The Fate of Planets Near Galactic Center

by Paul Gilster on November 1, 2011

It was Gregory Benford who used the wonderful phrase ‘the first hard science fiction convention’ to describe what happened at the 100 Year Starship Symposium. It was an apt choice of words. ‘Hard’ science fiction refers to SF that goes out of its way to get the science right, and in which the scientific and technical details play a major role in the development of the plot. Science fiction critic P. Schuyler Miller evidently coined the term in one of his reviews for Astounding Science Fiction back in the 1950s. In many ways, the Symposium operated under a science fictional meme.

Science fiction at its best exists to paint possibilities for us. Some scientific speculations may be remarkable in their own right but only become vivid when portrayed by writers who can make the background science into a scenario that plays out in fictional terms. An obvious case is the classic Isaac Asimov tale “Nightfall,” published in Astounding’s September, 1941 issue. Asimov took us to a place that knew night only once every 2000 years because of the configuration of the six suns in its planetary system. His craft painted a world few would have imagined, and showed us the consequences of its existence upon a civilization there.

All these musings were triggered by the latest news from the University of Leicester, and I’d love to see the science fiction story that might emerge when ‘hard’ SF tackles its findings. Surely someone will tell the story of a civilization too close to galactic center (speaking of places that are well lit!), and the consequences as radiation levels begin to rise to untenable levels on a world too near the central black hole. If interstellar flight is possible, surely it would happen here as a means of species survival.

For black holes seem to be common at galactic center in many galaxies, and in particular the supermassive ones that lurk at the center of galaxies like our Milky Way. Huge cloaks of dust obscure many of these, and a team of scientists led by Sergei Nayakshin at Leicester thinks that collisions between planets and asteroids occurring at speeds up to 1000 kilometers per second could be the cause. Nayakshin’s team argues that the accretion disc around supermassive black holes will eventually form planets, and that planets and asteroids that formed in the outer regions of the disc would be stripped away by the close passage of stars in the disc, given the tight quarters at galactic center.

From the paper:

Released from their host stars, these solids and planets orbit the SMBH independently. Since the velocity kick required to unbind them from the host is in km/s range, whereas the star’s orbital velocity around the SMBH is ∼ 1000 km/s, orbits of the solids are initially only slightly different from that of their hosts. AGN gas discs are expected to be very geometrically thin (e.g., Nayakshin & Cuadra 2005), and if they always lay in the same plane (e.g., the disc galaxy’s mid-plane) then the resulting distribution of solids would be quite thin and planar as well.

So far so good, but planets in this scenario seem bound to come to grief:

However, there is no particularly compelling reason for a single-plane mode of accretion in AGN as the inner parsec is such a tiny region compared with the rest of the bulge (Nayakshin & King 2007), and chaotically-oriented accretion may be much more likely…

Collisions are bad enough, but the planets would have already been sterilized as they orbited the supermassive black holes, says Nayakshin in a related news release:

“Too bad for life on these planets, but on the other hand the dust created in this way blocks much of the harmful radiation from reaching the rest of the host galaxy. This in turn may make it easier for life to prosper elsewhere in the rest of the central region of the galaxy.”

The team takes its lead from the zodiacal dust in our own Solar System, known to be the result of collisions between solid objects like asteroids and comets. And its work may help us understand how black holes grow and affect the galaxies within which they reside. The dust and gas in the inner regions of our own galaxy, much of which might have been expelled or destroyed by this process, would otherwise have contributed to the formation of more stars and planets. The black hole at galactic center would have thus played a significant role in the evolution of the Milky Way.

Image: ‘Light echo’ of dust illuminated by a nearby star V838 Monocerotis as it became 600,000 times more luminous than our Sun in January 2002. The flash is believed to have been caused by a giant collision of some kind, e.g., between two stars or a star and a planet. Collisions of smaller objects, such as asteroids or minor planets near a supermassive black hole could also be dramatic due to the huge collision velocities and would release a lot of dust. Credit: NASA/ESA.

The paper is Nayakshin, et al., “Are SMBHs shrouded by ‘Super-Oort’ clouds of comets and asteroids?” in press at Monthly Notices of the Royal Astronomical Society (preprint). The science fiction story based on it remains to be written.

