Maybe Earth isn't so special. "We could have easily been standing on Venus having this conversation," says Mark Jellinek, a geologist who studies how planets form.

I envision standing under Venus' thick atmosphere of greenhouse gases, broiling on the dusty planet's surface. Temperatures there soar high enough to melt lead. Sure, Venus and Earth are often said to be the most comparable planets—similar in size, makeup, and distance from the sun—but "easily" seems like a stretch.

Or maybe not. According to recent research, if we restart the "experiment" of our solar system, seemingly insignificant early changes—like internal heat, climate, or water content—could completely reroute planetary history, says Adrian Lenardic, a planetary scientist at Rice University.

Maybe a rewind would