Let’s start with the solar system. People very much want there to be life in the solar system. We’d like to think that the presence of life on Earth is not unique, and that life in some form also exists, or has existed, in other places. But where would this life have come from?
There are two main theories about the emergence of life from the soup of hydrogen, oxygen, carbon and nitrogen in primordial soup. One, the geocentric model, is that life emerged on Earth and is therefore unique to Earth. The other theory takes a more holistic view in that Earth is not central to the universe. We are uncommon, but not special or precious.
Panspermia has life originate in microbial form elsewhere in the universe and being spread through comets. While we have no definitive proof for either theory, there are some pointers. Firstly, the elements for life exist everywhere in the universe. In limited, but still reasonably numerous, places, temperatures are right for liquid water. Life on Earth evolved in liquid water. Secondly, certain bacteria are known to survive exposure to space for extended periods. Certain bacteria can lie dormant for many, many years. It is feasible to assume that these bacteria would not have developed these abilities unless there was a need for them.
How could we find out?
Sample return missions to places of interest are expensive. They will eventually be necessary, but there are other things we can do to pinpoint things to look for when eventually we send such missions.
We can look at comets. If comets carried the first microbes to Earth, they would also have carried them to other bodies. There is no reason to believe that if comets carried microbes millions of years ago, they have suddenly stopped doing so.
We can look at how and where organisms have survived in environments on Earth that are too hostile for most other life. This explains the interest in extremophiles, organisms that thrive in such places. It is with this in mind that scientists look for life in hydrothermal vents in the dark depths of the oceans, where the water welling up is extremely hot, where it is extremely dark and there is little oxygen to be had. It is for this reason that scientists look for adapted life in Antarctica, or in environments that are too poisonous for ‘normal’ life to exist.
We can look at bits of other celestial bodies that have found their way to Earth
That said, in recent months, we’ve seen a number of disappointing reports. First, there was the big kerfuffle over the discovery of bacteria that can substitute arsenic for phosphorus. While the research was officially announced, a big hoo-hah soon broke out over the validity of the results.
Then, last month, there was a report that fossil bacteria had been found in meteorites that are of Martian origin. Again, a big bunfight broke out, this time including claims against the reputation of the magazine in which the paper had been published.
This is starting to sound very much like the boy who cried ‘wolf’ too often, or maybe too soon, when the wolf was in fact the neighbour’s dog. It’s sad because as described above, a discovery would probably fall along those lines. This hyped-up (pseudo) news desensitises people to the real possibilities.
Meanwhile, a good number of well-informed scientists express the quiet opinion that we will ‘probably’ find evidence of microbial life elsewhere in the solar system. Candidates for such finds would be fossil life on Mars, extant life in the seas of Europa and possibly methane-based life on Titan. They also think that any such life is likely to be primitive, possibly no more evolved than bacteria. No one thinks we’ll be meeting little green men any time soon.