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{ 22 comments }

Christopher Phoenix November 1, 2011 at 17:30

A civilization to close to the galactic center would probably find themselves in the proverbial frying pan. Planets near the center would experience far more exposure to gamma rays, x-rays, and cosmic rays. My question is, how did complex life and a technological civilization evolve so near the galactic center in the first place? Some scientists theorize there is a “galactic habitable zone” which is friendly to complex life, like us, but anywhere close to the galactic center is well out of this theoretical GHZ!! Any thoughts?

This new research is interesting. Finally, someone is telling us why the galactic center is shrouded in dust- all they told us before is that you can’t see the black hole because visible light cannot penetrate the dust cloud.

Back to the civilization that must escape from the rising radiation levels near the galactic center- wouldn’t high radiation levels make space travel extremely hazardous? If the radiation levels have risen high enough that even a thick atmosphere is not shielding enough, how could a starship survive? If you want to escape dangerous radiation levels, you have to leave your neighborhood altogether.

The distance from Earth to the galactic center is 25000-28000 lys. How far would this civilization need to go to escape the dangerous radiation levels?

Space travel might be easier near the galactic center. There are large clouds of hydrogen near the galactic center, which will make the fusion ramjet a lot more practical. The stars are more densely packed, which will be an incentive for any budding starflight program. If you only have to travel one lightyear instead of four to reach a nearby star, interstellar travel will be easier than it is in our neighborhood.

Once stellar explosions start turning up the heat in their neighborhood, these aliens will turn their ramjets out from the galactic center and head for cooler, safer regions of space. Areas without dangerous gamma rays and x-rays, densely packed stars, nearby novas, and a giant black hole lurking nearby.

However, I think the dangerous radiation and nearby novas would wipe any life before they build interstellar ramjets. I prefer our neighborhood- there might not be any free fuel, but it is a lot safer.

Would a voyage to the galactic center in a starship with a hyperdrive be heroic exploration or insanity?

jensph November 1, 2011 at 18:23

Speaking of hard sci-fi and galactic centers surely a mention of Greg Egan’s Incandescence is warranted.

(I must mention that if Egan is new to you, then Diaspora is a must read.)

Adam Crowl November 1, 2011 at 18:40

Hi Paul
Stephen Baxter’s “Exultant” in its opening scenes is set amongst the asteroids around the Milky Way’s SMBH. The Core region does get more interesting, the more we study it.

Ron S November 1, 2011 at 22:02

I’ve only just cracked open this paper but something immediately struck me as odd in their reasoning.

First, where did the metals come from that formed these stellar systems near the AGN? Fragmentation beyond a certain threshold distance from the accretion disk. But how did they come to be in the accretion disk to begin with? That must have been from older novae near the AGN, and brought into the accretion disk along with non-metals (hydrogen).

So suggests to me there is a partial recycling of accretion disk material going on, where a portion falls into the SMBH, another portion into the SMBH jets, and the rest into (the paper’s hypothesized) fragments forming these stellar systems. I am having difficulty seeing how this would allow large clouds of metals to exist and have the effects suggested in the paper.

Second, while there are great differences in the opacity of metal (dust) clouds and hydrogen, even if all the material in these putative stellar systems gets pummeled into dust clouds it is still only a small fraction the mass of their stellar parents, and over time I would think that the stellar winds alone would greatly out-mass all the expelled metals.

It just doesn’t feel right. Maybe I’ll have to read further into the paper.

Gregory Benford November 2, 2011 at 23:53

I wrote a long series of novels about these problems (though without this recent research), the Galactic Center Series. I supposed the Center was the best place to live if you were a machine intelligence — plentiful energy resources, indifference to radiation levels, unconcerned with planets as life sites since one could live in raw, radiated space.
It still seems that way to me. I doubt life will evolve nearer than 1000 or more light years from the devouring black hole squatting at the Center.
Yet I supposed humans would go there, seeking the answer to why machine intelligences sought out organic life like us and sought to exterminate it. The astrophysics I did on the magnetic structures around the black hole — which interestingly don’t occur in the Andromeda galaxy — led me to write the rest of the series, after the first two novels.
The Center realm remains the most fascinating region of our galaxy, though not I think a zone for life.

