of manually looking at photographic

plates, contains over two million stars

before he discovered Pluto.

and its in our own solar system.

So how on earth, does one find a planet orbiting a distant star?

You could just look,

but stars are so far away,

and so bright compared to their planets,

that its really hard to just look at one, and see a planet orbiting it.

Direct discovery of new exoplanets by

just looking, is essentially limited to

relatively nearby stars,

with very large planets, very far away from them.

Think 10 times the size of jupiter, and an orbit at least as big.

The vast majority of exoplanets,

have been found indirectly, by observing their effects on their parents stars.

For example, a planet passing in front of a star will make

that star darker for a little while.

and the amount it darkens, will tell you about the size of the planet, relative to the size of the star.

of course this only works if the planets orbit is tilted perfectly to pass between us and its star.

and on average less than 1% of earth like planets will have this convenient orbital orientation.

We have managed to find a lot of exoplanets this way,

by exhaustive satellite searches.

but you could instead have a look at the effect planets have on the motion of stars.

as we know, planets don't orbit stars.

Rather, both orbit around their combined centre of mass.

Stars are so heavy, so often the centre of mass is inside the star,

but the star will nonetheless be moving.

That motion manifests itself as a teeny tiny wobble,

in the velocity of the star relative to us.

which we determine by carefully measuring the red or blue doppler shift

of the stars light

both these indirect methods are most effective at finding big planets,

close to their stars,

because the speeds involved, and the amount of light blocked

are greater, and also because close planets orbit more often

so we don't have to wait as long to notice their effects.

There are other more complicated and fancy methods which can find planets

which are harder to notice by a star wobbling or star light blocking

and all of these together have helped to discover

more than 1800 exoplanets

as of 2014.

Sorry pluto, you've been Ex-ed out!

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