Do Aliens Exist? 183
Now, this hypothesis naturally has a rebuttal.
If living creatures can continue to propagate even in harsh environments, why didn’t the
moon rocks brought back by the Apollo spacecraft show any traces of living creatures? Why
were no living creatures detected on Mars, which is certainly believed to contain water?
Even microbes have not been found, which means that primordial life (the basis of evo-
lution) might not exist on either the Moon or Mars, and might never have existed there. This
suggests that the probability of the existence of life on any given moon or planet might be
smaller than we would think. In other words, even if a planet currently has an environment
that could support living creatures, life might not necessarily exist there.
By the way, some scholars advocate the theory that life on Earth originated from
organisms that were transported across the universe by meteorites. Thus, to some extent,
the search for alien life is also research into the origin of life on Earth—not just astronomy.
Which Is the Closest Star System That Could Support
Extraterrestrial Life?
Although some of the previous discussion may have been somewhat pessimistic, we will
now try to find a heavenly body where extra terrestrial life may exist, taking into consider-
ation the environmental aspects.
Inside the solar system, Ganymede (a satellite of Jupiter, just like Europa) and Titan (a
satellite of Saturn) are promising candidates. This is because there is a high likelihood that
ice or water exists on both of them. In that light, the possibility of life on Mars where an ice
lake is reported to have been detected cannot yet be discarded.

Get The Manga Guide to the Universe now with the O’Reilly learning platform.

O’Reilly members experience live online training, plus books, videos, and digital content from nearly 200 publishers.