andy November 3, 2011 at 18:34

One recent study of the GHZ, Gowanlock, Patton and McConnell (2011) concluded that although the risk from supernovae to the biosphere in the inner galaxy was far greater, the favourable planet-forming conditions made it the most habitable region of the galaxy (i.e. there are sufficient planets there to compensate for the large fraction that experience supernova sterilisation events). From that paper:

From Sections 4.1 and 4.2, we see that SN sterilizations on their own make the inner Galaxy the least hospitable for complex life. However, regarding the planet-metallicity correlation without the effects of SNe makes the inner Galaxy the most hospitable for complex life. When both factors are taken into account, the inner Galaxy is ∼10× more hospitable than the outer Galaxy (Figure 9). This finding indicates that the impact of metallicity on planet formation appears to dominate over the effects of SN sterilizations. Furthermore, the inside-out scenario of Galaxy formation permits the inner region to be more habitable than the outskirts. Neither SN sterilizations nor metal-poor environments are capable of rendering any region inhospitable to complex life at the present day.

Of course this is unlikely to be the final word on the issue…

Eniac November 4, 2011 at 6:46

Andy: What form does this “sterilization” take? I thought a supernova was quite weak even just a few light years away, and a thick atmosphere like the Earth’s does not really let any ionizing radiation through, or does it? If this “sterilization” is just about potential damage to the ozone layer, that would hardly qualify as a life extinguishing event, I think.

stephen November 4, 2011 at 13:42

Cockroaches, and many bacteria, can tolerate high levels of radiation. I don’t know how they manage it, but life on planets near the galactic center could do the same. Life might take longer to develop, but once it does, *all* life on the planet might be like cockroaches, able to tolerate practically any environment.

A. A. Jackson November 4, 2011 at 14:01

Welp.
Larry Niven has us all almost trumped.
Pierson’s Puppeteers are taking their whole kit and caboodle of a Fleet of Worlds out of the galactic center, or where they were near the center.

Say, Greg, I think you said you had some idea to trump Larry?
Didn’t some Russian astronomer have an idea is moving the whole solar system with a big mirror around the sun?

Anybody ever written a story where ‘solid state’ intelligence evolves in a hostile radiation environment , such as near the galactic core.
Then goes on to build ‘wet ware’ things like humans?

The Milky Way and Andromeda galaxy are on a collision course. In about 3 billion years, the two galaxies will collide. That can’t be good for us!
How would one move the Milky Way?
I would say that would take a Kardashev type X!

andy November 4, 2011 at 14:34

Eniac: the model for supernova sterilisation used for that analysis is surprisingly enough discussed in the paper. Read section 3.1…

Rob Henry November 4, 2011 at 18:27

Fred Hoyle’s novel “The Inferno” featured the Milky Way’s centre becoming a full-on quasar. More importantly he used this unlikely scenario to illustrate a factor that I have never seen formally addressed. Unlike supernovas, the power of a galactic core event can be so great as to completely disrupt the galaxy’s magnetic field. In the novel this opened the possibility of every system, even in the galactic disc, being bathed in radiation from the quasar that was so intense that it could completely strip a planet’s atmosphere, but I can’t help but think of milder possibilities such as those highlighted by the cyclical occurrences of mass extinctions on Earth tending to coincide with periods when our system is at locations with the least protection from our galaxy’s field.

My question is how resilient is the bulge of our galaxy to such magnetic field disruptions and how real is the possibility that disruptions thereof by (a series of) core events can play a greater role in the development of higher life than this field does in the disc?

Eniac November 4, 2011 at 20:11

Andy: As I suspected, they appear to assume that a supernova closer than 8 pc will strip aways the ozone layer which will then “sterilize” the planet. I would like to submit these caveats:

1) The ozone layer might be more resilient than we think
2) Something else might replace the ozone layer
3) Life may survive the lack of ozone layer
4) There may be stars with less UV where no ozone layer is needed
5) Bigger planets may have a thicker atmosphere and no need for an ozone layer
6) … etc. etc.

I think there are so many caveats that it is unrealistic to assume that we have any idea about what distance a supernova needs to be to “sterilize” planets. Therefore the inner limit of the GHZ may well be the event horizon of the black hole, or at least not too far from it.

