Question

What makes Earth so unique in the Solar System?

Earth is unique in our Solar System most notably because it harbors complex, intelligent life that has survived and evolved through 3.9 billion years of the planet’s 4.5 billion year history. As discussed in Peter Ward and Donald Brownlee's book, "Rare Earth", there are a number of conditions that contribute to Earth's seemingly unique habitability.

Among them are: liquid water near the surface; a level of incoming radiation from space, filtered through our atmosphere, that is neither too much nor too little; a stable planetary orbit around the Sun; the presence of a gaseous atmosphere and liquid water ocean; enough internal heat from the planet’s molten core to allow plate tectonics (which are important for maintaining the balance of the carbon cycle); having Jupiter as a neighbor who protects us from comets and asteroids; the presence of a large moon that stabilizes tilt (keeping the seasons mild) and the tides; the relative absence of impacts from asteroids or other matter flying through space, after an initial bombardment period early in Earth’s history; our current position relative to the Sun, which provides our heat and energy; and the evolution of the process of photosynthesis within microbial life forms at a certain point in Earth’s history, who in turn enriched the atmosphere with oxygen, enabling life to evolve creatures like ourselves which depend completely upon it to live.

For more details, check out http://www.seti.org/science/rare_earth.html

No other planets to our knowledge have this delicate balance of conditions, making Earth rare indeed. But each planet has it’s own set of geochemical and atmospheric conditions which make it unique. Just because we don’t see Earth like conditions on other planets does not mean life as we know it cannot exist there. We must persevere and examine each new planet as it’s own balanced system, possibly suitable for some type of life that we may not know about yet. The NASA Astrobiology Institute’s JPL team, headed by Victoria Meadows, is currently working on what is called a “Virtual Planetary Laboratory.” Using mathematical algorithms on supercomputers, they enter some information about a planet such as its atmospheric composition and chemical spectra, and let the computer model decide whether or not it is likely to be able to support life, or indeed if life forms currently exist there. Stay tuned to this website to learn more about this fascinating story.
October 29, 2001