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

I new Al Gore was an Alien!

[DrTJ]

http://www.theonion.com/articl...

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?

[micahraleigh]

I was wondering the same thing. Maybe its referring to a nova or something like that so there's not much less.

I would think that in a red giant or even black hole stage it would still have the same mass.

Re:Star's rapid change in mass?

[Anonymous Coward]

From TFA:

As stars like our sun age, they puff up into red giants and then gradually lose about half their mass, shrinking down to 1/100th of their original size to roughly the size of Earth. This dead, dense star remnant is called a white dwarf.

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?

[troon]

It doesn't. The media have confused surface gravity of a much denser object with gravity at orbit distance, which will be unchanged.

Re: Star's rapid change in mass?

[gstoddart]

Well, the actual Nasa article talks about changes in mass, and stars pretty constantly eject material.

And this says:

This also means the massive stars (with masses greater than 1.4 solar masses) must shed most of their mass as planetary nebula or the final contraction to a white dwarf cannot be stopped by the degenerate electrons.

So, I'm more inclined to believe there is loss of actual mass going on.

It certainly sounds like changes in mass are part of the explanation for the mechanics of this. (Not that I claim to actually understand 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?

[Anonymous Coward]

This. A typical main-sequence star (like our Sun), will eventually burn through (fuses) it's hydrogen, it then fuses the resulting helium and other elements and expands in a red-giant (for low and medium mass stars, high mass stars will form supergiants). Eventually the star will shed its outer layers (forming planetary nebula) and the core of the star made up of carbon and oxygen will collapse into a very dense white dwarf. White dwarves cannot sustain fusion, and as a result succumb to immense gravitation pressure, as a result are very dense with high mass (comparable to the Sun, even though star would have lost a mass as it shed it's outer layers) but the volume of Earth. Since volume does not dictate gravity, mass does, the gravity from very dense white dwarf star is no more than the original star. In fact, if the Sun were today to form a very high density but low volume black hole, but with still the same amount of mass, the gravity would not change in relation to the Earth, all other things being equal.

Re: Star's rapid change in mass?

[DigiShaman]

As the saying goes - a candle that burns twice as bright burns for half as long.

If it's a white star, it's only because it's mass is so dense that it's able to burn through its fuel (hydrogen, helium, other elements) at a much MUCH faster rate. As such, the more it converts mass into energy, the less dense it becomes. That whole E=MC2 thing.

Re:Star's rapid change in mass?

[sexconker]

It's been nice proving you wrong.

Re:Star's rapid change in mass?

[ceoyoyo]

When a star starts to run out of hydrogen in it's core fusion slows and the core contracts, and gets hotter. If it gets hot enough, it will start fusing helium, then carbon. The core ends up small, hot and producing a lot of energy. That energy causes the outer layers of hydrogen and helium to expand and the star becomes a red giant. Material from the outer layers eventually gets blown entirely off the star to form a nebula. That's where the mass goes.

I guess it's fairly rapid in the context of a star's lifetime, but it's not like it happens overnight. Unless you're talking about a big star, where the core finds itself suddenly exceeding what can be supported by electron degeneracy pressure and collapsing. IIRC that process happens very quickly, and the energy produced blows the outer layers off in a supernova.

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.

Re:Star's rapid change in mass?

[TheTurtlesMoves]

Main sequence stars die in different ways depending on start mass. Near the end of its life the star, like our sun will expand to huge sizes. The outer layers are sheded and eventually the fusion process stops. What is left collapses. Our sun is not big enough to become a neutron star, so it eventually becomes a white dwarf. Electron degenrate matter basically.

We'll probably not see many of these

[Anonymous Coward]

regardless of what the summary says, because
A) The star itself is smaller than a 'normal' star, making an occlusion more unlikely, and
B) Most inner planets are likely gone after the red giant phase, leaving only planets further away which are less likely to occlude the line of sight from Earth, and
C) Any configurations like this particular one are fleeting and on a cosmological timescale it is exceedingly unlikely we'll catch it at the right moment.

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?

[Anonymous Coward]

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.

The mechanism is in the summary, along with the disclaimer that it's very unlikely but possible. I get not RTFA, but at least read to the end of the summary.

Re:habitable zone?

[Anonymous Coward]

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.

