First Planet Known To Orbit a White Dwarf Is Falling Apart (nasa.gov)

schwit1 writes: It's virtually certain that some white dwarfs still have planets in orbit despite their violent histories, but seeing those planets has proven difficult... at least, until now. Astronomers using the Kepler space observatory have spotted a planet circling around WD 1145+017, a white dwarf 570 light years away. Not that it's in great shape, mind you. The unusual light signature (PDF) from the dying star hints that the planet is disintegrating under the star's gravitational pressure, leaving behind a giant dust cloud. Researchers suspect it fell into its fatal orbit after the star's rapid change in mass triggered a planetary collision.

You should see more discoveries like this in the future, since the weaker light of a white dwarf is less likely to obscure planets. There's even a chance (however small) that collisions have bumped some planets into habitable zones, giving scientists an unusually clear view of worlds that could support life. Either way, it's evident that planetary systems don't vanish simply because their host stars are running out of time.

Comments

Star's rapid change in mass?

[sinij]

Can someone explain to non-physicist how this rapid change in mass happens?

Re: Star's rapid change in mass?

[troon]

Still doesn't change mass. Yes, density is a lot greater, but gravity doesn't care about that.

Re: Star's rapid change in mass?

[Anonymous Coward]

"gradually lose about half their mass,"

"Still doesn't change mass"

So losing half the mass doesn't change the mass?

Re: Star's rapid change in mass?

[Anonymous Coward]

The mass of the star has changed, in that perhaps half of its mass has been expelled into a planetary nebula. That expelled mass is beyond the planet's orbit, and the orbit of the planet is subsequently determined only by the remaining stellar mass. Also, that expelled mass is moving away from the remaining white dwarf, and is being diluted in interstellar space. I've forgotten enough college physics to be unable to say what the expected effect on the planet is due to the combined effects of the force of expulsion and the reduced mass of the central star on the planet's orbit should be (does it move in or out, etc).

Re: Star's rapid change in mass?

[cyn1c77]

I've forgotten enough college physics to be unable to say what the expected effect on the planet is due to the combined effects of the force of expulsion and the reduced mass of the central star on the planet's orbit should be (does it move in or out, etc).

Ah, I still remember that extremely painful integration in BC Calculus.

The planet's orbit increases due to the decrease in stellar mass: F = G M1 M2 / r^2 . (The stellar mass expanded around the planet will asymptotically cancel itself out as the mass expansion radius grows large relative to the planet's orbit.)

It's tricky to mentally model the effect of force expulsion on the planet. But if it interacted significantly with the planet, it would have forced it radially outwards with a relatively short impulse, which would result in a more elliptical orbit. It also could have induced drag on the planet's orbit during the expulsion time and the star's expansion phase, slowing the planet. But that effect would have been minimal compared to the mass reduction due to the low density of the star's outer layers.

Of course, it could also scorch the heck out of the planet's surface.

habitable zone?

[phayes]

Nope.

White dwarfs are stars that have gone through an expansion to red giants & then shrink back down once they run out of low atomic level fuel like hydrogen & helium.

All planets close enough to be in a white dwarf's "habitable zone" would have been well inside the star during the star's red giant phase.

Unless someone comes up with a mechanism for the planets to escape from the red giant & then migrate even further inward to the white dwarf's now much smaller & closer "habitable zone", its extremely implausible.

Somebody please reassure me that this is once again a "journalist" attempting to talk of matters that far outstrip his comprehension & not an astrophysicist gone barking mad.

Re:habitable zone?

[phayes]

Habitable zone == zone where surface temperatures would be such that liquid water could be found at the surface of the planet. Any significant dragging would also have the effect of heating all volatiles & stripping the planet's atmosphere. A waterless cinder with no atmosphere even at temperatures between 0 & 100C is not conducive to life as we now define it.

Re:habitable zone?

[Rei]

On the other hand, when you're subjecting planets to increased tidal forces, you're also unlocking a new source of energy: tidal flexure heating. You're bending a massive chunk of rock into a new shape, there's a tremendous amount of heat released in the process (you're probably also tidal locking it if it wasn't already).

Counterproductive if the body ends too close to the star, but useful if the body ends up too far from the star. Unless it's to the extremes covered in this article where the tidal forces are sufficient to rip the planet into a ring.

Re:What's "gravitational pressure"?

[Blaskowicz]

See the Roche limit, this term describes the boundary where you are at risk of disintegrating. A small moon around a gas giant would end up as big Saturnian rings.
Yes ocean tides would be a very tiny version, or Jupiter melting Io is more dramatic but not quite disintegrating.

If the moon came much closer I'm sure we'd have no danger of the Earth disintegrating but perhaps we would all be dead from earthquakes and tsunamis (or worse)

Re:It's a good thing we have seen this happen

[ceoyoyo]

Eclipses and transits are fairly common in the solar system. If you recall, a few years ago there was a somewhat rare transit of Venus across the sun. There was quite a bit of excitement about observing it because the information could be used to tune some of the models of exoplanet discovery.