The most important feature is to have manual settings. With just (semi)automatic, you can't do much. Since the best camera is the one you actually use and bring with you, don't get to cheap on it.

If you need (close-up) photos on jewels, you would most likely either need a great compact, or a DSLR combined with a macro lens. The latter one is very expensive.

The problem is, to get a great photo on the planets, or even the moon, you need either 1) A telescope (with the ability to mount a camera; no problem with a DSLR), or 2) a tele-lens (Say, 300-500m).

The sun is hard though, depending on how you want to shot it. But, the more you shot the more creative you get, and the more unique stuff you see. Sunrise, Sunsets... What you can do really comes down to your own imagination and creativity, combined with skill, luck and opportunities.

Some other example photos I've taken:

Moon through a 8" dobson, handheld with a cheap compact, Olympus X560WP

Startrails with D90, manual exposure 600+ seconds. I wrapped a rubber around the shutter and used books to keep it looking up.

With my 105mm lens (effectively 157mm since it's dx) I can only get this close to the moon.

And the last and hardest, planets. My D90 + 105mm only get me this close to the by far biggest planet in the Solar System. For me, it's so cool to be able to get the Galilean moons without any other equipment.

But all in all, there are so much other related phenomena to shot that you really should not be dissapointed if your camera does not mean all your constraint. The by far most important is that you end up use your camera, and that you are happy with it. Later, you can invest in better equipment (better camera, lenses, tripods etc).