The black hole question reminded me of an idea I wanted to implement at some point in a space campaign, but didn't go forward with because I was unsure whether it's merely statistically very implausible, or outright physically impossible:

Can a 'rogue' planet (in the sense of not orbiting a specific star in a close vicinity) be on such an orbit around the galactic centre that the amount of illumination it gets is equivalent of that gained in the Goldilocks zone? That is, is there such an orbit for which the total illumination provided by the densely packed stellar and similar objects is similar to that provided by our sun at 1a.u.

When I say galactic centre, I mean the central area of a galaxy, one hosting a denser selection of stars (and whatever other 'shiny' objects, including feeding supermassive black holes, nebulae or the like) than the rim. If it's doable in the Milky Way with its central area, great; if it's doable in some other galaxy we know to really exist, that's OK too; if it's only possible with a galaxy type we're not sure is plausible according to science, that's a so-so option but I'd still like to be informed about it.

I am fully aware that it'll have to lack seasons, and that its velocity will be odd. It's okay if the probability of such an arrangement naturally approaches zero; in fact even if it needs to be a result of Sufficiently Advanced civilisation meddling, that's okay, so long as such an arrangement can keep on existing with no reliance on superscience once it's in place.

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    $\begingroup$ You specified the hard-science tag. That places demands on the answerer - sure. It also demands that the question be very specific in it's requirements. Please specify the conditions in and surrounding your galactic center in a way that meets the hard-science requirements - otherwise this may get closed as "unclear what you are asking". $\endgroup$ Commented Jul 27, 2019 at 17:53
  • $\begingroup$ Its been a couple of months now... if neither of the answers below are good enough, perhaps you could let us know where they fall short? Otherwise it'd be nice to mark one of them as accepted... $\endgroup$ Commented Oct 10, 2019 at 7:21
  • $\begingroup$ @StarfishPrime Both answers are good. For a while I thought they don't put my question to rest, but ultimately I guess I should've read the careful statements of not meeting the requirements as more confident ones (that is, that it's impossible). I'm still not sure which answer is better of the two, so consider the Accept as a fleeting thing that might go away or move over to a different answer in the future. $\endgroup$ Commented Oct 10, 2019 at 11:32

2 Answers 2


This is a surprisingly difficult question to answer... related ones have been asked elsewhere, with few satisfactory answers. Turns out that simple questions like "what is the stellar density in such-and-such a region of space" doesn't often have a very good answer (multiple answers may exist, differing by at least an order of magnitude) let alone more specific things like "how bright is starlight there". I've tried my best, but I haven't been able to get a simple, citeable answer.

Instead of looking at the core itself, I'll look at some other astronomical objects which also exhibit very high stellar densities... globular clusters

There's a relevant article in Astronomy mazagine, which isn't a scientific publication, alas. There are excerpts from the article for free here, including this nice simulation of night on a planet inside the globular cluster 47 Tucanae, as viewed by a regular human eye.

Nighttime view from inside 47 Tucanae

To quote from the quote:

The cluster's suns would combine to give an average sky brightness some 20 times brighter than Earth's night sky at Full Moon

Our sun, as seen from earth, is about 400000 times brighter than the full moon. Clearly, stellar densities just aren't high enough here!

Now, a globular cluster ain't quite a galactic core, but it has some similarities. Near our solar system, the stellar density is about 0.14 stars per cubic parsec. The centremost cubic parsec of 47 Tucanae has about 150000 stars packed into it (though the density drops off sharply... its a tenth of that if you go more than 3 parsecs from the centre). The galactic core by comparison may have a density 100 times higher than that, but even with that it has been suggested that

...there would be a million stars in our sky with apparent brightness greater than Sirius. The total starlight in the night sky would be about 200 times greater than the light of the full moon; you could easily read the newspaper at midnight, relying on starlight alone.

That's a pretty impressive twilight, but one that falls short of your requirements by quite some way. The author does not explain their methodology, so it remains possible that they are incorrect but it seems plausible to a couple of orders of magnitude, and that would still fall a long way short of your requirements!

Lets look at something else that's quite import to your scenario, that you've not really thought about... the particularly hazardous nature of such locations.

When star densities are that high, the chances of you being near to a dangerous sort of neighbour are much higher. 47 Tucanae, for example, may or may not harbour a black hole but it most certainly does harbour 25 pulsars, many of which are millisecond pulsars (for comparison, the nearest pulsar to earth, PSR J0108−1431 is over 420 lightyears away, and it is very weak and spins much more slowly). The creation of such objects is a violent and dangerous event in itself, but they're associated with other unpleasant events such as gamma-ray bursts which are definitely planet-sterilisingly bad things to be near. Binary systems with neutron star companions are known to exist in 47 Tucanae, and that's definitely a forboding combination... certainly, I learned a new term "cataclysmic binary". There are stars which get brighter inexplicably, possibly being some new kind of nova.