Ronald November 4, 2011 at 20:43

@andy: yes, while most authors, such as Lineweaver, consider the GHZ as an annular ring at about 6-10 kpc from the galactic center, Gowanlock, Patton and McConnell (‘A Model of Habitability Within the Milky Way Galaxy’) model it from 2.5 kpc (or possibly even closer) to at least 12 kpc from the center, with the density of habitable planets in the inner region about 10x as high as in the outer region, also resulting in a rather high estimate of the number of habitable planets in the MW galaxy.

Stephen: scorpions are also known to tolerate very high levels of radiation.

A.A. Jackson: the ‘collision’ of the MW with Andromeda will not be a real collision, but rather a merging of the two galaxies, not uncommon in the history of the universe. Because of the huge distances between the individual stars, there will hardly be any real stellar collisions, in fact very few stars will even be caught in each other’s gravity field (to become secondary binary stars, a very rare event). What may happen is that stars may be disrupted in their galactic orbits and there will probably be a (relatively short duration) outburst of new star formation.

Adam Crowl November 5, 2011 at 7:01

The description of Hoyle’s “Inferno” makes me think of Donald Moffitt’s duology “Second Genesis”/”The Genesis Quest” about an off-shoot of humanity being made by aliens in a distant Galaxy (M101) from radio transmission data. The second book features a very interesting take on the Fermi Paradox based on periodic extinctions, akin to Hoyle’s concept.

The book is now rather out-of-date – eg. M101 is now known to be 21 Mly away, not 37 Mly as was previously thought – but periodic extinctions still drop out of the fossil data. One wonders just what Galaxies do every so often…

Eniac November 6, 2011 at 18:47

One more thought: Isn’t the ozone layer derived from atmospheric oxygen? That would mean that for most of its life Earth did not have one to begin with. Any calculations of GHZ based on supernovas stripping the ozone layer would then be blatantly invalid.

And please all remember that ionizing radiation does not penetrate the atmosphere, so no cockroaches or scorpions required. For UV, there could be a permanent cloud layer, or we could have life that is undersea or underground.

andy November 7, 2011 at 16:55

@Eniac: ozone has been detected on both Mars and Venus, there isn’t much of it though…

Eniac November 8, 2011 at 5:06

To me, it would make just as much sense that strong X-rays generate more ozone directly than they destroy indirectly by generating nitrous oxide.

Anyway, after all this and some reading I am now quite convinced that a supernova cannot have a lasting ill effect on life (much less “sterilization”) unless it is close enough to literally fry the place, i.e. provides as much energy as the sun. For the strongest supernovas, this would be less than one light year, which should be an unlikely occurrence even quite close to the galactic center. I would be happy to hear reasons why I am wrong.

Ronald November 8, 2011 at 11:33

Eniac, although I agree with you that supernovas are probably nor such a big risk, your 1 ly ‘safe distance’ may be a bit too optimistic: as I wrote in the previous post (Widening the Red Dwarf Habitable Zone), there are some studies indicating that the most dangerous supernovas, Type Ia, may harm a planet (atmosphere, biosphere) up to about 30 ly.

It shouldn’t be too difficult to make some base caslculations: how much radiation of various types (visible light + IR, UV, X-ray, gamma) reaching our upper atmosphere would be harmful per unit area?
It would be easy to calculate how much that would be at a given distance for a supernova of a particular type.

Eniac November 9, 2011 at 23:07

@Ronald:

there are some studies indicating that the most dangerous supernovas, Type Ia, may harm a planet (atmosphere, biosphere) up to about 30 ly.

Well, yes, but what I have come to realize (please tell me if I am wrong) is that those “some studies” are based on the destruction of the ozone layer as the mechanism for sterilization. To me, that is not conclusive (not even close) for all the reasons you will discover if you read my other posts on this matter.

Christopher Phoenix November 11, 2011 at 3:31

@Eniac

Anyway, after all this and some reading I am now quite convinced that a supernova cannot have a lasting ill effect on life (much less “sterilization”) unless it is close enough to literally fry the place, i.e. provides as much energy as the sun. For the strongest supernovas, this would be less than one light year, which should be an unlikely occurrence even quite close to the galactic center. I would be happy to hear reasons why I am wrong.

What about the expanding shell of high-velocity gas? Sten Odenwald answered the question of what would happen if Betelgeuse went supernova, and the expanding shell of gas would push the suns magnetosphere so far in it would touch the orbit of the Earth.

