A great deal of excitement was generated this year when NASA’s budget for 2016 was released and $100 million dollars was included to prepare for a mission to Jupiter’s moon Europa.

The icy moon may not seem, at first glance, like a good bet for life however among astrobiologists it is a perennial favorite. The moon, slightly smaller than our own, is 500 million miles from the sun and clearly covered in ice. However, there is also every indication of a liquid ocean and volcanic activity.

Well known astrophysicist and host of TV’s Cosmos regularly lists it as a place that he’d like to look for life.

“It’s icy on the surface, but the stress from Jupiter’s gravity pumps energy into Europa, melting that ice so there’s an ocean of liquid water that’s been liquid for billions of years. On Earth, any place we find liquid water, we find life,” said Tyson in a 2013 interview.

Included in NASAs budget is $30 million to study a specific plan called Europa clipper, which would send a probe to the moon to gather more detailed information about it. Though the craft wouldn’t land on Europa, according to the plan, it would perform 45 close flybys gathering more specific data about the icy moon.

However, Europa isn’t the only place in the solar system where life could be possible. The truth is that scientists don’t know where life might exist.

We know that on Earth life seems to exist just about everywhere. Last month a team working in the antarctic found a variety of species living in 30 feet of water between the ocean floor and a 2,500 foot sheet of ice. Microbial life has also been found in the La Brea Tar Pits. Life is fairly abundant in deep see trenches and near deep sea lava flows.

Because, to date, we haven’t found any examples of life beyond the Earth no one can say for sure where it might take hold.

For a very long time, Mars was considered the best candidate for extra terrestrial life. It is still the front runner among planets. Mars is the most “Earth-like” planet in the solar system. It is relatively close to Earth in terms of size and temperature and has large bodies of water frozen at the poles. It is also likely that there is water beneath the surface and signs that liquid water once flowed on the surface. However, none of the Mars missions to date have found any traces of life, past or present.

Beyond Mars the other leading candidate for life in our solar system are all moons.

Io, another moon of Jupiter, is a bit of a long-shot candidate. Europa’s neighbor is known to have an atmosphere and complex chemicals that are promising for life. Io also has volcanic activity, which provides some warmth when combined with the gravitational tug of Jupiter. However, Io is within Jupiter’s magnetic field which means that radaition levels on the moon are likely to be very high.

Enceladus, the sixth-largest of Saturn’s moons is covered with ice like Europa. However that ice is thought to be frozen water and based on readings from the Cassini probe, it is likely that carbon, hydrogen, nitrogen and oxygen are all present in Cassini. Enceladus, is also thought to have a molten core, which would create additional heat. So, Enceladus seems to have all of the basic ingredients that caused life to happen on the Earth.

Then there is Titan. Also a moon of Saturn, Titan does not contain the ingredients that caused life to happen on Earth but the ingredients that it does have are interesting. Titan has a thick atmosphere and lakes which remain liquid despite the incredible cold (minus 290 Fahrenheit). The bad news is that that atmosphere and those lakes appear to be primarily composed of ethane and methane, not water. However, on Earth, methane is usually produced by living organisms. That much ethane and methane would be toxic to humans but it is possible that other organisms might not feel the same way.

Beyond these usual suspects life could conceivably exist in any number of other locations. We only have one example of life and evolution so far and until there is a better pool of samples, nothing is impossible.