For Science! Well ok, science fiction is often based on science. So here's some things your science classes may not have covered, or did and it was a long time ago.
We'll start with the Earth, as much of what you'll come up with in a story is naturally going to start with earth and expand from there. When you create a new planet, consider some of these numbers before you go.
Stats about the Earth:
Age -> approximately 4.54 billion years
Weight (mass) -> 5.9736×1024 kg
Mean radius -> 6,371.0 km
Circumfrence -> 40,075.017 km
Area -> 510,072,000 km, 148,940,000 km2 land (29.2 %), 361,132,000 km2 water (70.8 %)
Distance from sun -> 150 million kilometers or about 8 light-minutes
Orbital speed -> 29.78 km/s (107,200 km/h)
Escape velocity -> 11.186 km/s
remaining 1.2% consisting of trace amounts of other elements
361.132 million km2 (139.43 million sq mi)
Currently the total arable land is 13.31% of the land surface, with only 4.71% supporting permanent crops
The elevation of the land surface of the Earth varies from the low point of −418 m at the Dead Sea, to a 2005-estimated maximum altitude of 8,848 m at the top of Mount Everest. The mean height of land above sea level is 840 m.
Three-quarters of the atmosphere's mass is contained within the first 11 km of the planet's surface.
78.08% nitrogen (N2) (dry air)
20.95% oxygen (O2)
0.039% carbon dioxide
About 1% water vapor (varies with climate)
Earth has at least five co-orbital asteroids, including 3753 Cruithne and 2002 AA.
A trojan asteroid companion, 2010 TK7, librating around the leading Lagrange triangular point, L4, of Earth in Earth's orbit around the Sun.
Common planetary 'tweaks' and their real-world affects
Orbits: Once you have one large mass in an orbit, there are only so many other places in the same orbit that other masses can be placed and still be stable enough not to knock each other off course. (Remember, planets have gravity too!) These are called Lagrange points. There are two close points (roughly 60 degree angles from the largest mass), and one on the far side. http://en.wikipedia.org/wiki/Trojan_%28astronomy%29
Moon Orbits: Moons also observe the rules on Lagrange points (though more than one object can occupy a point). Moons orbiting other moons is rare enough we haven't discovered any examples. Moons come in two flavors: 'standard' and 'captured'. A standard moon is one that formed at around the same time as the planet from the same material - it's leftovers. A captured moon is usually an asteroid that got too close and got pulled in. Most moons for anything smaller than a gas giant are standard. Gas Giants tend to collect things (it's a gravity thing).
Low gravity: A low gravity planet, presuming it is earth sized and rocky like earth - must therefore be less dense. Most likely less iron in the composition. The atmosphere will have more height on the surface - but thinner to breathe as gravity is less able to hold air in place. Lower air pressure means colder air and thinner air means less intense weather patterns. It is possible there will be no atmosphere at very low gravities because the air will have dissipated. The escape velocity will be lower. People will weigh less (though have the same inertia as they have the same mass), so expect bounciness in movements of those used to higher gravity. Plants and animals will be larger! With less risk of structural damage as volume increases, the limits will be higher. Limbs will likely be thinner, as they don't need to be thicker to support weight.
High gravity: Presuming earth sized and rocky - more dense. More iron, or a heavier metal. The atmosphere will be shorter and thicker. It may smell of ozone as the 'height' is shorter. Weather will be more intense, and generally hotter due to air density. Escape velocity will be higher, and lift off in general more difficult. At very high gravities, the air pressure may crush windows or people, or burst ear drums if directly exposed. People will weight more, and find it more difficult to move - though this is good resistance training if gravity is only a little heavier. Plants and animals will be shorter and rounder, as spindly supports are more likely to snap under weight.
Less oxygen: Conditions as per the top of Mt. Everest, or at less extremes Denver, CO. Harder to catch your breath, respirators may be required. Exercise will be harder.
Less oxygen due to elevation: The sky will be more intensely blue (or whatever shade your refraction on the sky is), as there will be less of it between the viewer and space. Alcohol has a greater impact on the human body. Humidity is lower and dehydration is a big deal, the thinner air wicks moisture out.
More oxygen: Mostly unnoticeable until it's really noticeable. Too much oxygen in the air causes a 'drunk' effect on humans. Way too much oxygen in the air can cause an explosion. Keep in mind, earth's atmosphere is only 20% oxygen. High oxygen content may also cause giantism on insect-like species: http://www.wired.com/2010/11/huge-dragonflies-oxygen/ This effect is specific to insects due to the structures that allow them to breathe.
No ozone layer: Hope you like cancer. Plants and animals will have defensive modifications to handle solar radiation - waxy coverings, thickened hides and scales. Water levels will mostly be unchanged, but planet may be colder as energy from the sun will not be trapped. So potentially more ice, and less humidity.
Extra ozone: No cancer! But temperatures will be higher as what radiation does get through will tend to be trapped. Higher temperatures, more water, more humidity in the air.
Multiple moons: Very complex tidal interactions. More extreme tides. Sea life and coastal life will be adapted to these phases and extremes. Possibly shells or air breathing adaptions.
Closer to the sun: Hotter on average. Within a certain range most of what determines a planet's atmospheric heat is composition (ozone and other gases) not proximity to the sun, but past that point the sun's own radiation will be intense enough to set the standard. The sun will be larger in the sky. Radiation more intense. The closer to the sun, the more likely solar wind (electromagnetic forces) will interfere with electronics causing short outs, unless the planet has a strong ionosphere to deflect intense solar winds. Ships in orbit need to be shielded.
