今天是第55个世界地球日，今年的主题为“珍爱地球，人与自然和谐共生”，保护我们赖以生存的地球。对于我们赖以生存的蓝色星球，你了解多少？
Earth, the only planet known to maintain life.
A product of scientific phenomena and sheer chance.
This blue speck in space holds the past, present, and future, of our very existence.
Approximately 4.5 billion years ago, the Earth formed from particles left over from the creation of our sun.
Gravity drew these particles together to form pebbles which then formed boulders, and eventually, the Earth.
At its heart is a solid inner core covered by a liquid outer core.
Above this sits the mantle, made of flowing silicate rocks, and a rocky crust.
This rocky mass is the third planet from the sun, orbiting the star from an average distance of about 93 million miles.
It's close enough to the sun to be warm unlike the cold gas giants.
But not so close that its surface is exposed to extreme heat and solar radiation as is the case with Mercury.
