This is covered in large part in the above racegen docs, but I can
make some comments about common questions that arise. Of all types of
planets, class M's are chosen for homeworlds around 75% of the time.
This is because they are the largest of any type planet except
Jovians. Thus, they can support a high population (for taxation and
tech purposes), and they usually have a fairly high number of
resources. Another common choice is desert, since it is on the same
order of size as a class M planet. Deserts have more resources, but
are less fertile and are rarer in some games than class M's (also, you
should take 100% desert as a sector preference, which means you can't
spend as much on the other, more common, sector types).

Big planets have two big advantages:

1) The larger the planet, the faster you will produce resources from
   it once you have covered the planet. So the more sectors you have, the
   better. Races that select small home planets tend to get left behind
   early on in the race to build the first ships for this reason.
2) The planet is much harder to conquer. Even if an enemy manages to
   field a temporarily superior force to your planetary population, odds
   are he won't have the AP's necessary to take the whole thing in one
   update, so you have time to marshal reinforcements if you're in a
   bind.

Here's a brief summary of the various planet types. The relative sizes
of each of these planets, as well as the benefits (and % of races
which choose each type of planet, etc.)

! will vary from game to game !

When using racegen, you'll typically pay (or get a bonus) for the type
of planet you choose to start with, and the deity will try his best to
see that your points are fairly spent.

Class M - - These planets are usually about 60% water, 20% land, and
an even mix of everything else. Once in a while, you'll find a class M
with an abnormal atmosphere (like heavy in methane content).

Jovian - - These planets are 100% gaseous, and they are usually twice
as large as the typical class M planet. They tend to be very high in
fertility, too, so you can easily build up a large population for
taxation and tech purposes. Also, ships in orbit around Jovians add
fuel to their holds every update (tankers are twice as efficient at
this), so even if you're not a Jovian-type race, having one of these
nearby can be a tremendous asset.

Water - - These are largely water planets, and I've found that they
are generally resource poor. They can be quite large, ranging up in
size to class M quality, but they are usually about 40% smaller. On
some versions there is also mountain sectors. That will raise the
resource depository a lot.

Desert - - These planets are usually 80% desert and 15% mtn/land. They
are _very_ nice as far as resource content is concerned, and their
size range is much like Class M planets.

Forest - - These planets are almost entirely covered in forest, which
is the rarest sector type. Resource content is fairly good. Size range
is between Water planets and Class M's. Fertility is higher than on a
normal class M.

Iceball - - These planets are generally small (1/5 the size of class
M's or less), and they consist of around 75% ice and 25% mountain.
Relative to their size, they are resource rich, but the small number
of sectors means that resources will come more slowly.

Airless - - These planets are mostly land (75%) with some mountain and
ice sectors. The atmosphere is almost always hostile, of course, but
these planets are probably the most resource rich relative to their
size (ie. 1/3 the res of a typical class M but 1/5 the size).

Asteroid - - These are just floating rocks in space, not good for a
heck of a lot. They don't count as planets in victory conditions, and
they usually have very few resources, and the small number of sectors
make it very difficult for most asteroids to ever contribute to your
cause, though having colonies on them have other advantages, such as
morale bonus. Sector types are random but range among land, desert,
ice, and mountain.