Most astronomers were pretty convinced that it was really a planet but

still seeing something like the dimming of the light as the planet went in front

would be a pretty amazing thing.

There's one final way in which astronomers were greatly helped by the fact that these

Jupiters were so close to their stars.

If I draw a star again and I draw Jupiter out here.

The distance, in terms of now diameters, those Jupiters and

3 each day orbits were something like 4 diameters away.

What that means is the probability of having a transit is much much higher

than if Jupiter were way, way, much further away.

Think about it this way, okay, we don't know what

this inclination is of the planetary orbit with respect to the star.

Could be this, no transit.

Could be this, no transit.

Could be this, no transit.

Could be this, no transit.

It could be this, there is a transit.

The transit will occur if it's anywhere between this and

the planet's right here, and this and

the planet's right here because this will transit just the very top of the star.

This will transit just the very bottom of the star.

This will transit the very middle of the star, and this is now simple geometry.

This distance here is the diameter of the star, d.

And the question is,

what is this distance compared to all the places where the orbit could have been.

So it is, this distance d divided by this entire circumference d over

2 pi times this radius which should had been 8d and

we should multiply by 2 because the transit would equal likely

occur of the orbit like this, as it well with orbits like this.

So we'll take away 2 here, and we get that the probability of a transit, for

one of these sort of 3-ish day orbits, is something like 1 over 8 pi,

with is pis about 3, 8s about 8 so that's about 1 over 25,

which is about something like 4% chance.

4% chance is not huge but it means you have detected 25 of these hot Jupiters.

You're likely going to come across one that happens to be going right in front.

And sure enough, within a few years at exactly the predicted time,

one star HD 209458, even I remember this one.

And this is now a very famous license plate number for a star.

The star HD209458,

exactly at the right time had a little 1% dip in its brightness and

that was the final dead certain proof that these exoplanets,

these hot Jupiters really were hot Jupiters going around the star.