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Comment by twic | Hacker News Reader

> KELT-9b is 2.8 times more massive than Jupiter, but only half as dense.

> Because the planet is tidally locked to its star

What does it mean for a gas giant to be tidally locked?

It means that if you average together the motion of all of the mass of the planet, it revolves once per orbit around the star. Which is the same as what it means for a rocky planet, if you think about it, it's just a little less obvious.

Another way to think about it is that the gas giant moves such that tidal friction caused by the gravity of the star is minimized... because that's why it's moving like that.

Makes me wonder what shape such a planet would take.

Very slightly elongated along the axis between itself and the star. Still nearly spherical and the deviation would be essentially ellipsoidal (not egg-shaped).

It's probably a bit more exciting than that when you include evaporation and the stellar wind! But yes, an ellipsoid is the basic shape, the tides elongate the planet both towards and away from the star, same as the tides on the Earth caused by the moon.

Yep. The original post even speculates the thing might have a comet-tail because of the evaporation.

I kind of imagine it looking sort of like a Hersey's Kiss shape :-)

"Tidal locking (also called gravitational locking or captured rotation) occurs when, over the course of an orbit, there is no net transfer of angular momentum between an astronomical body and its gravitational partner."[0]

[0] - https://en.wikipedia.org/wiki/Tidal_locking

One side of the planet is always facing its star. And I presume the gas swirling is on a much shorter/slower scale than the planet's rotation, so the notion of a side is meaningful.

https://www.google.com/search?q=tidally+locked&ie=utf-8&oe=u...

You have had several responses which are all basically correct. If you are still having trouble with the concept, think of the moon and how it always shows the same face to the Earth: you never see the "back." This is because the moon is tidally locked to the Earth.

I understand tidal locking as it applies to solid bodies. It was what "always shows the same face" would mean for something made of constantly moving and mixing masses of gas that puzzled me.

But, as you say, i have had several good answers - being tidally locked is about transfer of angular momentum, and gas giants still have angular momentum.

I imagine the gas flows in circles as if the poles were always perpendicular to the line between the star and the planet's center.

In other words, the gas on the dark side stays on the dark side, and vice versa, roughly.

I would imagine that you'd have a huge upwelling on the hot side, flow of gas to the dark(er) side, then cooling and sinking and flow along the inside over to the hot side.

Wouldn't those attributes mean the volume is exceptionally high? I thought Jupiter was near the limit for volume as increases in mass beyond that point just lead to more and more density.

As you heat gas it gets bigger. Jupiter is cool enough that it's got liquid and perhaps solid hydrogen in the middle (Juno will tell us the details soon!) I think this thing is so hot that it won't have a liquid or non-rocky solid core.

> It won't have a non-rocky solid core

Meaning a solid core, if it exists, will be rocky? How, at such high temperatures?

I'm uncertain whether I'm not misinterpreting your double negatives :)

High pressure does odd things even when it's hot, and I don't know off the top of my head how rocks work in that situation! I'm more of a black hole and accretion disk devotee.

I'd assume the minor spaghettification caused by extreme tidal forces us also a factor in its super low density.

I'd assume it loses material quite faster than a spinning one.

Here's a quick model: https://media.giphy.com/media/MBpKFBsQdeVIk/giphy.gif

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