for National Geographic News
Astrobiology is a relatively new science concerned with the frequency and nature of life in the universe.
So far, of course, we are aware of but a single planet that contains life: our own blue and green Earth. Yet it seems ludicrous to assume that this is the only planet with life in the Universe, given the immense numbers of stars in our Milky Way galaxy, and the immense number of galaxies revealed by humankind's powerful telescopes that relentlessly peer into the night skies.
Surely there are millions of other planets where life has either formed and has perhaps traveled to. The astrobiologists of planet Earth have every confidence that there will be evidence of life either in our own solar system, or in the planetary systems of neighboring stars.
A key to searching for life is the finding of other terrestrial, or "Earthlike" planets.
Yet what exactly is an "Earthlike" planet?
The obvious answer is a planet like our own, with oceans, continents, and temperatures that rarely or never exceed the boiling point, or descend below the freezing point of liquid water.
Yet our definition of "Earthlike" is for the Earth as we know it now, in our current Age of Animals, more than four billion years since our planet's origin.
Much evidence from the Earth's rock and fossil record makes it abundantly clear that our current conditions did not extend back to the time of our planet's formation.
If we go far enough back in time we can find a planet that was, at different times, frozen from pole to pole, or so hot that liquid water could not exist on its surface. Neither of these times would be even remotely "Earthlike" in our experience, and yet both existed for immense periods of time in the past.
In a similar fashion there is a dawning realization that the future of our Earth will certainly witness periods as "un-Earthlike" as these past episodes, and in many ways there will be times when the future recapitulates the cold and heat of the past.
A new discipline within astrobiology is emerging, one that we might dub "Planetary gerontology"the science of planetary aging. Our Earth is indeed aging, and a result of this will be the eventual end of all life on the planetand eventually the physical destruction of the planet itself. These are the subjects of our new book, The Life and Death of Planet Earth.
Much of the information that has led us to a hypothesis of what might be called "ends" of the world comes from the fields of geology and astronomy. From our planet's rock record we have learned that the early Earth had a much different atmospheric composition than it does today, and that even the atmosphere of a few million years ago was vastly different than that of now.
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