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For the purpose of science fiction, could an entire galaxy essentially be a planet's equivalent to our Sun for Earth?

If a planet were far enough away from other objects in the galaxy but still orbited the galaxy, especially one that was very bright, perhaps mostly composed of stars, could this planet feel much like Earth?

It would receive heat and light, and its inhabitants might view the entire galaxy as a brilliant point in the sky.

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    $\begingroup$ Galaxies and stellar systems are completely different entities. In stellar systems most of the mass is concentrated in the star, whereas in a galaxy most of the mass sits in the disc. The 'nucleus' of a galaxy does not determine how things are orbiting the galaxy, nor does the nucleus heat its surroundings... $\endgroup$ Oct 11, 2023 at 10:01
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    $\begingroup$ We are in a galaxy, what do you see at night? $\endgroup$ Oct 11, 2023 at 18:37
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    $\begingroup$ This is kind of like asking if you could have an entire city be your next door neighbour. Too far and nothing is your neighbour, too close and only the people right next door are your neighbours, never the entire city. $\endgroup$
    – DKNguyen
    Oct 11, 2023 at 22:41
  • $\begingroup$ I'm not in a state to want to do the math right now, but my guess would be that the amount of energy a galaxy would have to put off for a planet to receive the equivalent amount of energy as the Earth does from the Sun (and be the same relative size in the sky), that galaxy would basically have to be a galaxy-sized star in its own right, and I shudder to think of what a star that big would do to our poor unsuspecting laws of physics. $\endgroup$
    – Abion47
    Oct 13, 2023 at 17:21

7 Answers 7

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We are IN a Galaxy. It doesn't feel very warm to me when I go outside at night, compared with direct sunlight!

Your basic problem here is the enormous distance between stars compared with the distance between the Earth and the Sun, combined with the inverse square law of how much flux (power per unit area) a planet receives as a function of distance from a luminous source.

Another way of thinking about it is that the Sun occupied about 0.004% of the sky (when it is above the horizon), but the distant stars in a Galaxy, even though numerous, occupy a much smaller fraction because they are so far away and separated from each other by distances much larger than their individual diameters.

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  • $\begingroup$ My understanding is there is a distance from the galactic core yielding a life zone; except for the fact you're fried by radiation without some truly exotic solution. $\endgroup$
    – Joshua
    Oct 13, 2023 at 3:40
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To a very rough approximation the habitable zone of a star is $0.7\sqrt{L/L_\odot}$ to $1.4\sqrt{L/L_\odot}$ AU. So if we want a habitable zone that extends to $r$ AU the luminosity needs to be about $(r/1.4)^2$ to $(r/0.7)^2$ solar luminosities. So for an extragalactic planet $r$ will be several billion AU, requiring a total luminosity from the galaxy on the order of $10^{18}$ solar luminosities. This is way above even the largest galaxies (and they would of course require a much wider orbit).

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Even if the primary (galaxy around which the planet is orbiting, in this case) has an active galactic nucleus, so the galaxy is very bright, the interstellar or intergalactic planet would be very cold due to the inverse square law which says how light/energy from a point source decreases with distance: the light/energy is inversely proportional to the square of distance, so at distance 1 you get 1 unit of energy, at distance 2 you get 1/4 and at distance 3 you get 1/9;

The planet would have to orbit in the galactic disk for its orbit to be stable enough for a while, though it could easily be disturbed by a passing star; if the planet would have a high inclination, orbiting above and below the galactic disk, then the gravitational forces the planet would be subjected to would vary along its orbital path, which would change its orbit significantly during a single cycle.

An important thing to consider is that the orbital speed for planets around a star decreases with distance from the star - Keplerian orbits;

keplerian vs galactic orbit speed

In the case of galaxies, the orbital speed of stars (and therefore, of the interstellar planet in question) around the galactic center stays quite high (over 200km/sec) as distance increases from the center, and this also varies from galaxy to galaxy. Compare that to the orbital speed of the Earth around the Sun which is about 30km/sec, whereas Pluto orbits at about 4.7km/sec.

For this reason, besides being cold and dark, such an interstellar planet would have an unstable orbit, and it could be either captured or thrown out of the galaxy by gravity assist of a passing star.

agn

The only places where the planet might get enough light/radiation would be in the jet stream of an active galactic nucleus (AGN), except that is in the galactic north and south pole directions, and an orbit above or below the galactic disk would be unstable, and passing through the jet stream from the poles would happen once in a few hundred million years(the night), depending on the orbit, and would last probably a few hundred years(the day), if such an orbit were stable, which it is not.

