# Why would tidal forces on planets become more intense when a star becomes a white dwarf?

I was puzzled by this press release by the Royal Astronomical Society: "Small, hardy planets most likely to survive death of their stars" - phys.org, May 14, 2019

Astrophysicists from the Warwick Astronomy and Astrophysics Group modeled the chances of different planets being destroyed by tidal forces when their host stars become white dwarfs and have determined the most significant factors that decide whether they avoid destruction.

According to the shell theorem, the gravitational forces of a spherically symmetrical body's regions, in terms of how they act on external objects, sum up to the functional equivalent of a point mass. A star therefore acts like a point gravity source both before and after becoming a white dwarf (albeit a lower-mass one after, due to casting off matter that becomes a planetary nebula — a drop in attraction that I'd expect to cause any remaining planets, those not close enough to have been slowed and swallowed by the red giant phase, to migrate outward).

So why would a star, after turning into a white dwarf, exert stronger tidal forces on its planets?

• This is a side point, and doesn't answer your very good question about the wording of the Phys.org article. Gravity from a "point star" still produces a tidal force on a finite diameter planet, because one side of the planet is closer to the star than the other. – uhoh May 18 '19 at 19:33
• Just a guess, hence a comment, they may be modelling the whole process -- the effect of mass loss on orbits, and friction with the outer layers of the star (or the solar wind) in the red giant phase. The article has maybe summarised it badly. – Steve Linton May 18 '19 at 22:08
• @SteveLinton Yeah, friction/accumulation of low-angular-momentum gas seems like the likeliest explanation. It'd be nice if they explained that clearly. – Jacob C. May 29 '19 at 19:35

From the paper, p.2:

The star eventually transitions into a white dwarf, which is comparable in size to the Earth but has a Roche radius which extends outward to about one Solar radius ...How objects can be emplaced at one Solar radius from several au – and then circularised – is still subject to debate.

They are considering planets which have been observed in close orbits (orbital period as short as 2 hours) around white dwarfs.

• So it's literally just because planets migrate closer to the stars? – Florin Andrei May 19 '19 at 6:17
• Like Florin, I wish this answer would expand on this. It doesn't really make sense to discuss "planets being destroyed by tidal forces when their host stars become white dwarfs" without explaining how the new circumstances of the planets (e.g. being drawn into closer orbits) would happen as a result of the transition to white dwarf. – Jacob C. May 29 '19 at 19:16
• @JacobC. This seems to be an area of active research. This 2017 review article states (p.23, 4.3.1) "Although approximate, equation (4.18) suggests that giant planets and brown dwarfs should survive catastrophic disruption while inside stellar atmospheres". It should be a separate Question. – Keith McClary May 30 '19 at 4:04

They wouldn't become more intense when a star becomes a white dwarf provided the distance from the centre of the star to the planet doesn't decrease. On the contrary,tidal forces would diminish because in order to become a white dwarf the star would have to expel a certain amount of mass.

• but when the star expels a certain amount of mass the orbits get closer and the tidal force will increase. – uhoh May 19 '19 at 22:07
• @uhoh Why would the orbits get closer? Accumulation of low-angular-momentum gas being cast off by the star? Would that really contribute enough material to make up for the fact that, as seen If you do a simulation of a large and small body, and decrease the major body's mass, the orbit of the minor body gets farther, as its momentum caries it along farther without the path being bent as dramatically by gravity. academo.org/demos/orbit-simulator – Jacob C. May 29 '19 at 19:32
• @JacobC. Of course you are right. That's interesting, I can't figure out what made me think that. Okay +1 for this answer! – uhoh May 29 '19 at 21:43