Just saying there's no space is kinda misleading. There is space, it just isnt part of the same plane of existence as Runeterra. Those things you said, sun, moon, comets, shooting stars and meteors, they exist, but they're on the celestial realm. There isnt a space like we understand it irl, rather they are something in the celestial realm(that can include a lot of things, including Aspects, Aspect of War/Pantheon was literally a constellation) and are seen in the physical realm as objects in the night sky. That's probally why Interlocutioner said the Moon existing in the physical and spiritual realms is special btw, other celestial objects probally exist only on the celestial realm, so being part of Runeterra instead of only the celestial realm makes the Moon special.
And who needs Neil deGrasse Tyson when you have World Runes shaping reality?
Here are some quotes reinforcing that idea btw
An age ago, when time itself was young, the inhabitants of the celestial realm regarded the fledgling races of Runeterra with growing concern.
These creatures deviated wildly, unpredictably, and dangerously from the great designs intended for them by those above. The guidance and fates that had been woven into the night sky often went unseen...
Soraka bio
The Darkin’s god-killing blade was driven into Pantheon’s chest, a blow that carved the constellation of War from the heavens.
Pantheon bio
And though I am broken, and though the god is dead, I feel the power ignite once more in my spear, as the plume on my helm bursts alight. It is calling me to battle, as the Ra’Horak cast their spears once more.
And, for a moment, a star lost with the Constellation of War gleams brighter than the sun.
For Those Who Have Fallen
The celestial realm is not space. There is only one world in the setting, and that is Runeterra.
The Void is un-creation, and the celestial realm is creation. Mortals can't truly comprehend either, because we exist between them.
If you go "up" from the surface of Runeterra, you will not leave the atmosphere and enter an empty vacuum. It is a world, but not a "planet".
Imagine the way folks thought about our world in the 15th and 16th centuries. It's like that (Except they were wrong, and this isn't!)
https://twitter.com/lauriegoulding/status/1172912381875249152?s=21