David Kipping

So we asked for Hubble Space Telescope time, which was awarded in that case.

We were one of those lucky 20 that got telescope time, and we stared at it for about 40 hours continuously.

To provide some context, the dip that we saw in the Kepler data corresponded to a Neptune-sized Moon around a Jupiter-sized planet, which was another reason why I was skeptical.

I was like, we don't have that in the Solar System.

That seems so strange.

And then when we got the Hubble data,

It seemed to confirm exactly that.

There were two really striking pieces of evidence in the data that suggested this Moon was there.

Another was a fairly clear second dip in light, pretty clearly resolved by Hubble.

It was about a 5-sigma detection.

And on top of that, we could see the planet didn't transit when it should have done.

It actually transited earlier than we expected it to, by about 20 minutes or so.

And so that's a hallmark of a gravitational interaction between the planet and the Moon.

We actually expected that.

You can also expect that if the Moon transits after the planet,

then the planet should come in earlier than expected because the barycenter, the center of mass, lives between the two of them, kind of like on a balancing arm between them.

And so we saw that as well.

So the phase signature matched up.

The mass of the Moon was measured to be Neptune mass.

and the size of the Moon was measured to be Neptune radius.