Apparently, the detonation of Betelgeuse would not be too dangerous for life on Earth, but travelers in interplanetary space would need extra shielding.

Betelgeuse is 160 parsecs (520 light-years) distant. If you were much closer, say within a dozen light years, I’d imagine that the expanding shell of gas might have much worse effects on habitable planets. Do you have any thoughts on this damage mechanism?

Here is what Sten Odenwald hast to say about Betelgeuse going supernova.

Betelgeuse is 160 parsecs (520 light years) distant. If we just consider what could happen as a result of its expanding shell of gas, typical shell velocities are about 10,000 kilometers/sec. The shell would arrive here about 100,000 years after we see the star brightened. The shell would carry perhaps 10 times the mass of the Sun or some 2 x 10^58 protons. The flux of particles in a shell with a radius of 160 parsecs would be about 140,000 protons per second per square centimeter. The solar wind flux, by comparison is about 300 million protons/sec/square centimeter at the Earth’s orbit. So, although detectable, the flux from Betelgeuse probably won’t do much biological damage compared to what the Sun does. However, because the Betelgeuse flux is traveling at 10,000 kilometers/sec compared to the 450 kilometers/sec of the solar wind, the Betelgeuse flux has an effective pressure that is (10,000/450)^2 = 490 times stronger than the solar wind, and spread out over a region much larger than the size of the solar system. This would probably cause the Sun’s magnetopause to collapse from its present radius near 100 AU, to possibly less than the orbit of the Earth. Also, the Earth’s magnetosphere would be compressed, which would cause the energies of the particles in the van Allen belts to be amplified. The environment outside the van allen radiation belts would probably be ‘lethal’ for human exploration of the solar system.

There is also the x-ray flux to contend with. Inside this shell, which would probably take many decades to pass by, there is a bubble of plasma consisting of electrons and magnetic fields which produce copious amounts of X-ray light. We would be subjected to this x-ray flux for 10s of thousands of years until the expanding supernova remnant has aged sufficiently to quench this production mechanism. These X-rays, of the ‘soft’ variety’ would not get down to the Earth’s surface thanks to atmospheric shielding, but travelers in interplanetary space would need some additional shielding from the secondary electrons generated as these x-rays strike the skin of their spacecraft and liberate additional electrons.

I’ve just noticed that you and Ronald were only discussing radiation and the stripping of the ozone layer- but I’d think that the impact of the expanding shell of gas at close range might be bad. Am I wrong? Or is the expanding shell of gas only going to affect the ozone layer, which you don’t seem to think is going to be lethal for life?

Ronald November 14, 2011 at 11:08

Christopher Phoenix, very interesting information from Sten Odenwald, that you passed on.
However, there is one little flaw with the expaning shell/proton flux as described, or maybe not even a flaw, but a detail to bear in mind that makes this proton flux considerably less dangerous:
Quote:
“However, because the Betelgeuse flux is traveling at 10,000 kilometers/sec compared to the 450 kilometers/sec of the solar wind, the Betelgeuse flux has an effective pressure that is (10,000/450)^2 = 490 times stronger than the solar wind”.
This ’490 times srtonger than solar wind’ is *per proton*.
The amount of protons is only 140,000/300 million = about 0.0005

Which means that the total proton energy from Betelgeuse at this distance would only be about 23% of the solar wind.

It’s a pity Odenwald does not elaborate on the amount of X-ray (or did he?).

Eniac November 18, 2011 at 0:02

This from http://en.wikipedia.org/wiki/Supernova_remnant :

The shock continuously slows down over time as it sweeps up the ambient medium, but it can expand over hundreds of thousands of years and over tens of parsecs before its speed falls below the local sound speed.

Tens of parsecs is ~50 ly, so I am afraid Sten Odenwald is mistaken about the speed with which the Betelgeuse ejecta would arrive here. More likely, they would never arrive at all, having slowed down to a crawl and eventually dissipated before making it 10% of the way. Nevertheless, it seems likely that supernova ejecta could be comparable with the solar wind in density or energy somewhat further away than the 0.5 ly or so that it takes for the electromagnetic radiation energy to be reduced below solar levels. It is not going to kill anything on Earth, but it may cause radiation damage to space craft and unlucky astronauts.

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