Sorry if this has been explained before.

So, as I understand it, there are 9 planets in Mundus: Nirn, and the other planets which are actually the bodies of the 8 divines. Is that right?

But then I've heard other things like Nirn is the only "planet" planet and the other planets aren't really - um - planets, in the same sense...??

So what are they? Are they made of rocks and dirt and stuff?

is it proper to call the planets by their divine names (Akatosh, Dibella, etc.)?

And would any sort of space travel be possible in the TES universe? Like could you conceivably take a spaceship and go land on Kynareth and explore it? Do aliens exist on those other planets?

The divines are planets like Nirn, depicted in game through the http://www.uesp.net/wiki/Oblivion:Orrery. However, unlike Nirn, which is finite, they are infinite, but mortal mental stress from looking upon infinity makes them appear as spheres.

There have been manned flights through space (Oblivion) in the form of Imperial Mananauts, Vehkships, and the Alinori Sunbirds, which were quite literally "large birds made out of the sun". Those manning them were apparently able to, or at least attempted to, fly through the stars in order to reach Aetherius.

It's conceivable that the Aedric afterlives, should they exist in the same sense as their Daedric counterparts, exist on their corresponding planets.

Nirn itself is a finite ball of matter made from the bodies of fallen et'Ada, with the Earth Bones they left behind forming its physical laws. The Daedric planes are planets in much the same way the Aedric planets are, each formed of creatia, so it would be safe to assume that the Aedric planets are made of dirt and rock as well.

Plus you have stuff like the Battlespire, which is basically a space station if you think about it. A lot of stuff IRL has ingame counterparts, even the electromagnetic spectrum and quantum mechanics are accounted for in the form of Magicka and such.

Visit to Aetherius occur even less frequently than to Oblivion, for the void is a long expanse and only the stars offer portal for aetherial travel, or the judicious use of magic. The expeditions of the Reman Dynasty and the Sun Birds of Alinor are the most famous attempts in our histories, and it is a cosmic irony that both of them were eventually dissolved for the same reason: the untenable expenditures required to reach magic by magicka. Their only legacy is the Royal Imperial Mananauts of the Elder Council and the great Orrery at Firsthold, whose spheres are made up of genuine celestial mineral gathered by travelers during the Merethic Era.
http://www.imperial-library.info/content/pocket-guide-empire-third-edition-magic-aetherius

The planets are the gods and the planes of the gods, which is the same thing. That they appear as spherical heavenly bodies is a visual phenomena caused by mortal mental stress. Since each plane(t) is an infinite mass of infinite size, as yet surrounded by the Void of Oblivion, the mortal eye registers them as bubbles within a space. Planets are magical and impossible. The eight planets correspond to the Eight Divines. They are all present on the Dwarven Orrery, along with the mortal planet, Nirn.

Auriel-that-is-Akatosh returned to Mundex Arena from his dominion planet, signaling all Aedra to convene at a static meeting that would last outside of aurbic time.

http://www.imperial-library.info/content/nu-mantia-intercept-letter-4

It sounds to me like the planets are not literally the Aedra themselves, but something they hold dominion over. Not unlike how the Daedra Princes' territories may be interpreted as.

As for what they're like up close, I've no idea. I know of no account of any visits to the Aedric planets.

The language there doesn't exclude the gods are their planets.

Eight stars fell on Tamriel, one for each iniquity that Lorkhan made clear to the world...

Kind of like with Daedra Princes, they are their realm and the realm is them. Like for instance when Vile's realm was reduced in size because a piece of him was missing during the Umbriel Crisis. If anything the Prince/God is the representation of the conscious will of the realm made manifest, or at least that is how I have always thought of it.

One of the things that's always bothered me about ES lore vs games is the whole Aetherius / Afterlife issue. It seems like Aetherius is often equated with the afterlife (e.g. Sovngarde) and yet it seems like the afterlife in those depictions should more accurately be one of the Aedric Plane(t)s (again, looking at you, Sovngarde). Can this just be interpreted as confusion on the part of the narrator/author/player? Or is there actually a connection between the Aedric Plane(t)s and Aetherius?

Aetherius as ideal afterlife land might be a hold over of the Altmeri's take on ascendance. After all, Auriel is credited going there after the whole Mundus debacle, and the Imperial Cult is at least partially influenced by them. The Nord's themselves don't really talk about Aetherius, beyond the PC's journal saying that they're there, which is suspect at best.

But did Vile lose a piece of himself, or a piece of something he crafted from aetherial refuse and laid claim to?

“Don’t think I’m weak,” Vile said. “Everyone who comes here now thinks I’m weak, just because a wee bit of my stuff has been stolen. The trick is, if you’ve got less to work with, you just don’t spread it so thin. My realm may be a little smaller than in happier times, but in it I’m just as strong as I ever was.”

http://www.imperial-library.info/content/lord-souls-lore-notes

I think both, CP. Umbriel (?) was still his, but wayward. The Daedra are like havoc wellheads: they can lose pieces, because they spring from an eternal source.

Now when the Daedra Lords heard Shezarr, they mocked him, and the other Aedra. 'Cut parts of ourselves off? And lose them? Forever? That's stupid! You'll be sorry! We are far smarter than you, for we will create a new world out of ourselves, but we will not cut it off, or let it mock us, but we will make this world within ourselves, forever ours, and under our complete control.'

http://www.imperial-library.info/content/monomyth-cyrodiilic-shezarrs-song

I agree with Bactrian that it makes sense to be something of a combination. Along the lines of, yes, they crafted something tangible out of creatia that leaked into Oblivion, but their realms are instilled with their power and they have a measure of omnipotence, omniscience, omnipresence there "within themselves" that they do not have elsewhere. They "are" their realm because they can do anything there. But if a piece gets cut off, it's not so much damaging to the Daedra itself as it could be viewed as a weakening of their dominion.

Granted that if Vile were to shrink his realm, as he did, he would be just as strong as he was prior within that shrunken realm. However, with less to work with in total while Umbriel was out of sync with him, you would imagine that his influence outside of his realm would be consequently weaker as he has less resources to work with.

Also, do the Daedra really spring from an eternal source? I always thought that even though pieces of themselves might be sent to Mundus (such as Azura's Star) those pieces were still in accord with the Prince. Umbriel, on the other hand, does appear to truly be cut-off in that Vile was unable to access any of the power contained in it or Umbra.