Sagittarius A* is a black hole at the center of the Milky Way with at least 45 stars orbiting it. It weighs in at over 4.2 million solar masses and is likely rotating, which has interesting effects on the local spacetime. We've taken photos of it too!

What's of interest to me are the stars orbiting it - and more precisely, the planets those stars may have. I have a world (that I am building) that is set on the forty-five stars orbiting A*, and I'm wondering if it's possible that (assuming systems of habitable planets around the stars) life could have evolved despite the "issues" of living near a black hole. For example, S4714 is one such star and passes as close as 3.3 AU away from the black hole at periapsis (minimum end of the error bars, probably closer to ~12.6 AU according to online figures), which would cause time to slow by about 2%, and has an orbital period of 12 years. Additionally, at periapsis, the star - and all its planets - will be moving at around 8% of the speed of light, which is a gargantuan speed for an entire star system. I know about cosmology and the things black holes like that do to the rest of the Universe, but I don't know what effect that it would have on biological life.

What's the feasibility of habitable and life-harboring planets orbiting the stars that orbit the black hole? And what effect would the short orbital period of the star around the black hole have on the natural cycles on planets orbiting the stars (like how our moon affects life here on Earth)?


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Your problem is not the black hole itself. Some black holes have accretion discs that emit too much radiation for life to be comfortable while close to them, but as far as I know Sagittarius A* does not have one that might be (too much) dangerous, and it only extends to about 30 solar radii from it. Also notice that:

Additionally, at periapsis, the star - and all its planets - will be moving at around 8% of the speed of light, which is a gargantuan speed for an entire star system (...)

There is no priviledged frame of reference. From the planet's point of view, it is the rest of the galaxy that is moving at that speed. On its own frame it is static.

The real issue is not what you are orbiting in this case, but your neighbourhood. Some astronomers believe that just like stars systems have goldilocks zones in them, so does our own galaxy. See the Wikipedia entry for the Galactic Habitable Zone. The galaxy core in particular is full of smaller black holes, and heavy stars that may go supernova at any moment. Even if they don't, your system is bound to have a lot of close flybys, so planets may easily go rogue and asteroid and comet bombardments will be frequent.

The amount of things orbiting that black hole makes it possible that in geological time, even the system as a whole may be ejected from the core. A star system with life could, out of serendipity, dodge all the novas and other black holes as it is hurled outwards across the galaxy... in the end it wouldn't be orbiting around Sagittarius A* anymore.

  • $\begingroup$ Good points all around. I would think then that especially in the galactic core the chance of the star sticking around the BH for long enough for life to evolve would be closer to zero than to anything else? $\endgroup$ Commented Jan 12 at 23:23
  • $\begingroup$ @controlgroup I think so. Keep in mind that the galaxy core has plenty of red giants (I don't know if the Sagittarius A* cluster has any). Those far from the center but still in the core might have harboured life at some point (though it's much more unlikely than likely). $\endgroup$ Commented Jan 12 at 23:47
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    $\begingroup$ I suppose some "artistic license" (hand-waving) is thus required. Or perhaps I should choose someplace other than the galactic core to plant life. Thank you for the information! $\endgroup$ Commented Jan 13 at 1:20
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    $\begingroup$ The main "artistic licence" you will need is to be able to break the galaxy speed limit. Otherwise people from our world will never be able to meet the people from your world near the centre of the galaxy. $\endgroup$ Commented Jan 13 at 16:31

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