Imagine the Universe!
Imagine Home  |   Ask an Astrophysicist  |  
Ask an Astrophysicist

The Question

(Submitted February 21, 1998)

I am a student currently in the 10th grade. I have been curious to whether life could exist on Europa (or any other unexplored space body) in different forms. For example, since Earth is shaped around the element, carbon, could life exist revolving around a different element?

The Answer

Scientists have occasionally speculated that life could be based on an element other than carbon. Silicon, being the lightest element with an electronic structure analogous to that of carbon (having a half-filled outer shell with 4 unpaired electrons), is the most likely candidate mentioned. However, carbon's tendency to form the long chains and rings that form the basis for organic compounds that at some level of complexity begin to self-replicate is unique. Also, because older stars naturally produce carbon, along with nitrogen and oxygen (its neighbors on the periodic table), it is relatively abundant in the universe. Many astrophysicists who study the spectra of stars believe that complex chains and even rings of carbon appear in such unlikely places as stellar envelopes (e.g., in the form of PAHs, polycyclic aromatic hydrocarbons). When such compounds reach cooler regions of space where they can bond with readily available hydrogen, organic compounds as we know them are naturally formed.

Although other elements may form complex, covalently bonded structures, none has the rich molecular variety of carbon. It is the chemistry of carbon that allows us to consider the possibility of life "as we know it" in other parts of the Galaxy and the Universe beyond. We do not know whether Earth-like conditions exist elsewhere; but if they do, it is highly likely that life forms, if they exist, will be based on carbon.

One more point: The organic types of structures appearing in stellar envelopes are very hot and probably stripped of hydrogen, so that they are not themselves alive; it is only when carried off to a more hospitable environment, such as a much cooler planet 100 million miles away or so, that the kind of chemistry required by life becomes possible on a scale large enough to allow for stable development and replication. Water is also a factor, causing the hydrophobic proteins to clump together at all, and serving as a medium a conduit for new material, protection from temperature changes and harmful stellar radiation, etc.

Mark Kowitt and Damian Audley
for Ask an Astrophysicist.

Questions on this topic are no longer responded to by the "Ask an Astrophysicist" service. See for help on other astronomy Q&A services.

Previous question
Main topic

Imagine the Universe is a service of the High Energy Astrophysics Science Archive Research Center (HEASARC), Dr. Alan Smale (Director), within the Astrophysics Science Division (ASD) at NASA's Goddard Space Flight Center.

The Imagine Team
Acting Project Leader: Dr. Barbara Mattson
All material on this site has been created and updated between 1997-2012.

DVD Table of Contents
Educator's Index