Type 1b--Conditions are actively hostile to human (and probably any other type) life. A human would quickly die even with a space suit and other significant protection. Examples: Venus, Io, all gas giants
It's not like there's no place on Io that you could possibly stay. The way I see it, the main problem with Galilean moons is the very intensive radiation from Jupiter.
As for gas giants, just because they don't have a surface to stand on, doesn't mean you couldn't float at a particular altitude.
Type 2--Semi-Earth-like. Like 1a, but with sufficient atmosphere to produce a colored sky. Lacks resources to support human life, but environmental conditions may permit short-term survival without a full space suit. Examples: Mars[/quote]
Mars is about as hospitable to human life as empty space is. It may have enough atmosphere to produce a colored sky, to allow for erosion and sand storms, but the surface pressure is about 1000 Pa. It's 100 000 Pa on Earth. That's equivalent to about 30 km altitude on Earth.
What little air there is is toxic. 96% CO2.
The only thing you've got going for you is that it's not as cold as space is. It's......... slightly........... warmer.
Type 3--Earth-like. No space suit needed. Different environmental factors may require breathing equipment and/or or some form of protection (UV, radiation, toxicity, etc).
And hope that the atmosphere doesn't cause problems when coming in contact with skin. Like maybe high concentration of some nasty acid. Hydrogen Fluoride (HF), Hydrogen Chloride (HCl) or others, such as those present in trace amounts in the atmosphere of Venus.
Titan could be classed as either 1a or 1b depending on whether one regards the extreme cold as an active hazard, or simply a lack of heat energy needed to survive.
Surviving on Earth's poles during the winter is already an extreme hazard. And Earth has an atmosphere you can breathe...
Perhaps we should not try to classify planets by squeezing them into a fixed number of classes or types and instead take them "as they are".
We can't even classify the planets in our own solar system. Each has such unique features.
Terrestrials:
Mercury's a hot rock.
Venus has an abnormally thick atmosphere.
Earth has life and a very rich chemistry.
Mars is somewhere in between, but may have been more Earth like in the past.
Gas giants:
Jupiter is big, relatively dense and imposing.
Saturn is the least dense of all the planets and has a young ring system.
Uranus and Neptune seem the most similar to each other, but we really don't know much about them.
Icy bodies:
They seem similar at first sight, but we really don't know much about them.
Let's not even try to mention other solar systems.
Oh and have a look at THAT:
http://en.wikipedia.org/wiki/PSR_B1257+12
Yes, 3 planes orbiting a PULSAR were the first three extrasolar planets ever discovered.