I was actually just reading a great What If? article on this found here. Flight on other planets is possible. I think the included comic strip summarizes it wonderfully:
As for each valid body in our solar system (barring Earth of course), I'm going to paraphrase a bit:
The Sun: Attempting flight on the sun is more or less useless as any vessel close enough to feel its atmosphere would be instantly vaporized.
Mars: The article goes into a lengthy discussion about simulation via X-Plane. X-Plane, as it turns out, can be made to closely simulate the conditions found on Mars. Unfortunately, as was also found, flight on Mars is possible but difficult. To achieve flight on Mars, you need to be going fast. The article states that a speed of mach 1 is required merely to achieve flight. Problem is, once you achieve flight, the inertia makes it nearly impossible to change course.
Venus: Venus is interesting. The atmosphere on Venus is 60 times denser than Earth's atmosphere. You could easily achieve flight at incredibly low speeds (a Cessna 172 Skyhawk, the aircraft the article is based around, could achieve flight at running speed). Problem is, the air on Venus is hot enough to melt lead. You can always get around this by flying in Venus's upper atmosphere. The upper atmosphere is rather earth-like and would be quite easy to fly a plane in. Only, you'd have to ensure no metal is exposed as sulfuric acid in the upper atmosphere introduces the threat of corrosion.
Jupiter: Flight on Jupiter is unrealistic. Jupiter's gravity is much too strong. The power required to maintain flight is about 3x that of Earth making flight there highly unrealistic.
Saturn: Weaker gravity and slightly denser atmosphere than Jupiter means an aircraft might fair better but ultimately would succumb to cold or high winds.
Uranus: Flight on Uranus could be sustained slightly longer but ultimately the aircraft would still succumb to the conditions found there.
Neptune: The temperature and turbulence make it impossible to achieve flight on Neptune. It's assumed your aircraft would quickly break apart in the atmosphere.
Titan: Titan is perhaps the best plan to attempt flight on. To quote the article:
"When it comes to flying, Titan might be better than Earth. Its
atmosphere is thick but its gravity is light, giving it a surface
pressure only 50% higher than Earth’s with air four times as dense.
Its gravity-lower than that of the Moon-means that flying is easy."
Flight on Titan IS easy. A human could theoretically achieve flight with a wingsuit and mere muscle power. The problem is, Titan is cold, 72 Kelvin cold. Flight would require some major heating modifications but, barring the heat factor, Titan is the absolute best place to attempt flight in our solar system. It's even better than Earth. As an interesting note, Titan, thus far, has actually been too cold for even unmanned probes to explore. Again, quoting the article:
The batteries would help to keep themselves warm for a little while, but eventually the craft would run out of heat and crash. The Huygens probe, which descended with batteries nearly drained (taking fascinating pictures as it fell), succumbed to the cold after only a few hours on the surface. It had enough time to send back a single photo after landing—the only one we have from the surface of a body beyond Mars.
Earth: Earth's conditions are quite optimal for flying. Earth's gravity is 9.78 m/s². As a comparison, Jupiter's gravity is 24.79 m/s² and Titan's gravity is 1.352 m/s². Earth's atmosphere is, at sea level, 1 standard atmosphere or 101.3 kPa or 14.7 psi compared to Mars's average which is about 0.006 standard atmosphere or 600 Pa or 0.087 psi and Venus's average which is about 9.2 mPa or 1,330 psi. Takeoff speed for our Cessna 172 Skyhawk is 64 KIAS (Knots Indicated Air Speed) and the best rate of climb is 73 KIAS. Normal cruise speed in a Cessna 172 Skyhawk is 122 knots (140mph, 226 km/h). As a comparison, flight on mars would require speeds over Mach 1 which translates to 768 mph or 1,236 kph.
- Sun: Instant vaporization.
- Mars: Atmosphere's too thin to fly below mach 1, above mach 1 you
essentially can't steer.
- Venus's Lower Atmosphere: Flight is possible but the air's hot as
lead. You'd melt.
- Venus's Upper Atmosphere: Flight is possible but corrosion is a
factor due to sulfuric acid so no exposed metal.
- Jupiter: High gravity makes flight extremely unrealistic.
- Saturn: Flight's possible but your aircraft might ultimately succumb to the
cold and weather conditions.
- Uranus: Same as Saturn but you MIGHT last a bit longer.
- Neptune: Your aircraft would break apart quickly from the extreme
- Titan: Flight could be achieved with artificial wings and mere muscle
power. Unfortunately, Titan's cold. To quote the XKCD article:
If humans put on artificial wings to fly, we might become Titan versions of the Icarus story—our wings could freeze, fall apart, and send us tumbling to our deaths.
- Earth: We know flight on Earth works due to firsthand knowledge. We
don't have the most optimal conditions in our solar system, but
the conditions here are still great for all types of manned aircraft.
- Anywhere Else: No atmosphere, so you would crash ballisticly.
As a small note:
Titan is the absolute best environment for flight using a conventional aircraft if you don't factor in the cold. I imagine it would be much easier and less costly to attempt flight in Venus's upper atmosphere by protecting all exposed metal from corrosion than it would be to make major modifications to a conventional aircraft so that it and its pilot can withstand the extreme cold found on Titan.
Another Small Note: Mach 1 is measured relative to earth so 340.29 m / s. The speed of sound on Mars is different. The speed of sound is 226 m/s.