Can a planet survive being engulfed by a red giant star?

[Halc]

News article I saw today where they found an anomalous planet orbiting too close to a helium burning star.

https://www.nature.com/articles/s41586-023-06029-0

Abstract:

"When main-sequence stars expand into red giants, they are expected to engulf close-in planets. Until now, the absence of planets with short orbital periods around post-expansion, core-helium-burning red giants has been interpreted as evidence that short-period planets around Sun-like stars do not survive the giant expansion phase of their host stars. Here we present the discovery that the giant planet 8 Ursae Minoris B orbits a core-helium-burning red giant. At a distance of only 0.5 AU from its host star, the planet would have been engulfed by its host star, which is predicted by standard single-star evolution to have previously expanded to a radius of 0.7 AU. Given the brief lifetime of helium-burning giants, the nearly circular orbit of the planet is challenging to reconcile with scenarios in which the planet survives by having a distant orbit initially. Instead, the planet may have avoided engulfment through a stellar merger that either altered the evolution of the host star or produced 8 Ursae Minoris b as a second-generation planet. This system shows that core-helium-burning red giants can harbour close planets and provides evidence for the role of non-canonical stellar evolution in the extended survival of late-stage exoplanetary systems."
 

So the question is, how did it get/stay into that orbit?  If it was always there, it should be further out. If its orbit was disturbed post-nova by the passing of a massive object, one would expect a very eccentric orbit.  Perhaps it was right at the edge of getting captured and not. It got close enough (say it got within 0.7 AU initially) to lose some energy to friction, but not enough to fall in. As the star radius decreases, the planet orbit by chance also decreases, just barely enough to not be swallowed. When the star shrinks further, the planet eventually gets left behind at 0.5 AU.

That seems like a very implausible scenario since it requires riding the edge of a very unstable equilibrium (between swallow and left-behind) for an awful long time, so I'm probably full of BS on that call, but any other ideas?

[OceanBreeze]

On 6/29/2023 at 11:33 PM, Halc said:

News article I saw today where they found an anomalous planet orbiting too close to a helium burning star.

https://www.nature.com/articles/s41586-023-06029-0

Abstract:

"When main-sequence stars expand into red giants, they are expected to engulf close-in planets. Until now, the absence of planets with short orbital periods around post-expansion, core-helium-burning red giants has been interpreted as evidence that short-period planets around Sun-like stars do not survive the giant expansion phase of their host stars. Here we present the discovery that the giant planet 8 Ursae Minoris B orbits a core-helium-burning red giant. At a distance of only 0.5 AU from its host star, the planet would have been engulfed by its host star, which is predicted by standard single-star evolution to have previously expanded to a radius of 0.7 AU. Given the brief lifetime of helium-burning giants, the nearly circular orbit of the planet is challenging to reconcile with scenarios in which the planet survives by having a distant orbit initially. Instead, the planet may have avoided engulfment through a stellar merger that either altered the evolution of the host star or produced 8 Ursae Minoris b as a second-generation planet. This system shows that core-helium-burning red giants can harbour close planets and provides evidence for the role of non-canonical stellar evolution in the extended survival of late-stage exoplanetary systems."
 

So the question is, how did it get/stay into that orbit?  If it was always there, it should be further out. If its orbit was disturbed post-nova by the passing of a massive object, one would expect a very eccentric orbit.  Perhaps it was right at the edge of getting captured and not. It got close enough (say it got within 0.7 AU initially) to lose some energy to friction, but not enough to fall in. As the star radius decreases, the planet orbit by chance also decreases, just barely enough to not be swallowed. When the star shrinks further, the planet eventually gets left behind at 0.5 AU.

That seems like a very implausible scenario since it requires riding the edge of a very unstable equilibrium (between swallow and left-behind) for an awful long time, so I'm probably full of BS on that call, but any other ideas?

I'm not sure I understand the scenario, since only the abstract of the paper is freely available. Maybe you can find a full free version?

What I find confusing is the statement that this planet is orbits the host star at a distance of only 0.5 AU. I am assuming that is the present state of affairs.

How far was it from the host star before the host expanded into a red giant? If it was sufficiently far away why would it not survive the expansion and be left on a closer orbit?

For example, Jupiter is 5.2 AU distant from the sun, while Mars is only 1.5 AU away. If the sun were to expand in radius by 4.7 AU, all the inner planets would be engulfed and Jupiter would be left to orbit at a distance of 0.5 AU, similar to what you are describing.

If the planet was orbiting at 0.5 AU before the star expanded into a red giant, surely that planet would have been absorbed!

The key question is how far away from the star was that planet orbiting before the star went red giant, or is there any way to know that?

 

 

[Halc]

8 hours ago, OceanBreeze said:

I'm not sure I understand the scenario, since only the abstract of the paper is freely available. Maybe you can find a full free version?

That's all I saw as well. I imagine the paper goes into some of the unobservables such as how they arrived at 0.7 AU. One of the first suspicions is that the star was never that big, so they have to demonstrate it.

8 hours ago, OceanBreeze said:

How far was it from the host star before the host expanded into a red giant? If it was sufficiently far away why would it not survive the expansion and be left on a closer orbit?

Unknown. If it was far away, it would get further away since the host star loses mass during the whole process. That's one of the 4 things affecting Earth's orbit, and it's getting further away due to the sun losing mass as it burns it. Of the 4 things, I think that one currently has more effect than the others, but in the end, it is the weakest one of them that never goes away and wins in the long haul.

My proposal (highly unlikely) in my OP was that it was right at 0.7 (at least at perihelion), enough to lose orbital energy, but not lose it fast enough to fall into the shrinking star. Losing energy at perihelion doesn't diminish perihelion until the orbit becomes circularized. It only brings aphelion down.

8 hours ago, OceanBreeze said:

For example, Jupiter is 5.2 AU distant from the sun, while Mars is only 1.5 AU away. If the sun were to expand in radius by 4.7 AU, all the inner planets would be engulfed and Jupiter would be left to orbit at a distance of 0.5 AU, similar to what you are describing.

I think the 0.5 AU is the orbital radius, not the altitude from the stellar surface. Ditto with the 0.7 figure. The article would be totally unremarkable if it was the latter.

What if Jupiter was close enough to be absorbed? Would the sudden influx of new hydrogen from it into the star somehow arrest the giant phase by giving it something other than helium to burn? Maybe that's why the star didn't have time to finish absorbing orbital energy from it. That actually seems more likely (in a naive sort of way) than my original suggestion.

8 hours ago, OceanBreeze said:

The key question is how far away from the star was that planet orbiting before the star went red giant, or is there any way to know that?

Maybe it was captured after the 0.7 phase. The abstract dismisses that as unlikely since the odds of a circular orbit from a recent capture is remote. I agree.

 

On 6/29/2023 at 5:06 PM, SuppedAutodidact said:

Its going be scorched on the surface.

It being scorched implies it has a solid surface that can be meaningfully scorched. A big planet often doesn't have a distinct surface.

Earth has been scorched already and is no worse for the wear from it. The melted crust reforms in short order once the heat is removed.