When you look up into the sky on a nice and clear night, looking at around 2,000 stars
in our galactic neighborhood, you're not really seeing the full picture. While you do see
thousands of stars on a good clear night, there are actually planets orbiting those
stars, which are invisible to us.
On this graph contains how we've found exoplanets since 1988. The red parts of the graph mean
we found a planet by direct imaging. With direct imaging, actually looking at the planets
through a telescope taking such a small percentage of this graph, how are we actually finding
these exoplanets?
First off, why does direct imaging not work very well? The reason is because unlike stars,
planets aren't actually creating any light. If you were a fair distance away from both
the sun and earth, the earth would be millions of times less bright than the sun, which means
actually seeing Earth would be extremely hard. Stars from a distance case a glare which usually
means you can't see other planets orbiting around it very easily.
So if we can't actually observe most of these planets, how are we finding planets? The main
ways to find exoplanets is to look at the effects which these planets are causing on
other things.
The most popular way we have found planets over time is called transit photometry, which
is the easiest method to wrap your head around. Using the transit method we can get the size
of these exoplanets and also their orbit period. The Transit method involves pointing a space
telescope at a region of space and watching the stars. When a planet crosses over one
of the hundreds of thousands of stars we're looking at, it will cause a slight drop in
the amount of brightness that we are receiving from that star.
Planets such as Jupiter, Saturn and other quite large planets cause a 1% brightness
drop, compared to a planet the size of earth causing a mere brightness drop of 0.01%. You
can confirm whether or not it was a planet which you observed, by seeing if there's a
similarity on how often that star is experiencing causing a brightness drop. If this is happening
at regular intervals and the brightness decrease lasts the same amount of time you can begin
to confirm that there is quite likely a planet orbiting that star.
With the Kepler telescope measuring the brightness of about 160,000 stars every couple of minutes
we are able to detect these small dips in the amount of light we are receiving from
these stars.
Just because you've found a new exoplanet using this method doesn't mean you should
be out partying just yet (picture of stick figures holding balloons). In single planetary
systems there might be a 40% chance that you've received a false positive, which means we
have to use other methods to confirm if this planet is really there or not.
Radial Velocity is another method we can use, and also is a way we can confirm whether or
not a planet we found using the Transit method is actually there. The Radial Velocity method
is looking at the very slight change of velocity a star exhibits when both the exoplanets and
the host star are orbiting around their common center of mass. This is because all the planets
and sun are orbiting not the sun, but the center of mass of the whole solar system.
For example you can see in this that the two objects are orbiting around the center of
mass which is the little cross in the middle of your screen.
Radial Velocity, or the more technical term of Doppler spectroscopy takes a lot of time.
Jupiter for example causes the velocity of our sun to change by 12.4m/s over the period
of four years and tiny planet earth causes the sun's velocity to change by 0.1 m/s over
a whole year. This means we have to take very long observations of these stars to pick up
this change in velocity, which potentially mean you're searching for years. Pluto wouldn't
be observable if someone was to be looking at our solar system wondering if there were
any planets in it, with it being so small and so far away from the sun, it only causes
a 0.00003 m/s change in velocity of the sun.
But how are we actually seeing these changes in speed if they're actually quite tiny? We
can detect the shift in the wavelength of the light which the stars emitting, with the
light being blue shifting when it's moving towards us and red shifted as it moves away
from us.
The magic all happens when we bring relativity into finding planets, with gravitational microlensing
being the third way we can detect exoplanets. Gravitational microlensing really relies on
one star passing in front of another. So we have a star far off in the distance, and then
a closer star coming in from the side. When the star is in line with the distant star
and us on Earth observing this, the light from the distant star will be bent and also
magnified.
On a brightness against time graph we would be seeing something like this as the closer
star passes in front of the distant star, causing an increase in brightness, because
the closer stars gravitational field creates a gravitational lens which magnifies the light
of the further star.
But what if there's a planet orbiting the star? If there is, the planet will cause a
small blip on our graph, due to the planet also acting as a gravitational lens for a
really short period of time, which might only last a few hours.
The problem though with gravitational lens is that you only really have one shot. That
sort of alignment is unlikely to happen again, compared to the radial velocity and transit
method which is happening all the time, as you're not relying on a distant star or the
off chance of stars aligning and you being able to catch it in time.
With only about one thousand of these events have happened over the past ten years, we
have a lot more success with the transit method, which is our main method of finding exoplanets.
With these methods working together we have managed to confirm the existence of 974 planets
and there being 3846 candidate planets just found by the Kepler space telescope, we are
beginning to learn about our galactic neighborhood.
We've only discovered 0.0000002% of the four hundred and eighty billion exoplanets thought
to exist in our galaxy alone. It's amazing to think that there could quite possibly be
about like 64 septillion planets out in the whole universe, just waiting to be discovered.