation of planetary systems
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Finding Exoplanets
Finding exoplanets is tough because they are very
dim compared to the star that they orbit, and
they are very close. Jupiter viewed from 5 pc
away will just be separated by 1 arcsecond and is
just 0.000000001 times as bright! Therefore one
away to detect the planet is by the gravitational
pull of a planet around the star it orbits. As
planets orbit a star they tug it causing the star
to wobble.
Bigger planets cause bigger wobbles so we can
generally only observe the largest planets using
this technique. The first exoplanet was observed
in 1994 by detecting a wobble in the pulse of a
pulsar!
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The wobble can be seen directly in some nearby
stars by measuring the shift in the stars
position in the sky (astronometry) or be
detecting a red-shift / blue-shift wobble due to
the Doppler effect (radial velocity).
Like eclipsing binary star systems exoplanets may
be discovered by observing tiny dips in the
lightcurve of a star
Microlensing can also be used to spot the
existence of a planet around another star
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Nulling Interferometry
We could find much smaller planets, and find out
more about their nature if we could directly
detect the light from them. One technique to do
this is by nulling interferometry which uses an
array of telescopes in space separated by
millions of miles to improve the spatial
resolution and hence separate out the light of
the planet from the light of the star. It is
known as nulling interferometry because by
cancelling out lightwaves from the star it blocks
all of the star's light allowing the planets
light to be detected.
From this we can observe its spectrum and hence
see what its atmosphere is made of and possibly
detect the presence of water vital for life to
exist.
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So far we've found over 100 planets mostly
Jupiter sized or bigger