What is life? What counts? We know cats and dogs are alive, but what about a fire or a hurricane? NASA’s current working definition of life is “a self-sustaining chemical system capable of Darwinian Evolution”. With that in mind, what do we expect from living organisms on other planets?
The Star Wars and Star Trek universes have shown us a vast collection of ideas for what life could look like, from Ithorians to Vulcans. Perhaps extraterrestrial life won’t be so obvious, but more subtle and found in the microscopic realm like bacteria and phytoplankton. We may not even agree that we’ve found life when we see it. After all, paleontologists can’t agree on the oldest form of life here on Earth, even though there are plenty of candidates. The only example of life we have to work with is on our planet where most of the oceans haven’t been explored yet, so we are not even aware of all of the potential life forms that exist on our home turf. So far, we’ve found several common traits amongst life on Earth, some of which are listed below…
- It’s Carbon-Based
98% of all living organisms use five key elements: carbon, hydrogen, nitrogen, oxygen, phosphorus, and sulfur. As far as we know, there are no other elements that can serve as alternatives to these five key elements. So, until we know more, we can assume that extraterrestrial life is most likely carbon-based. - It Requires Water
Why water? Specifically, why H2O? Water is one of the most common molecules in the universe. Hydrogen is the most common element by far, followed by helium (which is a noble gas, so it does not bond with other atoms), then followed by oxygen. Using the two most common elements, you can create the most common molecule. However, there are possible molecular alternatives to water that may serve a similar purpose for life other than our own.
Logarithmic scale showing the most common elements in the universe. Hydrogen and helium are the two most abundant, with oxygen following close behind. - It’s DNA-based
This is mostly true. The exceptions to this trait are viruses, which can use either DNA or RNA, but never both. Therefore, if we consider viruses to be alive, then life does not necessarily need to be DNA-based. - It’s Cellular
This simply means that all of the material that composes the cell is contained within something that separates it from the other cell materials.
To find out more of the common traits for life on Earth and how we can develop our expectations for aliens, checkout Dr. Matthew Pasek’s First Friday lecture linked below!
Boise State First Friday Astronomy – Dr. Matthew Pasek – September 2022