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    $\begingroup$ I became interested. Is the blue curve an observation or a calculation? Why does it have such an oscillatory character? $\endgroup$
    – dtn
    Oct 12, 2023 at 7:54
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    $\begingroup$ @dtn the blue curve represents orbital speeds of stars around the galactic center at different radiuses, so it is observation data; at the source I put in the post, you can see the above diagram, and further below, another one, showing the orbital speeds of stars in 4 separate galaxies, and you will see they are all different, so this curve varies from galaxy to galaxy $\endgroup$
    – jmarina
    Oct 12, 2023 at 8:03
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    $\begingroup$ Yes, I understand the meaning of this graph, but I still do not understand why the speed of rotation of stars around the center of the galaxy has such an oscillatory character. $\endgroup$
    – dtn
    Oct 12, 2023 at 9:56
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    $\begingroup$ @dtn this is an unsolved problem in astronomy/astrophysics - the galaxy rotation curve difference between expected and actual orbital speed as distance increases from the center is why dark matter was invented, to try to explain why galaxies rotate so fast $\endgroup$
    – jmarina
    Oct 12, 2023 at 11:24
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    $\begingroup$ Why the planet should necessarily be interstellar? Why it can't orbit non-luminous source? like a brown dwarf. neutron star of even a black hole. at the same time it could be illuminated by the jet, as you propose. Moreover, the density of stars per unit of volume is different in different parts of galaxy/universe., so why jet, why central halo can't illuminate such a planet? $\endgroup$
    – dEmigOd
    Oct 12, 2023 at 12:44
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I think that the closest you could get to this is the black hole TON 618. Due to its immense mass (forty billion Suns' worth of mass) it has an accretion disk that kicks out 1.4 x 10^14 times as much energy as our Sun.

Basically, you could be roughly 11.83 million astronomical units away from TON 618 and its apparent brightness would be the same as we experience from our Sun, one astronomical unit away.

11.83 million AUs works out to about 187 light years, much smaller than our own galaxy, however. Someone with a serious education in astronomy would have to tell you whether a galaxy that small could have a black hole of that mass at its center, but as a layman I rather doubt it.

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  • $\begingroup$ +1 I think this is ok - your planet orbits 200 light years from the black hole. But the caveats might be (I) the spectrum of the radiation and (ii) whether the high luminosity quasar phase will last very long. $\endgroup$
    – ProfRob
    Oct 13, 2023 at 12:37
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Apart from being in a dark, lonely place: Galaxies are, relative to their own size, quite close to each other.

The nearest galaxies to the Milky Way are so close that they experience tidal forces, i.e. they are deformed or torn apart. A star so far outside the Milky Way that it does not interact much with its single stars would eventually interact with neighboring galaxies. There are probably no stable orbits.

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Our star orbits around a galaxy, which you can see if you look up at night: The Milky Way. As you can see, it's not bright enough to really call a sun.

The problem is basically luminance density, or you can say candelas per cubic meter, or total flux over the smallest blob that envelops the whole galaxy. It needs to be a lot higher for what you say.

Making stars a lot brighter doesn't make much sense. However, you could pack them tighter. Most galaxies are not that packed, so yours would be unusual and perhaps you may need to explain that, but hey, it's science fiction.

Nothing obviously bad or weird would happen if the stars were a bit closer to each other.

One estimate would be to copy paste 20 solar systems, all at 40 AU from the Sun (orbits of Uranus touching). That gives you 20/1600=1/80 the light of the Sun from the first shell. The second shell, at 80 AU, would have 4x as many, so you get 80/6400=1/80. Shell 80 would be 3200 AU away, would have 6400*20=128000 stars, so the whole system is about 250k. For comparison, our galaxy has ~250 billion, a million times more. Of course, if you pack solar systems that close, something bad or weird probably would happen - the Uranii might collide, the Oort clouds, the orbits of the Suns... But you could space them out more and compensate by having more stars in the whole galaxy, so it seems plausible. Keep in mind that in this oversimplified calculation, the entire galaxy is only 0.1 light year across, which is odd, but it doesn't seem impossible.

Another way is to take the Milky Way and compress it. How much would you need to compress it, to give as much light as the Sun? Taking the galaxy at -5 and the Sun at -26, we it needs to be about 10^8 times brighter. That means things need to be 10^4 times closer. It's about 87k light years across, and instead would need to be 8.7 light years. Again, certainly odd, but perhaps not obviously impossible, and luckily "sort of" close to the other estimate.

One problem I see is that while the Sun makes a lot of nice visible light, a lot of objects in the galaxy make all sorts nasty stuff like X-rays. By compressing them and making them 10^8 stronger, you're also increasing the galactic radiation by a similar degree. That doesn't sound like good news for the people living there.

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A blanet is a member of a hypothetical class of exoplanets that directly orbit black holes.

Blanets are fundamentally similar to planets; they have enough mass to be rounded by their own gravity, but are not massive enough to start thermonuclear fusion, just like planets that orbit stars. In 2019, a team of astronomers and exoplanetologists showed that there is a safe zone around a supermassive black hole that could harbor thousands of blanets in orbit around it.


Is there an habitable zone for black holes?

So planets could potentially form around black holes, but that's no guarantee that they offer a life-friendly environment. On Earth, living things are hugely dependent on the light and warmth from the Sun to survive. Without the glow of a star, life around a black hole would likely need an alternative source of energy. https://www.sciencefocus.com/space/can-life-exist-around-a-black-hole

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