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View Full Version : How can a pulsar have planets?



Tom Mazanec
2009-Feb-03, 04:54 PM
I suppose for every generation of astronomers, there is a discovery which causes them to say "There ain't no such animal". For the first generation of astronomers, it was two "new stars" in a generation, both of which faded away. For a later generation, it was the density of white dwarfs. For me, it was pulsar planets.
Scenario 1: The planets were there during the star's lifetime.
Pulsars form in supernovae that eject most of the mass of the star. Assuming that the planets survive the blast (a big if in itself), how do they stay gravitationally bound to the star? They should go flying off like the rock from David's slingshot.
Scenario 2: The planets form after the supernova.
Out of what? Supernovae are the "snowplows of the sky". They push the supernova remnant and any surrounding gas out at a great rate of speed, clearing the neighborhood and eventually leaving a void like the "Local Bubble". And the pulsar should not accrete any gas...an atom heading towards the neutron star will either hit it or go into a hyperbolic orbit and escape.

antoniseb
2009-Feb-03, 09:14 PM
Imagine that out there somewhere is a millisecond pulsar that is being orbited by a star that is being stripped by the pulsar's gravity and radiation. At some point, all that is left of the stripped star is a Helium core too small to ignite. Maybe the differential gravity tears this core into three of four lumps.

As it happens, the pulsar with planets is the only millisecond pulsar without a stellar companion to feed it.

Tom Mazanec
2009-Feb-03, 10:07 PM
Do the planets' orbits agree with this? I thought the were circular and did not intersect, and too far away for tidal disruption. But it sounds more plausible than my two scenarios.

RalofTyr
2009-Feb-03, 10:27 PM
There are pulsar planets. However, these three planets orbit the pulsar closer than Mercury does, so, most likely, they formed after the supernova event. Probably some left over matter didn't escape and collapsed to form planets.

EDG
2009-Feb-04, 05:58 AM
Tis a bit of a mystery isn't it... the other issue is that the stars that become supernovae shouldn't be around long enough to allow planets to form around them anyway (plus even if they were, their solar winds are so strong that they should just blast away any planet-forming material around them).

So I'd guess that the planets pretty much had to form after the supernova. I wonder could they actually be bits of the exploding star's own core that got blasted out? Hrm, maybe not, I'd imagine that the explosion would send those hurtling away from the rest of the core faster than its gravity could hold onto it.

I'd say that it looks like the hypothesis that they coalesced in situ after the supernova from debris that was somehow still there seems the most likely. Or they're not really there and we're mistaking something else for planets.

CuddlySkyGazer
2009-Feb-04, 08:41 AM
Supernovae are the "snowplows of the sky". They push the supernova remnant and any surrounding gas out at a great rate of speed, clearing the neighborhood and eventually leaving a void like the "Local Bubble".
The process is not totally efficient - there will always be some material that doesn't attain escape velocity. Some will fall back onto the core, and the rest will go into orbit. This is likely to be material that was nearer the core before the explosion, and will be of heavier atomic weight than the average.

PSR B1257+12 has (according to Wikipedia (http://en.wikipedia.org/wiki/Pulsar_planet) at least!) four planets together massing about 8.3 times that of the Earth. That's 0.002% of the mass of the Sun, which itself is much smaller than the star that formed this pulsar would have been.

EDG
2009-Feb-04, 09:26 AM
That article does mention one magnetar that is known to have a circumstellar disk:
http://en.wikipedia.org/wiki/4U_0142%2B61

So evidently there can be enough material left over from the explosion that can potentially form planets.

Middenrat
2009-Feb-04, 11:22 AM
I wonder if during the cataclysm shock waves produce fields of harmonic resonance where matter can rest in nodes sympathetic to its atomic weight? Later to coalesce as planets.

eburacum45
2009-Feb-04, 12:41 PM
A supernova produces about a hundred thousand Earth masses of iron (I think- don't take my word for it), so only a tiny amount of that mass needs to fall back and coalesce into planets - and 'hey presto!', a new planetary system. Pulsar planets are quite small, IIRC.

Tom Mazanec
2009-Feb-04, 03:53 PM
PSR B1620-26 b is at least 2.5 Jupiter masses.

eburacum45
2009-Feb-05, 07:10 AM
Even if that planet is entirely made of iron (which is probably not the case) that represents only about`1% of the total iron produced by the supernova, assuming my original rough estimate was correct.

I hesitate to guesstimate supernova iron production on an astronomy board; firstly I don't know anything about the subject, and secondly there are apparently several different types of supernova, so the figures would presumably be different in any case. I was hoping a real 'stronomer might come along and give a more accurate range of figures. All I know is that it is a lot.

Tzarkoth
2009-Feb-05, 08:27 AM
Given the above examples of supernova remnants forming planets, how long would it take for these planets to clear out their orbital paths, and how long would it take for a potato shaped hunk of rock to squish down under its own gravity to more closely resemble a sphere?

timb
2009-Feb-05, 08:33 AM
Even if that planet is entirely made of iron (which is probably not the case) that represents only about`1% of the total iron produced by the supernova, assuming my original rough estimate was correct.

I hesitate to guesstimate supernova iron production on an astronomy board; firstly I don't know anything about the subject, and secondly there are apparently several different types of supernova, so the figures would presumably be different in any case. I was hoping a real 'stronomer might come along and give a more accurate range of figures. All I know is that it is a lot.

Dunno about the quantum but a substantial amount of nickel is also formed. Optical Photometry of Type II-P Supernova 2004dj in NGC 2403 (http://arxiv.org/abs/astro-ph/0512526) states that about 0.023 Msun of 56Ni was ejected from that supernova. 56Ni decays to iron, of course, but the amount of 58Ni could also be substantial.

Tom Mazanec
2009-Feb-05, 03:14 PM
Thanks for the help. Anyway, this has taught me a lesson. If it does not contradict basic scientific knowledge (like a planet moving faster than light), do not say a scenario is absolutely impossible.
I had a good laugh with an astronomy professor about a suggestion to look for planets around pulsars using timing. Something about the old joke of a drunk looking for the keys he lost in a vacant lot...under a lamppost because the light was better there. I might like to remind him of this, but it was thirty years ago and he was old then, so he has probably gone to a different heaven than the one we discuss here.

Romanus
2009-Feb-08, 03:56 PM
Scenario 2: The planets form after the supernova.
Out of what? Supernovae are the "snowplows of the sky". They push the supernova remnant and any surrounding gas out at a great rate of speed, clearing the neighborhood and eventually leaving a void like the "Local Bubble". And the pulsar should not accrete any gas...an atom heading towards the neutron star will either hit it or go into a hyperbolic orbit and escape.

The confirmed pulsar planets we know about are around millisecond pulsars, which are a slightly different beast from "normal" pulsars. The theory is that millisecond pulsars have been "sped up" by accreting material from a lost companion, long after they originally went supernova--material that perhaps formed a disk, rich in metals from which planets could form.

Grashtel
2009-Feb-08, 05:13 PM
PSR B1620-26 b is at least 2.5 Jupiter masses.
According to the Wikipedia page (http://en.wikipedia.org/wiki/PSR_B1620-26_b) on it Methuselah is apparently a special case in that it originated around what would become the white dwarf half of the binary pair it now orbits in the normal fashion of planetary formation, then it and its primary were captured by the neutron star rather than the planet having formed around the neutron star initially.