The brighter the starlight, the more short-lived giant stars and dangerous supernova remnants there are out there, and the greater the chances of something terminally unpleasant happening to your rogue planet.

Other potentially interesting sources which I didn't manage to back up with more hard data:

  • This physics.SE answer suggests some sort of bright twilight, though doesn't compare to the moon and I was too lazy to do the maths to compare them myself.

  • Another physics.SE answer suggests half as bright as moonlight "near" the galactic centre.

  • This quora answer, suggesting a much brighter sun-like twilight in the galactic centre, though also suggests a more moon-like lights at the edge of the galactic core

You may also wish to look up "Ahad's Constant", which is about how much light from stars other than the sun falls upon earth. The original paper seems to have disappeared, but you might have more luck finding it than me!


Your planet is traversing an emission nebula. Here is the Orion nebula.

orion nebula

By ESO/Igor Chekalin - http://www.eso.org/public/images/eso1103a/, CC BY 4.0, https://commons.wikimedia.org/w/index.php?curid=27880539

Nebulae are interstellar clouds of gas. Some are bright and give off light; these are emission nebula. Orion is a big one; 22 light years across and bright enough to be visible from Earth with the naked eye. I liked this description of the Orion nebula from Wikipedia

The first published observation of the nebula was by the Jesuit mathematician and astronomer Johann Baptist Cysat of Lucerne in his 1619 monograph on the comets (describing observations of the nebula that may date back to 1611). He made comparisons between it and a bright comet seen in 1618 and described how the nebula appeared through his telescope as: "one sees how in like manner some stars are compressed into a very narrow space and how round about and between the stars a white light like that of a white cloud is poured out"


Emission nebulae glow with their own light, energized by stars and also stranger things like

supersonic "bullets" of gas piercing the hydrogen clouds of the Orion Nebula. Each bullet is ten times the diameter of Pluto's orbit and tipped with iron atoms glowing in the infra-red. They were probably formed one thousand years ago from an unknown violent event.

Your rogue planet is traversing such a nebula. Hopefully it has a magnetic field to deflect the iron atoms! Those would sting. As can be seen in the image, the nebula is not uniformly bright and there are stars in there too, which could be seen from your planet. The entire sky will glow to a greater or lesser extent, with different colors dependent on the local energies and composition of the nebula.

I could imagine that a planet traversing a gas cloud might gravitationally collect a tail of gas in its wake. The locally denser gas trailing behind the planet could be brighter, especially if there were aurora-like interactions between collected gas and the planetary magnetosphere. If your planet rotated (surely it will to some degree), rotating thru the bright "tail" and the relatively darker front side could give a day and night.

The hard science: absolute magnitude! http://astronomy.swin.edu.au/cosmos/A/Absolute+Magnitude Absolute magnitude is a way of normalizing brightness of various celestial objects by assuming a fixed (32.6 light year) distance from the viewer. Our sun as an absolute magnitude of 4.8 although looks much brighter because we are a lot closer than 32.6 light years. The Orion nebula has an absolute magnitude of 4 which is comparable to the sun, but looks much dimmer because it is 1344 light years away. Given that the nebula and our own star have comparable absolute magnitudes I think it is safe to assert that they would be comparably bright when seen from the same distance.

The tricky thing for calculations is that the nebula is also much larger than the sun, and also the rogue planet is inside of it.

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    $\begingroup$ Hmm. A nebula. An unexpected answer, almost a frame challenge, but certainly a useful way to implement the thing. I'll keep this option in mind. $\endgroup$ Commented Jul 27, 2019 at 18:56
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    $\begingroup$ Question: If Orion is 1344 light years away, and the "gas bullets" were formed 1000 years ago, how can we see them? $\endgroup$ Commented Jul 27, 2019 at 22:42
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    $\begingroup$ @BilboBaggins: ‘ago’ is a relative term. In this case i suspect its Relative to things as we see them. If we jumped back a thousand years and looked again we’d observe the gas bullets forming. It’s a consistent point of annoyance to me that science educators (and often cosmologists) mix and match timeframes when they’re trying to make a point. It’s like a history teacher suddenly switching to the Mayan calendar without telling anyone. $\endgroup$
    – Joe Bloggs
    Commented Jul 28, 2019 at 10:29
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    $\begingroup$ It is interesting that in such nebula a planet without atmosphere still would have all its sky bright. $\endgroup$
    – Anixx
    Commented Jul 31, 2019 at 21:22

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