Astronomers spy an iron planet stripped of its crust around a burned-out star

It reminded me of the meeting location at the start of The Hydrogen Sonata by Iain M. Banks.

[I seem to remember it was a sliver of a planet held around an exploding star used as a meeting location - largely because doing that kind of thing is cool if you are godlike AIs].

Available here: https://sites.uni.edu/morgans/astro/course/TheStar.pdf

"stray" implies they get closer, eventually coming close enough for tidal effects to get nasty. Everything I understand agrees with a white dwarf not ripping apart planets whose orbits are not distirbed from their previous ones.

IANA astronomer/astrophysicist.

To expand on @jvanderbot excellent reply, there's "Roche Limit" [1] - region of space close to massive bodies where gravitation gradient will break up the orbiting body.

Given that white dwarfs remain in the cool-down phase for trillions of years, any orbiting body's orbit will slowly decay and eventually it ends up inside the Roche limit.

[1] https://en.wikipedia.org/wiki/Roche_limit

It's a question of density and tidal forces. The red giant has that mass spread out across a much larger space, so the object that is "straying" has a much more uniform gradient of gravitational attraction across its mass than the white dwarf. An object can only be close to a very small part of the red giant at a time, while an object getting close to a white dwarf is subjected to much more of the gravity well all at once. That makes the gravitational gradient much more pronounced across the object, which means tidal forces are much stronger and more likely to rip the object apart.

A good way to think about it is by looking at where the Roche limit exists for a mass of that size. Since the red giant is much larger but has roughly the same mass as the white dwarf, the Roche limit probably exists somewhere behind the surface, while the same orbit would extend far past the surface of the white dwarf.

Let's say we have an orbital body around a giant star whose mass dictates that its Roche limit is 0.1AU, and then lets say that the star's diameter is 0.3AU. The object could never get close enough to the star to break apart. Now, that same star collapses while losing a negligible amount of mass, and now is a white dwarf with a diameter of 0.01AU. The object could now approach close enough that it gets pulled apart by tidal forces since its unimpeded by the stars surface all the way down to 0.1AU.

https://en.wikipedia.org/wiki/Roche_limit

EDIT: After reading, I realized that I unnecessarily introduced a non-astronomical term in gravitational gradient when I could have just said tidal force. It's the same thing. My apologies to any sticklers out there :)

I am also curious about the cool-off period of white dwarf stars being measured in 'trillions of years' - given that the universe is only 13.8 billion years old, I assume we are just modeling that?

I've been reading about space to my six-year old and there are a lot of neat real photographs of the solar system. Then we followed it up with a book on dinosaurs where he proclaimed "That's a drawing!"

I emailed a link to this comment to the mods using the Contact link in the footer so that they see your comment and edit the post to be better.

OK, we've changed to that from https://www.nytimes.com/2019/04/04/science/white-dwarf-fragm....

The title is wonderful. Poetic. It's hard not to imagine a mind drifting into reverie on reading it.

What? Why even say this?