There isn’t a crystal clear boundary; both science and science fiction achievements require imagination:
There is an Earth-like planet orbiting our Sun’s closest star.
Found by ESPRESSO, part of the Very Large Telescope Array in Chile, it is currently only a planet candidate — that is, it awaits confirmation from other astronomers — but the researchers are confident that it will check out:
Astronomers have discovered a third planet orbiting Proxima Centauri, the star closest to the Sun. Called Proxima Centauri d, the newly spotted world is probably smaller than Earth, and could have oceans of liquid water.
“It’s showing that the nearest star probably has a very rich planetary system,” says Guillem Anglada-Escudé, an astronomer at the Institute of Space Sciences in Barcelona, Spain, who led the team that, in 2016, discovered the first planet to be seen orbiting Proxima Centauri.
Davide Castelvecchi, “Earth-like planet spotted orbiting Sun’s closest star” at Nature (February 11, 2022)
Proxima Centauri d is much smaller than Earth, at 26% of Earth’s mass. Its star is 4.25 light years away, so we would need to reach near light speeds to physically explore the planet within reasonable time constraints. But it’s something to aim for and science fiction around it could be grounded in reality.
News, “Recent science papers support science fiction premises” at Mind Matters News (February 12, 2022)
Takehome: Of course, science can deal only in fact but many of the facts scientists are unearthing can support science fiction premises. Five examples are offered here.
You may also wish to read:
Is life from outer space a viable science hypothesis? Currently, panspermia has been rated as “plausible but not convincing.” Marks, Hössjer, and Diaz discuss the issues. Famous atheist scientists have favored panspermia because there is no plausible purely natural explanation for life on Earth that would make it unnecessary.
and
Science paper: Could octopuses be aliens from outer space? It’s the octopus’s intelligence that causes such usual theses to float in the science literature. There is no simple way of accounting for how smart the eight-armed invertebrate is. So, even if we dismiss an extraterrestrial origin, we still face a mystery.