The red giant phase causes the start to swell up to massive sizes, albeit at insanely low density. This actually could have an aero-braking effect on planets further out, causing them to spiral inward, and they may survive this phase and end up close enough to be in the habitable zone BUT...
Because the luminosity of white dwarves is so much lower than main sequence stars, the habitable zone is much much closer than a main-sequence star. This causes two issues. One is that as the habitable zone get closer, it also shrinks in width, making the chances of a planet residing there less and less likely. The second issue is that tidal forces start to kick in as you get closer and closer to a star as well. Within the habitable zone of white dwarves tidal locking is almost guaranteed, meaning one face of the planet will always be bathed in light, while the other resides in darkness. From what we know about tidally locked systems within our own solar system, that kind of situation doesn't seem very conducive to life.

Re:habitable zone?

[phayes]

The posited mechanism is implausible to the point of being ridiculous but then your post was probably typed by a bunch of monkeys so I suppose that implausible is no longer a problem...

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:habitable zone?

[Anonymous Coward]

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.

Why have you assumed that a planet must escape from the "habitable zone" before the red giant phase, and then migrate back in afterwards? There's no need for the planet to have been close to the star before the red giant phase. If a planet begins well away from the star, and only comes near it after it's become a white dwarf, you avoid half the problem: you need to explain it migrating inwards, but you don't need a mechanism for it to migrate outwards earlier.

An inward migration is the easier problem to solve, too. All you need is friction - say, drag in the intense stellar wind produced by the star during the red giant phase. Drag can't raise a planet's orbit, but it can lower it, and circularise it too.

Something like this must have happened in this case, because this planet, per the linked press release, has an orbital period of 4.5 hours. That's really short - Mercury, for comparison, has an orbital period of 88 days. If this star has a mass similar to the Sun's (which it should, being a white dwarf ... at least to within a factor of three or so), this planet would be at an orbital radius of ~1 million km. That's barely bigger than the radius of the Sun - orders of magnitude less than the radius of a red giant. So either (a) this planet was inside the star when the star was a red giant, or (b) this planet was further away, and migrated closer afterwards.

The full article is, unfortunately, paywalled, so I can't go into any more detail than that.

Re:habitable zone?

[phayes]

No-one doubts that there are planets orbiting white dwarf stars & nobody cares that there are burnt-out cinders orbiting far off stars. It's the possibility of life being able to exist on the discovered planet "in the habitable zone" that would make it news.

Your BOTE calculations forgot to take into account the following points:
- No inner planet will survive being englobed by a red sun. Stellar density will be high enough to slow them down so that they impact the stellar core long before the red giant phase is over.
- For planets further out to migrate inward to the point that they are in the habitable zone they would have to be englobed by the sun in it's red giant phase as well. Thus the solar flux cannot be calculated as a more or less one point source but from all points. Add at least 3 zeros to your 15% and it changes the outcome. By the time the star evolves on to be a white dwarf, all volatiles would have long been baked off & stripped away.

Re:habitable zone?

[phayes]

That there are mechanisms that make it possible for some animals to fly doesn't mean that pigs can fly. You're trying to perform a similar leap of logic.

You should see more discoveries like this

[xxxJonBoyxxx]

>> You should see more discoveries like this in the future

I'm sorry, I don't currently have access to the Kepler space observatory. Perhaps YOU'LL see...

Re:Doc Brown is tearing space-time apart!

[AmiMoJo]

This was happening 570 years ago. Some kind of /. record?

The Dink

[PopeRatzo]

We don't call them "White Dwarf" any more. Now, they're known as "White Little People".

Re:The Dink

[PopeRatzo]

Wha?

What's "gravitational pressure"?

[wonkey_monkey]

the planet is disintegrating under the star's gravitational pressure

I'm guessing that's not the actual scientific term for whatever's happening to it. So what is? Is it a tidal forces thing?

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.

The star isn't running out of time

[spauldo]

Considering white dwarfs live an insanely long time, the star isn't "running out of time."

If a habitable planet was orbiting a white dwarf, life on that planet could potentially go on for billions and billions of years, barring any planet-killing catastrophes. The star would slowly cool, but life once formed might be able to adapt to the cooling temperatures over billions of years.

A red dwarf would be better, though. They're practically immortal and keep a steady output over their lifetime. Only problem there is that the habitable zone is really, really close to a red dwarf.

Re:The star isn't running out of time

[spauldo]

If that's the case, then surface life would probably be difficult. I don't think radiation wouldn't be very harmful for life in an ocean, however.

And yeah, I'm not suggesting a habitable planet would stay habitable after being inside a red giant. However, if a planet became habitable after the red giant phase (imagine the amount of chaos going on after the star collapsed - planets would change position, possibly collide, lose their atmospheres, possibly gain new atmospheres and oceans from outgassing, etc.), it could potentially allow life a longer time to evolve than it would have had before.

I'm sure that's not horribly likely, but hey, there's a whole lot of stars out there.