Strange as it may seem, there isn't really one standard definition of life. We can't say that life requires something as specific as breathing or growing; there's just too much variety to living things to nail down one specific criterion.
What we can do is talk about some things that all living things on Earth share: They're carbon-based; they require water; they use energy. Perhaps most important for survival, they can grow or reproduce in some way.
So is that all scientists and astrobiologists are looking for when searching for signs of life outside the Earth? Let's take a deeper look at what is required for life to exist.
As we just said, a main requirement is water, which is necessary for many chemical reactions [source: NASA]. Liquid water allows for chemicals to be transported or dissolved, so we do need the water to be between 59 and 239 degrees Fahrenheit (15 and 115 degrees Celsius) so it doesn't vaporize or freeze [source: NASA].
Energy -- either in light or chemical form -- is also required for life. Both forms fuel the metabolic reactions that allow life to reproduce. Along with energy, we must make sure that any planet has a protective atmosphere that keeps the radiation from a sun out, while still keeping the planet warm.
Of course, life also needs nutrients that will help sustain it. The atmosphere of a planet or moon can even provide these. Methane, for instance, can produce carbohydrates and fats, which might contribute to my mandatory-cheese-and-wine planet [source: NASA]. These systems need to be able to replenish nutrients, which is no problem if your planet has events like volcanic eruptions or weather systems that produce water.
But here's the rub: While scientists can pretty confidently assert those things might be necessary for life on Earth, we should remember that we're basing all our assumptions on, well, Earth. We can't know for sure if other planets or moons could be harboring a "life" form that doesn't require the same things Earth-bound systems do. In the meantime, we'll look for the planets with nutrients, energy and water.
Author's Note: What conditions are required for life to exist?
We should all think way more outside the box when it comes to wondering about "life" on other planets. What if we're only looking for carbon-based life and thus fail to recognize the cool planet made up of swirling, cognizant gases? It could happen.
- Genetic Science Learning Center. "Conditions that support life." University of Utah. 2014. (June 23, 2014) http://learn.genetics.utah.edu/content/astrobiology/conditions/
- NASA. "Life on Earth . . . and elsewhere?" NASA. 2006. (June 23, 2014) http://astrobiology.nasa.gov/media/medialibrary/2013/10/Astrobiology-Educator-Guide-2007.pdf
- Lunar and Planetary Institute. "What Makes a World Habitable?" Universities Space Research Association. 2014. (June 23, 2014) http://www.lpi.usra.edu/education/explore/our_place/hab_ref_table.pdf