The Earth Is Unique Within Our Solar System. It Is The Only Planet With Intelligent Life And It Has Features Which Suggest That Planetary Situations Similar To Ours Are Quite Rare.Earth Falls Within What Is Usually Called The Life Corridor Of Our Sun, Which Lies Within Distances Ranging From About The Orbit Of Venus To About The Orbit Of Mars. Mercury Is Too Hot Ever To Support Life, And The Gravity And Coldness Of The Gas Giants (Jupiter, Saturn Uranus And Neptune) Preclude Life From Arising On Them.Neither Mars Nor Venus Has Produced Intelligent Life. Mars Appears To Be Too Small To Hold An Atmosphere, And Venus, While About The Right Size, Suffers From A Runaway Greenhouse Effect Totally Inimical To Life.The Gas Giants Mentioned Above Do Seem To Protect The Earth From Excessive Impact From Comets, Stray Asteroids And The Like. Remember Schumacher-Levy 9, The String-of-Pearls Comet That Impacted Jupiter In 1994? Have No Doubt Had It Impacted The Earth None Of Us Would Be Here Today. Earth Has Other Special Qualities. Firstly, It Wobbles. The Gentle Rocking Motion Of The Earth During Its Orbit Around The Sun Gives Us Our Seasons. Secondly, Our Moon Is The Largest Satellite In Proportion To The Size Of Its Planet In Our Solar System. Large, And Relatively Close By, The Moon Gives Us Our Generous Tides.The Regular Tidal And Seasonal Rhythms Are In My View Most Likely To Give The Kind Of Selection Pressures And Opportunities To Promote The Arrival Of The Earliest Life Forms Capable Of Emerging From The Seas, The First Amphibians. If Life Does Not Make It To Land, Then There Will Be No Communicative Intelligent Life.As We All Know, The Earth Has Vast Oceans Of Liquid Water. Liquid Water Does Function To Sustain Life, And Arguably To Allow Life To Originate In The First Place, But It Does Serve Another Purpose. Much Has Been Written Lately About The Problems Associated With The Greenhouse Effect, Caused By Atmospheric Gases Absorbing Radiation In The Infrared Band, And Causing The Earth To Warm Unacceptably. We Do, However, Need Some Of This Effect. With No Greenhouse Effect At All We Would Freeze. Water Vapour In The Atmosphere Supplies That Necessary Level Of Greenhouse Gas To Keep The Earth's Surface Temperatures Liveable. We Not Only Need Liquid Water, But Plenty Of It.Time Scales Are Another Problem. We Have No Information On The Survival Time For Civilisations Such As Our Own, Except That We Seem To Be Doing Our Best To Provide Some Statistical Data On That Point.We Do Know We Have Had Some Forms Of Life Here For More Than 3 Billion Years, But We Have Been Here Less Than A Quarter Of A Million Years, And About 100 Years With Radio Communications. We And Other Intelligent Life Forms May Arrive, Flourish And Die With No Opportunity To Contact One Another.We Will Not Be Chatting With The Extraterrestrials.
