David Kipping

any serious connection of planet binarity to habitability.

Certainly when we investigated it, that wasn't our drive, that this is somehow the solution to life in the universe or anything.

It was really just, like all good science questions, a curiosity-driven question.

What's the dynamic?

Are they legit orbiting each other as they orbit the star?

So the formation mechanism proposed here, because it is very difficult to form two protoplanets close to each other like this.

They would generally merge within the disk, and so that's why you normally get single planets.

But you could have something like Jupiter and Saturn form at separate distances.

They could dynamically be scattered in towards one another and basically not quite collide, but have a very close-on encounter.

Now because tidal forces increase dramatically as the distance decreases between two objects, the tides can actually dissipate the kinetic energy and bring them bound into one another.

So that seems...

When you first hear that, you think, well, that seems fairly contrived that you'd have the conditions just right to get these tides to cause a capture.

But numerical simulations have shown that about 10% of planet-planet encounters are shown to produce something like binary planets, which is a startling prediction.

And so that seems at odds with, naively, the exoplanet catalogue, for which we know of

so far, no binary planets.

And we propose one of the resolutions to this might be that the binary planets are just incredibly difficult to detect, which is also counterintuitive.

Remember how they form is through this tidal mechanism, and so they form extremely close to each other.

So the distance that Io is away from Jupiter, just a few planetary radii, they're almost touching one another, and they're just tidally locked facing each other for eternity.

And so in that configuration,

as it transits across the star, it kind of looks like you can't really resolve those two planets.