Science & Technology

Why Are Alien Hunters Looking for the Same Things That Support Life on Earth?

Whether you're talking about finding carbon-based molecules on Mars or exoplanets that sit in their star's habitable zone (where liquid water can exist), astronomers are downright obsessed with the search for telltale clues of extraterrestrial life. Those clues make sense: Life on Earth is carbon-based and requires liquid water to exist. But does all life require that? Why can't life be silicon-based and thrive in oceans of liquid mercury? The answer: It could, conceivably. But there are two good reasons we look for life that's more like ourselves.

Related Video: If You Find Aliens, Follow SETI's 9 Rules to Announce It to the World

Dude, Where's My Carbon?

The first reason that alien hunters, like the astronomers at the Search for Extraterrestrial Intelligence (SETI), look for carbon-based lifeforms, comes down to logistics. It just makes more sense to look for something we already know is an ingredient for life. If you found silicon-based molecules on some distant planet, what then? It might be life, sure, but because of everything we know about what gives rise to life, it's more likely not.

For example, imagine if you were unable to find your car in the parking lot. It's certainly possible that it was stolen, that you left the parking brake off and it rolled away, or that it was beamed into space by a Toyota-Camry-loving extraterrestrial. But it's much more likely that it's there, and you just haven't found it yet. If you went investigating all those other options first, well, you'd never get home in time for dinner.

"Carbon-based chemistry works great — we know what to look for," Seth Shostak, a Senior Astronomer at the SETI Institute, told us on our podcast, Curiosity Daily. "So maybe we can be forgiven, as it were, for looking for life that's more like life as we know it than life as we don't know it."

Family Bonds

The deeper reason we're looking for carbon-based life, and the liquid water and oxygen that comes along with it, is all about chemistry. When people seriously consider alternatives to carbon when it comes to life, silicon is the biggest one: It's in the same group on the periodic table as carbon; it has a valence of four — that is, it can bond with up to four other atoms at once — just like carbon; and it can form long chains called polymers.

But carbon has the advantage of being incredibly small, which means it can make double bonds easily. That's why when carbon oxidizes (bonds with oxygen), it can become a simple molecule known as carbon dioxide: Each of the two double bonds is satisfied with one oxygen atom. But when silicon oxidizes, it forms a single bond with four oxygen atoms. Since oxygen atoms have a valence of two, they look for another silicon atom to bond with, which looks for more oxygen, and before you know it, you have the monstrous molecule known as silicon dioxide — also known as sand. This all might sound like a minor difference, but it means everything for life (on Earth, at least).

Still, that doesn't mean silicon-based life isn't possible. As Raymond Dessy, a professor of chemistry at Virginia Polytechnic Institute and State University, wrote in Scientific American, scientists are trying their darnedest to create silicon versions of the life-giving carbon-based molecules we know. "Many silicon analogs of carbon compounds just cannot be formed," he writes. "Thermodynamic data confirm these analogs are often too unstable or too reactive."

"The complex dance of life requires interlocking chains of reactions," Dessy continues. "And these reactions can only take place within a narrow range of temperatures and pH levels. Given such constraints, carbon can and silicon can't."

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The first time this writer learned about the Search for Extraterrestrial Intelligence, it was in Carl Sagan's classic novel "Contact," since adapted into a movie with starring Jodie Foster and Matthew McConaughey. As they always say, the book is better. We handpick reading recommendations we think you may like. If you choose to make a purchase, Curiosity will get a share of the sale.

Curiosity Daily Featuring SETI Senior Astronomer Seth Shostak

Written by Ashley Hamer July 20, 2018

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