Recently the Kepler telescope in its study of white dwarfs detected the first planetary object transiting a white dwarf in the data from the K2 mission. It was consistent with earlier theories' prediction that a planetary object orbiting a white dwarf would slowly disintegrate. Why would a planetary object that is orbiting a white dwarf disintegrate? I read it here.

The original paper published in Nature (preprint): A Disintegrating Minor Planet Transiting a White Dwarf

  • $\begingroup$ I would think that it is disintegrating because it orbits very close to the WD. 4½ hour period I think means about 1 million km from a one Solar mass WD. Mercury is about 50 times further from the Sun. That close, the gravitational pull is significantly larger on the side of the planet facing the WD than on the far side, ripping the planet apart. So I don't think it directly has anything to do with it being a WD instead of an active star. $\endgroup$
    – LocalFluff
    Oct 23 '15 at 6:02
  • $\begingroup$ Actually Not just this planet but the relative effect on the planets after white dwarf conversion. What i want know is.is it just the gravity(relatively how much?) or some other forces acting on it. $\endgroup$
    – r2_d2
    Oct 23 '15 at 6:14
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    $\begingroup$ Here is the full text original peer reviewed article published in Nature cfa.harvard.edu/~avanderb/wd1145_017.pdf It will take me a while to read it. Check it out. $\endgroup$
    – Eubie Drew
    Oct 23 '15 at 23:54
  • $\begingroup$ @LocalFluff see comment above. That link is to a preprint. $\endgroup$
    – Eubie Drew
    Oct 24 '15 at 0:20
  • $\begingroup$ @Aabaakawad thank you for the link. I've only read part of it, but very informative. $\endgroup$
    – userLTK
    Oct 24 '15 at 1:12

I think Aabaakawad's link gives a complete answer, but to give an astronomy for dummies answer, there's nothing about a white dwarf that causes a planet's orbit to decay at least, not directly. Your article (I've pulled quote below the caption):

Slowly the object will disintegrate, leaving a dusting of metals on the surface of the star.

That's only talking about this particular situation and there's a difference between disintegrate and decay. This planetoid is enormously close to the white dwarf. So close, that what we think of as normal white dwarf/planet dynamics (very cold) is no longer true. This planetoid is slowly being vaporized.

Looking at the orbital period of 4.5 hours (about 1948 orbital periods in 365.25 days). The orbital distance to orbital ratio relation is exponential to the power of 2/3 (this varies a bit due to eccentricity, but it's generally correct), so an orbital period 1948 times faster means about 156 times closer, and giving the white dwarf equal mass to our sun, that puts the planetoid at a bit under 1 million KM. If this white dwarf is lighter than the sun, the planetoid would need to be even closer. That's close to the Roche limit and would be inside it if the planetoid wasn't dense and rocky/metallic.

If we estimate the white dwarf to be about the size of the Earth, which is a common size given for white dwarfs, An Earth sized object from 1 million KM would be larger in the sky than the sun appears from Earth, and presumably quite a bit hotter than the surface of our sun too, so this isn't a tiny white dwarf in the sky from the perspective of the planetoid. It's a blazing furnace of a sun, so hot, it vaporizes metallic gas and dust off the surface of the planet.

The article mentions this (end of page 3), that Poynting-Robertson drag see here and here, and that may be a factor in any orbital decay in this scenario. The article is clear that there's a good deal of uncertainty on with that effect, and that only affects tiny particles, but enough tiny particles could create a drag over time. . . . (maybe). The general scenario with this orbit is a planet scorched and as a result, is losing material. It's likely the very high heat that's driving any orbital decay, not gravity.

Gravitational decay / orbital decay does happen, usually much more slowly. That's probably not what's happening here.

There are some interesting orbital effects that can happen when a main sequence star goes red dwarf and later when it creates a planetary nebula, significant increases in tidal forces due to the star's greater size in the first case and increased drag in the 2nd, but at the white dwarf stage, there's no significant orbital decay effects.


Why not Poynting Robertson drag and Orbital decay effect the planetoid when white dwarf was a star or even red giant? Is there any "interesting orbital effects" when a star undergoes red giant?.Can you update your answer to summarize the forces and their effect on the planetoid in each phase of the star. and also what do you mean by orbital decay? Does it have something to deal with the Roche limit.

OK, I think, having read more about it, Poynting-Robertson effect only matters when the orbiting objects are very small. I've linked it twice above, but the simple explanation is that objects in orbit move and so any light or debris from the sun hits the moving object at an angle, not direct on. If the object is small enough, this over time drives the dust and maybe grain of sand sized particles into the sun. This doesn't affect larger objects, so it's not really relevant to any planets or planetoids.

As far as "interesting red dwarf" effects. That really has to do with tides. Using the Moon/Earth example, the Moon creates tides on the Earth, a tidal bulge towards the Moon, but because the Earth Rotates faster than the Moon orbits, this tidal bulge is always ahead of the Moon and this creates a gravitational tug on the moon that pulls it away from the Earth - very slowly.

The same thing happens with planets around stars, but even more slowly, lets pretend it's just the Earth and the Sun - a 2 body system (in reality, with several planets it's much more complicated), but just Earth and sun, teh Earth creates a tidal bulge on the sun, the sun rotates ahead of the earth, this causes the earth to very slowly spiral away from the sun - so slowly that it might take a trillion years for the Earth to spiral away.

Now when the sun goes Red Giant, the sun is essentially the same mass but much more spread out and parts of it, much closer to the Earth and less gravitationally bound to the sun. This creates a far larger tidal tug. Also, as the sun expands it's orbital velocity drops, because orbital momentum is concerved, so when the Sun is Red Giant, the tidal bulge will be behind the Earth which drags it in towards the Sun. Due to the size and proximity of the Red Giant star, this draws the remaining near-by planets towards the sun fairly quickly, at least compared to main sequence stages which, provided the sun rotates faster than the planets orbit, has a much smaller outward tidal pressure on the planets.

