Here's a fun topic to kick around. We have little-to-no data to date, but as always, we can speculate away! The percentages given are my WAGs at probability of finding various kinds of life. Note that any kind of life outside of the inner solar system would be very interesting, as it would almost definitely be of independent origin. We need something of independent origin - a second data point - to accurately speculate about the odds of finding life in the universe.
Finding a second occurence of bacterial life in the Solar System would be cool, but would not falsify the single-source hypothesis - the bacteria could hitch a ride in the rock when it was ejected from one planet to another via impact. I think this is a feasible scenario, hence my relatively high WAGs.
Note that my WAGs are not independent. If my assumption that bacterial life can hitch a ride from one planet to another (panspermia) is wrong, for example, my estimates for Mercury and the Moon (and Io) go to zero, and even my estimate for Mars drops some. If it is correct, then almost every asteroid could harbor some bacteria.
There's at least three stages of life we'd care about:
1) Bacteria
2) Multicell
3) Intelligent (whatever that is!)
Mercury: Single-cell, bacterial: 5% Anything else: 0%
Life could be found here, in close-to-polar regions. Because the (current) sun-facing side is burning hot, and the permanently-shadowed craters at the poles are very cold, there must be someplace between, under the surface, where bacteria similar to the ones found in the rocks on Earth *could* live if they were transported there. And, they could have been transported there via Earth impacts.
Venus: Bacteria: 10%
I had this at close to zero - and then ran across http://abc.net.au/news/scitech/2002/09/item20020926135029_1.htm
We may find that every place in the solar system that can support
life already does, due to "pollution" from Earth.
Moon: Single-cell, bacterial: 10%
All the reasons that life could exist on Mercury apply to the Moon also, but more forcefully. The Moon has been hit by a lot more Earth-debris than Mercury ever has been, and there could be more lunar water as well.
Mars: Single-cell, extinct: 60% Single-cell, extant: 50% Multicell, extinct: 1% Multicell, extant: .5%
Mars is the most likely other place in the inner solar system to look for life. In fact, it is possible that we're all Martians - it's certainly a lot easier to kick rocks off of Mars than it is Earth. Early Mars was a lot like early Earth (relatively warm, thicker atmosphere). Early Earth shows signs of life about as soon as possible - as soon as it cooled enough, at least 3.5 billion years ago. If early Earth did, why not Mars? It could have developed separately or been kicked there from Earth (or been kicked here from Mars). If life ever arose on Mars, I'd bet that some form of it is there today - especially given the exciting new news regarding the possible presence of liquid water near the Martian surface.
I just boosted the odds as a result of the newest findings of possible Martian life.
Jupiter / Gas Giants: I don't even have a bad guess. All the gas giants are warm enough (in places), but they have such bizarre chemistry that they are completely outside our experience. The energy supply is from within (mostly), rather than from the Sun. The life would have to live in a high-radiation gaseous environment. But - it is certainly possible; lots of energy, lots of complex molecules. Wouldn't it be ironic if Jupiter harbored life & we crash Galileo into it to avoid contaminating Europa? We should smash it into Amalthea instead (A moon of Jupiter).
Callisto: Single-cell, bacterial: 1%
IF one of the other gallilean satellites has bacterial life, Callisto could have been "polluted". If there's liquid water underground (very dubious), there could be more.
Ganymede: Single-cell, bacterial: 5% Multicell: 1%
Chance of water is higher. If it exists, it's warmer.
Europa: Single-cell, bacterial: 20% Multicell: 5%
It's currently easy to get excited about possibilities on Europa. Lots of warm water, lots of energy input.
Io: Single-cell: .01%
To hot, too much radiation.
Titan: Single-cell, bacterial: 10% Multicell: 1%
Nice atmosphere, methane could exist in solid/liquid/gas, filling much the same role that water does on Earth. All the ingredients are there, but it is very cold. This isn't just a concern from an anthropocentric point of view, low temperatures decrease the speed of chemical reactions. We'll get a lot more data when Huygens splashes down in January 2005. I've been waiting to see this for years!
Anywhere else: .1%
Too cold, not enough chemistry.