by Ray Villard
By all accounts Earth should be a "snowball planet" like the frigid world Hoth in the 1980 Star Wars film "The Empire Strikes Back."
Why? Because common theories of stellar evolution predict that the sun was only 70 percent of its current brightness when it first lit its fusion engine 4.5 billion years ago. The sun has been steadily growing brighter since then and will continue so into the future, eventually evaporating away Earth's oceans.
ANALYSIS: Stellar 'Speed Bumps' Could Shape Baby Star Systems
Once Earth amassed an ocean 4.3 billion years ago it should have quickly frozen over and reflected so much sunlight back into space that it squelched Earth's ability to thaw out for billions of years.
The dilemma, called the "faint young sun paradox," has been know about since the 1950s and was popularized by Carl Sagan. Geochemists and solar physicists have wrestled for answers all these years.
Lowering Earth's reflectivity by reducing cloud cover doesn't work. Models also show that a greenhouse effect from dense carbon dioxide and methane can't warm the Earth enough either. In some simulations, methane and carbon dioxide combine to make a photochemical smog that would have chilled Earth even further. Read More
