This was a really fun project! I spent quite a bit of time on it and took pictures of some of my trial & error of the planets. The cake itself was pretty straightforward. The birthday boy (not my son; a friend's son) wanted chocolate (and, he wanted the cake to be space-themed). So, since the cake was supposed to feed a crowd of 50-60 people, I used five Devil's Food cake mixes.
One standard cake mix fits perfectly into a half-sheet pan, so I baked four of those and the fifth cake mix was for the sun. The final product was quite heavy: about 22 pounds. It needed a strong base; so I cut up a large cardboard box and hot-glued three layers of regular corrugated cardboard together (and then hotglued foil over the whole thing). Three layers thick of cardboard ended being just right for this cake.
Most of my cake layers didn't break when I transferred them, but of course the one I took pictures of was the broken one :) Since I think it helps cakes to be moist, I usually bake-ahead-and-freeze them, but there wasn't room in my freezer for these. So, to ensure that they were nice and moist I brushed each layer with a simple syrup: just water & sugar brought to a boil, and I added a little vanilla to it before brushing it on.
Following the simple syrup I covered each layer with a full can of store-bought vanilla frosting.
I used canned frosting between each layer, but I made my own buttercream to cover the entire cake (four layers) in a thick crumb coat. I used part butter and part shortening for a nice flavor and consistency. After the frosting had had a chance to dry out & set up, I patted it nice and smooth all over with a paper towel so that it would be a smooth finish for the ganache, later.
Once the cake was coated (the whole thing; I just hadn't finished the sides when I snapped the photo above), it was time to figure out how to make the planets. I looked online for ideas but ultimately only had success with lots of my own trial and error. I'd planned on making the smaller planets out of balls of rolled fondant, but for the bigger ones (Jupiter & Saturn) I wanted them to be hollow so that they'd be light enough to stand up on a sucker stick above the surface of the cake.
So... how would you make hollow, edible planets???
I mixed up a batch of royal icing and tried covering a greased balloon with it, with the idea that I could let it harden and then pop the balloon. Well, this was a small, thin, flimsy water balloon and it popped under the weight of the royal icing just a few seconds after I took this picture. Maybe a more sturdy balloon would've worked. But I just gave up on that idea.
Then I thought, I'll make two half-spheres and stick them together. I decided that a tennis ball was a good size, so I made a half-sphere mold out with a tennis ball and greased foil.
And, I tried the "inverted" version. Neither of these worked. Even letting them harden overnight, and doing several of them with different thicknesses of royal icing, either they didn't harden completely, or they were too crumbly and delicate.
So how about melted sugar? The standard recipe for homemade hard candy is 2 parts granulated sugar to 1 part light corn syrup, cooked until hard crack temperature. First I tried using the inverted (bowl-shaped) foil molds, and drizzling melted sugar back & forth to create sort of a nest. I've seen this done on TV so I thought it would work :) Well, it wasn't a bad idea but the results all broke -- too delicate. And, definitely too delicate to cover with fondant, as was my plan.
Then I tried covering the tennis ball with waxed paper and spooning a thicker layer of sugar onto it. The results were better: a sturdier half-sphere, BUT the waxed paper stuck to it (which meant it would not be 100% edible).
Finally! The solution!! Greased foil wrapped around a tennis ball: I spooned the melted sugar (which was over-cooked by this time, hence the amber color) over the greased foil, and after it hardened (which only took a couple minutes) it was easy to squeeze the tennis ball out of there. And, the foil peeled off quite nicely. Hooray!! The sugar was thicker so it would be strong enough, so it was heavier than I really wanted but it was nice and sturdy. In the background of this picture below you can see Saturn's ring, which I made by drawing a circle onto a piece of paper and putting it underneath my Silpat, then spooning melted sugar onto the Silpat using the circle as a guide, and then quickly running a toothpick that had been dipped in food coloring through it to give it a marbled color.
Using more melted sugar I glued two halves together to make a circle, and for Saturn I adhered toothpicks (with more melted sugar to make them nice and strong) on either side to later support its ring (after I covered Saturn with fondant, I glued the ring to the toothpicks using - you guessed it - melted sugar).
For the smaller planets, I'd planned to make balls of fondant, but my fondant was too fresh and not firm enough to hold a good shape. So, I used suckers for the form and covered them with fondant. The smallest planets (Mercury & Pluto) were dum dums, and the medium-sized planets were Tootsie-Pops. Once I covered them in fondant I stuck their sticks into a piece of floral foam so their surface could dry out a little before I painted them.
Painting them was the most fun part of the whole thing! I decided this method would be easier to get the colors I wanted, rather than mixing up several colors of fondant. I used mostly food coloring gels, thinned out with a little water.
I made the sun the same way as when I made Noah's basketball cake, a few years ago: baked 3 small rounds, stacked them, and had my husband carve them into a half-sphere for me. I didn't take any pictures of the creation of the sun, but it's just chocolate cake with vanilla frosting between the layers, crumb coated all over, and I put it in the fridge to firm it up for a while before it was time to put it on the cake.
Oh, and as you can see, the cake is covered all over with chocolate ganache (chocolate+cream). I wanted the backdrop to be dark, without having to make something like black fondant. Ick. This was much better and I think it was dark enough for the "space" effect. A little ganache goes a long way, as it's quite runny (I made WAY too much of it) -- but I knew it would be viscous enough to flawlessly cover the white frosting underneath it.
Once the ganache was on and had firmed up a bit, I put the sun on, and frosted it with yellow buttercream that I'd made to a pretty thick consistency so that I'd be able to make it all spiky-looking. I added some dabs of orange food coloring gel to try to make it look more sunny.
Since most of the planets were on suckers and therefore already on sucker sticks, I hot-glued longer sucker sticks to the ones that I wanted to stand up higher. Jupiter & Saturn proved too heavy to be more than barely above the surface. I rolled out some yellow fondant for the stars and painted the letters on them with food coloring gel.
This was a really fun cake and it turned out pretty similar to the sketch I'd drawn out ahead of time. I heard that the birthday boy loved it (I wasn't able to be there to see his reaction) and I heard that it tasted good, too :)