Interstellar travel now seems a lot more hazardous than it did before. To quote one of my TAs in college, "it's the stuff you can't see that gets you" ...
The Milky Way has 4E^11 stars.
It has an upper estimate of 4E^16 planets.
It has a lowest estimate diameter of 100kly.
It has an estimated thickness of 2kly.
It's lower estimate volume is 63,000 kly^2.
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If planets are uniformly disrupted (they surely aren't), there are ~17 planets in every cubic light year. Sure, that's a hazard.
The change of hitting one while traveling one light year is in the order of 10e-16.
[1] at least in Newtownian physics. The argument is geometry, so I suspect but am not qualified to say it isn’t exactly inverse square in GR.
1. My numbers are on uniform distribution of planets. The milky way is not uniformly distributed, and I suspect this isn't either. I hypothesis most of the planets will be on the spokes, just like most of the stars are. In addition, while it is 2kly tall, there is not a uniform distribution across the height.
2. Interstellar travel doesn't involves traveling a minimum of 4 lys, and a maximum distance without being intergalactic would be about 100,000lys.
3. Your assuming instantaneous travel across the light year. In reality we would consume at least 1 year in travel time, and likely much more.
4. The amount of resources it takes to build an intersellar ship is very large. The cost of a collision is high. This doesn't change the risk of a collision, but it does change what we consider acceptable risk.
The earth itself is only 0.02 light seconds in diameter, yielding a cube of 8000 cubic lightmilliseconds. Compared to the 2.910^31 cubic light milliseconds in a cubic lightyear... It's a rounding error inside a rounding error.
For light seconds, you're looking at 2.910^22 (if my math isn't wrong). That's the amount of cubes fitting between earth and moon to fill a cubic light years.
Even superplanets with 15 times the size of the earth are simply dwarfed by the sheer volume of a cubic light year.
The largest stars we found measure 73 light minutes, a cubic size of 389017 cubic light minutes. How much of them fit in a cubic light year? 346 billion.
I guess the point people are trying to make here. If you can't see light coming off an object, then while traveling at interstellar speeds means it will be harder to spot objects until they get too close.
I think if you were to encounter a black hole, esp an inactive one, in interstellar space, you'd probably just get sucked in if you didn't notice the black disk...
If a black hole has been travelling without eating for long enough the accretion disk can disappear. If a black hole is large enough it won't form an accretion disk either as it swallows stars whole.
So while I agree it's still probably very small numbers I think you may be overcooking it.
Why travel in a spaceship? If there's 100,000 starless planets for every star in the galaxy, then travel between stars on one of them instead. Build the habitations 10 km below water to avoid radiation. Travel at 0.1% light speed instead and get to the nearest star in 5000 yrs. Enjoy the journey, and slip into an orbit around the target star.
Speaking of pea-sized rocks, periodically fire one laden with electronics/optics/quantumics at the target star at near light speed and receive its transmissions for recon.
These would act as space weather beacons, forming some kind of illuminated space (high)way. An interesting idea!
As for the rogue planets, although I envision humans having some use for them, I doubt it will be for interstellar travel the way you describe.
If most planetary systems are anything like this one, planetary orbits are quasi-stable, and a visitor could easily destroy that stability.
It would take a while for that to play out - anything from months for a very unlikely very near miss, to many millennia for a distant visit.
Collision with planet sized object in space would be extremely rare. Even if you would travel blind it would be very unlikely that you would hit star or plant while traveling trough galaxy.
I'd imagine you could detect it blocking a circular patch of sky more easily and from farther off than you could detect the gravitational pull on your spacecraft.
Just don't steer twords the black disc and you'll be fine.
What’s one wavelength per minute if the formation is 1000km?
Or changes in density or relative velocity of the interstellar medium, etc.; gravity isn't the only feature that can alter the formation.