I put more thought into this than is probably evident and I want that to be appreciated. :P
The book doesn't say much about the planet Adrian, at least that I can recall — it has a surface gravity of about 1.4g, and its upper atmosphere is largely carbon dioxide and opaque on the human visible spectrum. However: it also contains astrophages, the most ludicrously overpowered phototrophs in the known universe, as the base of its ecosystem.
The maths should be doable, though I haven't bothered to actually do them. An organism's trophic level is calculated based on how many links of the food chain are between it and its basic energy producers. Only about 10% of any creature's food is retained after digestion, which means the total energy available decreases exponentially; the apex predators of Earth's biosphere cap out at about trophic level 5 at most, because between them all combined they have only 1/100000th the energy of all the plants in the world and that isn't enough for anything else to survive by specialising in hunting them. But Adrian's base trophic level aren't mere plants, they're migratory sky plankton that sup directly from the solar corona — they'd easily overshadow the energy input of ordinary photosynthesis.
That isn't even getting into the fact that astrophages didn't evolve in the vacuum, and there are certainly other species in its family tree with the same specialised organelles for absorbing light, that just didn't develop the adaptations that turned astrophages into a celestial plague. Adrian's ecosystem must be ludicrously energetic.
Back to the premise. U wasn't sure why going to space would help, but it makes sense to me that if the problem is poisonous rocks falling from space, the obvious solution is to throw them back. Technically, that doesn't require space travel — if you don't care about the integrity of the payload, all you need is a really big cannon that can fling shrapnel at escape velocity. But, once you put all that effort into building a giant railgun... are you just going to expensively dismantle it again? What if we put a computer in it and shot it a little more slowly so it could look around and then fall back into the atmosphere and tell us what it saw? What if we gave it a little fuel so it could alter its ballistic trajectory and stay up there? What if we put someone in it? And I'm really enticed by the idea that the native Adrians' first spacecraft would make Jules Verne proud, because that means humans, Eridians, and Adrians would all have three completely different methods of basic orbital spaceflight informing their approach to the stars.
Humans gave up on the space gun idea because it frontloads all the acceleration into a single sudden impulse that would pulp anyone in the projectile. But do you know what kind of multicellular organism evolved to handle incredible g-forces without effort? Peregrine falcons.
So what I'm imagining as the structure of Adrian's biosphere is something like Earth's oceans. All the major energy producers are atmospheric microbes, drifting on the surface of the clouds where the sun still shines, being fed on by larger and larger organisms that slowly pass the energy all the way down to the abyssal zone; except, unlike Earth, there's so much energy to begin with that even the abyss is vivacious. And Adrians descended from flock-hunting bats-of-prey (feathers are a bizarre evolutionary fluke), with a touch of Humboldt squid: they nest on the surface but regularly soar up to several kilometres above it, and their ancestors could dive-bomb their prey en masse while shrugging off the pressure changes from the altitude. Large eyes are far-sighted and sensitive to longer wavelengths that aren't absorbed by the clouds, while the smaller lateral eyes are short-sighted. Their analytical intelligence derives from the need to quickly react to subtle changes in the wind or far below them (they can't throw, unless you count "spitting" or "swooping-and-dropping", but this does give them a instinct for ballistic motion in common with humans that Eridians do not share), while their tool use is originally nest-construction related, and their social instinct emerged from the advantages of flocking to gang up on larger prey with worse reflexes and to protect each other from competing predators.