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> Inside a 'thick' nebula, obscuring all other entities in the sky except for their own star and planets. Imagine an interstellar object like Oumuamua passing through and the complete perplexity of a civilization observing it and wondering where it could have come from.

This is almost the exact premise of Life, the Universe and Everything, the third book in the Hitchhiker's Guide to the Galaxy series.

The start of the cricket wars.

Thickness (or rather its optical depth) of this nebula would be wavelength dependent - in IR you could see through all the distant stars, while in all other wavelength, including X-rays, that nebula would glow.

> Near the center of a galaxy, where stars are so closely packed together that the night sky is filled with bright lights, perhaps illuminating their world to what we experience around twilight, or even brighter.

My understanding is this is unfavorable for life due to any planets being constantly sanitized by too many nearby stars going supernova

> Far removed from the center of a galaxy, or even on a rogue star that has escaped its galaxy early on. Very few stars visible in the sky.

My understanding is this is also unlikely since a total lack of nearby stars would deprive a star system of the heavier elements needed to make molecules out of anything more interesting than helium and hydrogen.

It’s possible that a happy medium between too many or few is needed for life (or at least highly favored along a bell curve along which our conditions are more likely to be near the top)

In a big-enough universe, all sorts of unlikely things end up happening.

“Even when you’re one in a million, there are 6,000 others just like you” at cosmic scale.

According to a 2022 study, there are up to ten rogue planets for every one orbiting a star.

A rogue planet has pretty much zero chance of harboring life. It needs a sun.

There's plenty of chemosynthetic life on earth that doesn't need a sun - even if earth style life needed the sun to evolve, if the earth became a rogue planet those organisms might be fine. And life might not have needed the sun to evolve.

Yeah, that doesn't work when you're not receiving any heat whatsoever. There needs to be an energy input. For Earth, that's the sun. Where is the energy coming from otherwise?

> Inside a 'thick' nebula, obscuring all other entities in the sky except for their own star and planets.

Similarly to the alien planet in Eric Flint's "Mother of Demons"; there was a permanent thick cloud cover, and the the slowly moving sky illumination due to the unseen sun was called "the mother of pearl" by the locals.

I also like the idea of living in a dense globular cluster. Interstellar probes and, later, manned missions would be feasible quite early into civilization's technological development - since you're just a few light-weeks (or less) from another star, not a few light-years like us.

I thought the consensus was that stars in globular clusters are exceedingly unlikely to host planets, since they'd be ejected by the frequent interactions with other stars.

Quite unlikely, but see e.g. [1]:

> Therefore, it is still possible that planets exist around main sequence stars in globular clusters, although at small numbers because of the low metallicity, and at orbital periods of >~10 days.

[1] Expected Planets in Globular Clusters (https://arxiv.org/abs/0704.1067)

Oh my gosh, I loved that book! The creative depth with the native creatures there and their experiences with humans was captivating.

What would it be like for civilization to have evolved on a world with a moon (or other large natural satellite) in geosynchronous orbit. There’s just this … thing, in the sky, hovering over one spot. How sacred would that spot be to those people

Good one. The novel Courtship Rite by Donald Kingsbury had such a moon (the Scowlmoon) as part of the worldbuilding.

For another, imagine having a moon so big it can completely cover the sun... and just the right size and distance to only block the photosphere, leaving the corona visible without instruments. What a show that'd be, what wild myths the life there might make up.

Imagine that life is universal and fairly common, but the notion of 'sacred' is rare.

Another cool places for habitable planets to be:

- Active galaxy with a jet stretching perpendicularly to the galaxy plane for distances much bigger than the galaxy's size, shinning in all the wavelengths.

- Planet around a double star one of which is a black hole or a neutron star.

> shinning in all the wavelengths

Just a precaution so we don't get sued.

It took me a couple of passes to catch this subtle Simpsons reference. :-)

Planet around a double star one of which is a black hole or a neutron star

I'm not sure that would be very habitable. Unless they're far apart, the neutron star/black hole would be accreting matter from its companion and the accretion disk would be emitting large amounts of x-rays. In some cases, they can occasionally increase in luminosity by a factor of 10, emitting powerful bursts of x-rays.

I believe anywhere near a neutron star is going to be extremely inhospitable. A very compact massive object, spinning really fast, and with a strong magnetic field, will produce intense synchrotron radiation, right?

Wikipedia says the magnetic field of a neutron star is between 10^8 and 10^15 times the Earth's. The amount of energy in a magnetic field that strong is mind-boggling. Anything anywhere near a neutron star will be bathed in intense broadband radiation, and whipped about by a rapidly-rotating magnetic field. Not a nice place to set up camp.

How about the surface?

https://en.wikipedia.org/wiki/Dragon%27s_Egg

The starquakes can make life difficult. https://en.wikipedia.org/wiki/Starquake_(novel)

>A double planet system

Did you mean a double star system? Or something like Earth-Moon where the size difference between the two bodies is much lesser?

I'd imagine something like the Anarres/Urras double planet system described by Ursula K Le Guin in The Dispossessed (one of my favourite novels) - https://en.wikipedia.org/wiki/The_Dispossessed

> Far removed from the center of a galaxy, or even on a rogue star that has escaped its galaxy early on. Very few stars visible in the sky.

Wouldn't this be approximately the same star density we see when NOT looking in the plane of the milky way?

We are still inside the milky way galaxy. It surrounds us, even if it is denser in one direction.

> Inside a 'thick' nebula, obscuring all other entities in the sky except for their own star and planets. Imagine an interstellar object like Oumuamua passing through and the complete perplexity of a civilization observing it and wondering where it could have come from.

You'd be able to peer through the nebula with higher frequency bands than visible light, no?

If a civilisation is able to notice an interstellar object like ’Oumuamua, they are probably advanced enough to observe what's outside their nebula.

But without a long history of looking up at the stars with our own eyes, how would we ever have thought to point radio telescopes up there?

There might be interesting phenomena going on right now that we don't know about just because they're too far awar from what our natural senses can detect.

A multi-star system where at least one star is almost always in the sky. They'd never see other stars, and their estimate for how many stars exist would be wildly off. And if there ever were a moment where all suns set together, they'd be overwhelmed. (There's a scifi story like this, I think it might have been from asimov, but I'm not sure)

reasonable inputs into Drake's yield 1. So this is just fanciful

That equation is just a back of the envelope guess, not a proven law. We have a sample size of 1 so we can make no generalizations about the trillions of star systems in the universe.

Reasonable inputs into Drake's also yield millions, so that's not helpful.