"Are there others out there? Yes. And they're like us. We should try to meet again some day."
It's almost like a modern day Atlantis myth, but true.
If you want a really interesting setting for a game/book, imagine a major regression on Earth (standard post-apocalypse trope), and the colonists finally find a way to visit again...
By the time we managed to meet them again, there would be no reason to expect them to be like us.
> If you want a really interesting setting for a game/book, imagine a major regression on Earth (standard post-apocalypse trope), and the colonists finally find a way to visit again...
This is a regular feature of Motie civilization in the series beginning with The Mote in God's Eye.
Depending on technological advancements and how "hard" you want to consider your sci-fi or future science (purely based on likely developments of current technology, etc), how close to Earth conditions you want to assume the other planet was, how much you believe humans are still evolving, whether you think humans are likely to evolve along similar paths if segregated, whether we are considering mental models only or physical likeness...
So yes, no reason necessarily to expect them to be, but some small chance depending on a lot of factors lining up.
> This is a regular feature of Motie civilization in the series beginning with The Mote in God's Eye.
Yes, it's not an original trope. The rogue star explanation is one of the more feasible I've heard though.
It could easily be the origin of a story of the first interstellar star-ship from Earth traveling across the cosmos, only to reach a planet already inhabited by a human civilisation. (This would presume that knowledge of the traveler world was lost at some point)
Possibly easy to address... maybe, it was small and dim, and in line with a bright star, so we'd interpreted it wrongly.
What's a reasonable ∆v? We get some free ∆v from Earth's orbital velocity of 107,000 km/h, or 29.7 km/s. Our fastest spaceship (which is around sun, https://wikipedia.org/wiki/List_of_vehicle_speed_records#Spa...) is 252,792 km/h, or 70.22 km/s - which isn't including the ∆v to cancel that, ∆v to return to Earth, cancel again.
This 70.22 km/s is about 2 ten thousandths of the speed of light (70.22/299,792). It would take about 20,000 years for alpha centuri (4 ly) to arrive at that speed, and 200,000 years to travel 40 ly, 2,000,000 years for 400 ly.
Of course, maybe we can go faster in future. but propellant is a problem, you have to come back, and a manned mission is orders of magnitude heavier.
Space is big.
funexercise: propose a means of propulsion that could do this.
EDIT a new one is going 692,000 km/h, over twice as fast. At that rate of improvement, maybe we'll get closer to it sooner than I thought. https://www.space.com/41447-parker-solar-probe-fastest-space...
At some point in time (past or future, or magically also present), a star passed in very close vicinity to our sun. It had a habitable planet in its system, full of alien plant and animal life. For the brief moment it was Δv-accessible, it was studied, and eventually colonized. The people going there knew it was going to be a one-way trip, as the star eventually moved further away. Possible stories to set before, during, or after encounter (or even long, long after).
(This works for no-FTL, hard sci-fi setting. By "very close vicinity" I mean, close enough for us to reach it via sublight propulsion, but still far enough not to mess up the inner Solar System gravitationally.)
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EDIT: Wow, turns out this isn't even that implausible - https://news.ycombinator.com/item?id=18123098.