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Astrobiology and the Search for Extraterrestrial Life
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The Search for Habitable Exoplanets
How do we search for Earth-like planets orbiting distant stars and how would we detect life on them?
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Charles CockellProfessor
Astrobiology
[BLANK_AUDIO]
There's little hesitation saying that one
of the most remarkable developments in Astrobiology
over the last few decades has been the discovery of planets orbiting other stars.
Exoplanets and we're going to have a look at some of the methods
that people use to look for these planets and some of the recent discoveries.
It's worth remembering that speculations about the
possibility of planets orbiting other stars goes
back a very long time.
Giordano Bruno said in the sixteenth century in
space, there are countless constellations, suns and planets.
We see only the suns because they give light.
The planets remain invisible for they are small and dark.
There are also numberless earths circling around their suns.
Well unfortunately, Bruno is burned at the stake
in 1600 not just for his astrobiological speculations
but also because he did other things that irritated his religious superiors.
But nevertheless, his speculations about the possibility of planets
orbiting other stars, even earthlike planets was truly remarkable.
The search for planets around other stars really
became an empirical science in the late twentieth century.
In 1992, 2 radio astronomers announced the discovery of 2 planets orbiting a pulsar,
a rotating neutron star.
And in 1995, Michelle Meyer and Didier Queloz
of the University of Geneva announced the first definitive
detection of an exo planet orbiting a main sequence
star, a G type of star called 51 Pegasi.
These discoveries ushered in the era of the search for extra solar planets.
And now many of these planets are known and continue to be discovered everyday.
The holy grail for astrobiologists at least is to try and find a second Earth,
a planet, identical maybe to Earth in
terms of having life and habitable conditions that
may be orbiting a distant star, to be able to detect that planet and study
its composition, its atmospheric composition, and the
conditions that it might have, to sustain life.
The search for planets around other stars has focused
on main sequence stars.
If we plot stars according to their luminosity
and their temperature, we end up with a
band of stars that you can see in
the center of this diagram, the Hertzsprung-Russell diagram.
And the majority of planets we might be
looking for are orbiting these so-called main sequence stars.
One method that we can use to look
for planets around stars is the transit method.
When a planet orbits
around its star, some of these planets, if they're in the field of view
of the observer will pass in front of the star and block out its light.
As they block out the light, if you're measuring the light
from that star you'll see a dip in the light given off.
And this dip can tell you there's a planet there
and something about the nature of the planet orbiting the star.
This is an example of a light curve from a planet that has transited in front
of a star.
And you can see very clearly the
light of the star continues quite continuously along
the top of the graph and then drops when the planet passes in front of it.
This is one method used to search for exosolar planets.
Another method to search for planets is called astrometry.
And the best way to think about this is think
about 2 people holding hands and swirling around each other.
When 2 people hold hands and begin
to circle around each other, they will circle around a common center of mass.
And we can use the same sort of method to search for planets around other stars.
If a planet is orbiting around a star, they
will in fact circle around the common center of mass.
And the center of mass will not be exactly in the
center of the star because of the mass of the planet.
In fact the star will wobble ever so slightly as the, as the planet and
the star revolve around their common center of mass.
This wobble in the planet can be detected,
and used to infer the presence of a planet.
It needs very high sensitivity but telescopes can
be constructed, to find planets by the astrometry method.
Another method that we can use is called doppler shift.
This rotation of a planet around the star that causes this wobbling will also
cause a slight change in the wavelength of light given off by a star.
As the star moves away from the observer,
the light will be slightly shifted towards the red
and as the star moves towards the observer,
the light will be slightly shifted to the blue.
This wobbling caused
by the presence of a planet around the star can therefore be
detected by the slight change in the wave length of the star.
And this method called radial velocity or Doppler shift
has been very effective in being used to find
extra set of planets and by mid 2011, over
500 stars had been detected using the Doppler shift method.
Yet another method that is recently been
developed is called gravitational lensing.
And the idea here is that a star will
distort the light from stars or galaxies behind it.
This is because of the gravitational influence on the
light being bent around the star, first predicted by Einstein.
So instead of seeing distant stars behind stars
that you're observing the light will be distorted
and you'll start to see a sort of blood image, almost a ring if you like
of the light that is being deflected from a
distant star around the star that you're interested in.
If there's a planet in the way, a planet orbiting that star, it will
have a very subtle effect on the
light that's being distorted from distant starts.
And that subtle effect can be picked up and again
used to infer the presence of a planet orbiting that star.
Gravitational Lensing is a new method but it's being used very effectively for
finding exoplanets orbiting distant stars.
So what have we learned in this brief review?
Well, we've learned that there are different
ways to find planets around other stars.
Each method has some advantages and disadvantages but taken together, they
allow us to search for a large number of planets orbitting around other stars.
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