geophysical factors
XXX combination XX these astronomical and XXXXXXXXXXX XXXXXXX XXX XXXXXXX a stable XXXXXXXXXXX XXXX XXX XXXXXXX life XX XXXXXXXX XX earth for billions XX years. while earth has XXXXXXXXXXX XXXXXXX XXXXXXXXX and XXXXXXX changes throughout XXX history, XXXXX XXXXXXX have provided a XXXXXXXXXX XXX the planet's overall XXXXXXXXX.