Neptune may have swallowed a planet | Hacker News Reader

Neptune may have swallowed a planet

newscientist.com

17 pointsKliment6 comments

Uranus and Neptune, the two outermost planets?

Still not used to the fact(?) that Pluto is not a planet anymore. :-/

Yeah, it's confusing. I grew up being told it was a planet too. And I'd state it as "still not used to the current definition of planet".

Welcome to hacker news :).

A part of me still wants to call it a planet, especially since I grew up where Pluto was discovered, in Flagstaff at Lowell observatory.

My parents teach astronomy to kids - and the #1 favourite question is about Pluto and why it's not a planet.

And usually one kid will try and explain at length why it still is :)

Though it's likely impossible, I can't help but imagine a Neptune where the core is quite simply this theoretical planet it swallowed- i.e., since it is a gas giant, the super-earth just passed into it's center. Now super-earth has a really thick atmosphere, and we call the result Neptune.

This is part wishful thinking, part imagination, and part me having a really hard time wrapping my mind around the idea of a ball of gas with no solid core.

A few other notes; what happens to solid matter introduced to a gas giant? Does it just go to the core after all? And, how the heck does gas colliding with a planet create excess heat? This makes sense if the planet is thermally active like the earth, and adds to the total heat, but while gas exhibits friction they'd have to collide pretty fast to generate much heat, right?

It does go to the core, because it's heavier and it sinks.

But that's not the whole story - the mass of the solid planet is enough that it becomes the new planet. You can think of the solid planet as sinking into the core. OR, you can think of the solid planet as stealing the gas from the old planet and wrapping itself in this gas.

And both are correct, and both happen at once.

The collision speeds they are talking about here are indeed immense. Truly stupendous amounts of energy. However it only works if one of the two planets was in a very elliptical orbit, otherwise they are both in the same orbit when they collide, and little energy is released - they will just gradually merge. And the heat is not really from friction, it's more collision heat. (Like when shoemaker-levy crashed into jupiter.) But elliptical orbits are not "natural" they are created by action of some other body, or by capturing a non-orbital planet from far away.

Another thing they just passed right over is how the pre-neptune moved to a new orbit. Are they suggesting some sort of tidal exchange with the sun? Friction with the leftover gas doesn't work since the gas would move at the same speed as the planet (they are both in the same orbit).

It's much better to think in terms of momentum rather than energy. To move pre-neptune to a new orbit you have to transfer the momentum somewhere. And I can't think of anyplace except slowing down the rotation of the sun (much as the moon slows the earth, and moves farther away in the process). Maybe transfer to a different planet? I'm not sure that would work.

Edit: they link to an article with more detail, where they suggest jupiter did it.

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