And when the sun goes planetary nebula, any debris in the planet's path can also cause the planets to slow down slightly - the precise process there I'm less clear on, but in general, any orbital debris creates drag and can slow down a planet's orbit. This may be a key factor in the formation of hot jupiters, cause they can't form close to their suns but enough orbital debris can drive them in closer to their suns. (or planet to planet gravitational interactions can too).

That's the gist of the Sun-Planet orbital relation. When the sun is young, planets are mostly driven outwards, and young suns can have far greater solar flares and stronger solar wind. How much that effects the planets, I'm not sure.

During the Main sequence stage, stars tend to push planets outwards (unless they rotate very slowly, in which case the tidal effect is reversed), but this tidal effect is very small and very gradual.

During the Red Giant stage, stars tend to drag planets in wards, and I assume, during the planetary nebula stage as well. This effect is larger for closer planets.

You also asked about Orbital Decay - if you click on the link, there are examples of that. That probably gives a better explanation than I could. In general, Orbital decay happens very slowly unless you're talking Neutron Star or Black hole in which case the relativistic effects can cause orbital decay to happen quite fast. There's nothing about a white dwarf star that would cause faster than normal orbital decay but a white dwarf star would lose any tidal bulge tugging that a main sequence star has, so there would be essentially no tidal outwards pressure either which could in theory speed up decay cause you've lost a small outwards pressure but you would still have any debris or space dust clouds causing a small inwards pressure. (if that makes sense?)

That's my layman's explanation anyway.

  • $\begingroup$ why not Poynting Robertson drag and Orbital decay effect the planetoid when white dwarf was a star or even red giant? Is there any "interesting orbital effects" when a star undergoes red giant?.Can you update your answer to summarize the forces and their effect on the planetoid in each phase of the star. $\endgroup$
    – r2_d2
    Oct 25 '15 at 7:58
  • $\begingroup$ and also what do you mean by orbital decay? Does it have something to deal with the Roche limit. $\endgroup$
    – r2_d2
    Oct 25 '15 at 7:59
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    $\begingroup$ @r2_d2 The Roche Limit is in a sense, just a boundary usually quite near the object being orbited. Orbital decay (or the opposite, orbital escape) can happen at any orbit, far outside the Roche limit or close to it. As to the effects at different stages of the star, I can give a summary, but not tonight. I also probably need to update what I said about Poynting-Robertson. Need to tidy that up a bit. $\endgroup$
    – userLTK
    Oct 25 '15 at 8:08

Let's assume the white dwarf has a mass of $0.6 M_{\odot}$ (there's probably a more accurate value, but most white dwarfs are close to this...). With a period of 4.5 hours we can use Kepler's third law, assuming the planetary mass is negligible compared to the white dwarf, to infer an orbital radius of 0.0054 au ($8.1\times 10^{8}$ m).

The tidal forces this close to a white dwarf are very large. The Roche limit for the total tidal disintegration of a satellite, in synchronous rotation, held together only by its own gravity is roughly $$ d = 1.44 R_{WD} \left( \frac{\rho_{WD}}{\rho_p}\right)^{1/3},$$ where $R_{WD}$ is the radius of the white dwarf (similar to the radius of the Earth), $\rho_{WD}$ is the average density of the white dwarf (a few times $10^{9}$ kg/m$^3$) and $\rho_p$ the density of the planet (let's assume 5000 kg/m$^3$).

Thus $d \simeq 6 \times 10^{8}$ m and is very similar to the actual orbital radius of the planet. i.e. It will be tidally disintegrating.

I guess it will be an observational selection effect that such objects will be detected at the tidal breakup radius, since if they were further way they would not be disintegrating and would not be detected, and if they were closer they would have already disintegrated and wouldn't be seen!

EDIT: On reading the paper - the authors claim that these objects are not tidally disintegrating. In fact they argue that this must be debris from a rocky planet precisely because the density must be large enough to avoid tidal disintegration according to the formula above. However I find the whole discussion rather incoherent. They specifically talk about "disintegrating planetesimals" (note the tense) which are being evaporated in a Parker-type wind due to heating by the radiation from the white dwarf. I cannot see where they explain then how the planetesimals disintegrate.

  • $\begingroup$ why didnot this orbiting body disintegrate when white dwarf was a normal star? if we consider distance(Roche limit which applies to al celestial bodies) $\endgroup$
    – r2_d2
    Oct 24 '15 at 20:15
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    $\begingroup$ @r2_d2 I think you are correct that the Roche limit for the progenitor (e.g. a 2 solar mass main sequence star with a radius of $\sim 10^{9}$ m would be quite similar). Therefore we must conclude that the planet was not that close during the progenitor's main sequence lifetime. That is not surprising, since prior to becoming a white dwarf, the progenitor would have engulfed such a close-in planet during its red giant stages. In fact at $10^{9}$ m, such a planet would have been inside the main sequence star. $\endgroup$
    – ProfRob
    Oct 24 '15 at 20:35
  • $\begingroup$ So what could be the possible explanation for this. $\endgroup$
    – r2_d2
    Oct 24 '15 at 20:46
  • $\begingroup$ @r2_d2 Drag in the wind of the progenitor during the AGB phase perhaps. $\endgroup$
    – ProfRob
    Oct 24 '15 at 20:56
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    $\begingroup$ @r2_d2 The Nature paper suggests that mass loss from the AGB star causes planet migration outwards that can then lead to an instability and interaction between planets that projects a planet inwards towards the host star. The orbit is then circularised due to wind drag. $\endgroup$
    – ProfRob
    Oct 24 '15 at 21:12

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