David Kipping

So they're the ones that lurk more in the shadows in terms of how difficult it is to detect.

They're much harder.

They're harder for several reasons.

The method we primarily use is the transit method.

So this is really eclipses.

As the planet passes in front of the star, it blocks out some starlight.

The problem with that is that not all planets pass in front of their star.

They have to be aligned correctly from your line of sight.

And so the further away the planet is from the star, the cooler it is, the less likely it is that you're going to get that geometric alignment.

So whereas a hot Jupiter, about 1% of hot Jupiters will transit in front of their star,

only about 0.5%, maybe even a quarter of a percent of Earth-like planets will have the right geometry to transit.

And so that makes it much, much harder for us.

There's not a direct connection, but they're connected by an intermediate parameter, which is their separation from the star.

So the planet will be cooler if it's further away from the star, which in turn means that the probability of getting that alignment correct is going to be less.

On top of that, they also transit their star less frequently.

So if you go to the telescope and you want to discover hot Jupiter, you could probably do it in a week or so because the orbital period is of order of one, two, three days.

So you can actually get the full orbit two or three times over.

Whereas if you want to set an Earth-like planet, you have to observe that star for three, four years.

And that's actually one of the problems with Kepler.

Kepler was this very successful mission that NASA launched in