Further from the sun: Smaller sun, potentially small enough to be the slightly-larger dot in the sky. Past a certain point the sun's radiation is so lessened the planet will be cold, or the atmosphere extremely heavy to retain heat.
Water world: What? 70% water wasn't enough for you? The earth is actually a higher percentage of water than average planets. In a fully water world expect all the depth and variety of life on earth - only water based. No other major changes. "Land" may be found in the form of ice.
Bipedal lifeforms: Walking on two legs is evolutionarily *EXPENSIVE*. It is limiting, and as far as the animal kingdom is concerned, nearly crippling. Bi-pedal movement is slow, awkward and makes falls very dangerous. The only reason to go bipedal is to support use of two limbs for tool use. In general if nature can avoid that - it will. For example - octopuses use tools, and while they tend to reserve two of their tentacles for that use - they have plenty to spare and so their tool use has not changed their body-form.
'Similar to Earth flora and fauna': Incredibly unlikely. Some things are fairly standard in evolutionary terms - but the outward appearances of things aren't. Keep in mind on an alien planet what looks like grass *isn't*. Things that seem fairly standard: biplanar symmetry (the left side and the right side look alike), though radial symmetry such as seen in starfish is possible as well. Bipeds tend to be tool users. Sexual reproduction is common on higher order lifeforms as a way of increasing genetic diversity. An extremely successful asexual species is possible as well, but will likely be very vulnerable once something comes along that *can* kill it. (Hence why they are less common in higher order lifeforms). Basically - if the description of your planet includes "very earth like" for flora / fauna, or there are people there - then expect to be asked to include some backstory regarding terraforming or colonization efforts.
Climate Differences for Animals Colder areas tend to result in larger breeds of animals with shorter limbs compared to the same animal in warmer areas.
See: http://en.wikipedia.org/wiki/Bergmann%27s_rule Bergmann's rule is an eco-geographic principle that states that within a broadly distributed taxonomic clade, populations and species of larger size are found in colder environments, and species of smaller size are found in warmer regions. Aka: Locally adapted breeds closer to the poles are usually larger than those closer to the equator. For example: Cats! Cats totally follow this rule. There's a reason the largest cat breeds in the world - the Maine Coon, the Norwegian Forest Cat and the Siberian are all from cold areas.
Also see: http://en.wikipedia.org/wiki/Allen%27s_rule The rule says that the body shapes and proportions of endotherms vary by climatic temperature by either minimizing exposed surface area to minimize heat loss in cold climates or maximizing exposed surface area to maximize heat loss in hot climates. Aka: short and thick limbs in cold environments mean less heat leaking and less risk of frostbite. Long thin limbs in hot environments means less over-heating! So there's pressure to evolve that direction in both environments. For example: Again, cats! The Forest Cat vs the Siamese.
Moons orbiting gas planets: The gas planet will dominate the sky. And in general there is no night time - the planet will reflect enough light that 'night' will simply be 'the dimmer time'.
Planets with rings: They will be visible in general from everywhere. They will reflect light all the time. And the night will be 'dim but not dark'. The rings seen from the equator will be thinnest - a sliver. The rings seen from the poles will be a curve on the horizon. There is very likely more meteorite activity and shooting stars as chucks of matter from the rings fall into the gravity well.
Gas planets: These need to be large in order to have enough density to form a stable ball. The top surfaces will be chaotic (fueled by miles upon miles of lower levels of weather patterns). High winds. The deeper you go the higher the pressure - like going under water. In fact, think of it like a ball of water and you've got the idea - only at higher depths things get hotter as pressure increases. Heat causes movement, movement causes weather.
General dangers: Atmospheres can contain many many elements - most of them unpleasant or dangerous for humans to breathe. Heavy metal poisoning sucks, can be lethal and is possible in both aerosol and water contamination forms.Molecular handedness - the human body can only make use of same-handed molecules. Complex molecules (not Elemental molecules) are your nemesis on an alien planet - in particular *organic* molecules. Remember how well the New World fared on exposure to small pox.
Flying a Cessna -> http://what-if.xkcd.com/30/
About simulating the formation of planetary systems:
The thing to look for is the PIMF (Planetary Intitial Mass Function) on page 15, showing the distribution of planet masses - they seem to form a few nice bumps, so one could likely model this by rolling for overall type on a table and then rolling a size (in such a way you get a normal distribution). Their probability estimate of the size is:
(Super)-Earths (<7 earths) are 58%
Neptunians (7-30 Earths) 17%
Intermediate (30-100) 6%
Jovians (100-1000) 14%
Super-Jupiters (>1000) 4%
Page 13 has a very neat diagram showing the distribution of planetary orbits and masses: I would love to figure out how to roll on this with dice. The obvious method is to divide it into a matrix, give each cell a measure based on planet density, and then linearize it into a cumulative table, but it would be nice with a smarter approach.
His site also has interesting work on planet formation - young gas giants are *enormous* fluffy things for about a million years before quickly shaping up. They cool exponentially (any moons habitable because of just the giant will have had a long period of being too hot), and the maximum diameter is about 30 Earth radii (for 1000 Earth masses) - above that the gas giant shrinks because it is really dense: super-Jupiters are smaller than Jupiter.