When you think about movies like Avatar or Alien: Covenant, or the new Syfy series The Ark (or other colonizing the planets sagas, like the Dragonriders of Pern or Larry Niven's "Known Space"), the significant problem is getting there. Now, a different example was the world of Firefly, because it had several colonized worlds in the same system -- and something similar to that has already been found, TRAPPIST-1, which recently got a look from the James Webb Space Telescope. So if there were a several habitable planets in a single system - not necessarily likely - getting between them would be easier, as interplanetary space is a lot smaller than interstellar space.
TRAPPIST-1 is only 39 light years away, and the JWST very recently found it's first exoplanet, another rocky world, LHS 475b, only 41 light years away. Very short distances, speaking galactically. But still a very long way to travel, speaking realistically.
NASA’s Webb Confirms Its First Exoplanet
So figure - if it were possible to get up to speed, say 90% of the speed of light (which realistically is still the law), it would still take a colony ship around 40 years to get to either TRAPPIST-1 or LHS 475b. So there would have to be a really great and reliable hybernation/hypersleep system, and it would have to be paired with a propulsion system the likes of which does not exist now (and then there's also the problem of slowing down when you get there).
So while we can write the stories and make the movies, actual implementation -- and a backup Earth -- isn't very realistic.
There's always starseeding, as was done in 2001: A Space Odyssey, and which has been part of other sci-fi envisionings, but that's a different idea with a very long lead time and requiring a lot of patience.