To date, astronomers have discovered about 209 extrasolar planets; that is, planets outside our solar system. From Earth, it is quite difficult to even detect them, much less learn anything about them. We haven’t been able to determine much about these planets—usually only the mass—though we’ve been able to get glimpses of atmosphere in a couple cases.
Of course, we have no idea if there is life on other planets, or how we might detect it. One way to think about it is to imagine what other life forms might think of Earth if they were to stumble across it.
Some scientists at NASA put together a cool summary of what Earth might have been like during different periods in its history. Here’s an excerpt:
At Epoch 0 (3.9 billion years ago), the young Earth possessed a turbulent, steamy atmosphere composed mostly of nitrogen, carbon dioxide and hydrogen sulfide. The days were shorter and the Sun was dimmer, shining as a red orb through our orange brick-colored sky. The one ocean that covered our entire planet was a muddy brown that absorbed bombardment from incoming meteors and comets. Carbon dioxide helped warm our world since the infant Sun was a third less luminous than today. Although no fossils survived from this time period, isotopic signatures of life may have been left behind in Greenland rocks.
Read the rest at NASA’s web site.