How will we recognize life on other planets? Scientists don’t yet know how to address this tricky question but at the P32B Sagan Lecture; Life at the Common Denominator: Mechanistic and Quantitative Biology for the Earth and Space Sciences, biogeologist Tori M. Hoehler presented a different way of thinking about the answer.

Hoehler—who much like the lecture’s namesake, Carl Sagan, is both a gifted scientist and communicator—feels that we are getting tantalizingly close to discovering life elsewhere in the universe. He believes that within the next few decades there will be evidence for extraterrestrial life. As we learn more about water on Mars and new exoplanets are uncovered every day, the chances of finding life increase.

But alien life will be, well, alien and identifying it could be difficult. Hoehler wants to create a schema for definitely identifying life from non-life. For him, it comes down to probability.

Life, he explained, makes improbable events commonplace. The odds are slim that a group of amino acids will string themselves into a giant polypeptide chain. For this to repeatedly happen trillions of times is nearly impossible. Yet, here on Earth such events are ordinary.

How does life accomplish this wondrous task? By exploiting energy, Hoehler says. With the input of energy, improbable events become probable. Organisms on Earth use molecules such as ATP to power their endeavors. As the famed physicist Erwin Schrödinger once pointed out, life makes order out of the universe’s disorder, drawing energy from the environment to keep itself alive

Hoehler has generated equations that quantitatively model how much energy is needed for life to do its magic. Looking at the fundamentals of physics and chemistry allows him to make predictions about the habitability of different environments. He can assess a wide range of habitability scenarios for planets, from barren to highly productive, all based on the amount of energy available.

In the talk, Hoehler urged scientists to look for a many different biosignatures when searching for life outside the Earth. Instead of narrow-mindedly pursuing familiar things such as water, oxygen, and amino acids, scientists need to look at a planetary system as a whole. By considering the energy in and out, we will avoid missing something potentially groundbreaking.

-Adam Mann, UC Santa Cruz Science Communication Graduate Student