This illustration shows the relative sizes of the habitable-zone planets Kepler-22b, Kepler-69c, Kepler-62e, Kepler-62f and the Earth.
Last week NASA scientists put the space telescope Kepler in a kind of technological coma. The craft, designed to search for Earth-like planets orbiting stars in our cosmic neighborhood (within a few thousand light-years, that is), failed and seems to be unfixable. (Hope remains, though.)
Launched in 2009, Kepler has found a total of 132 potentially habitable planets and scientists have a long list of another 2,740 candidates awaiting more detailed analysis. Terrestrial telescopes will undertake the confirmation, since they now know where to look.
At a cost of $550 million, Kepler changed our view of the Universe and of how we fit in, very much like the homonymous brilliant German astronomer from the seventeenth century, an arduous defendant of a sun-centered cosmos and the first to provide mathematical laws describing planetary orbits.
The idea that stars have planets orbiting around them is very old, dating back at least to fourth-century BCE Greece, where philosophers like Epicurus suggested the existence of other worlds:
Moreover, there is an infinite number of worlds, some like this world, others unlike it. For the atoms being infinite in number, as has just been proved, are borne ever further in their course. For the atoms out of which a world might arise, or by which a world might be formed, have not all been expended on one world or a finite number of worlds, whether like or unlike this one. Hence there will be nothing to hinder an infinity of worlds.
In the sixteenth century, Giordano Bruno elaborated on this notion in his On the Infinite Universe and Worlds. To Bruno, such worlds would be like Earth, inhabited and full of good and evil.
Of course, if other Earths existed, the centrality of ours would be threatened. This was, and remains, an essential question in the debate on the plurality of worlds: are we unique and hence important in some sense, or are we the norm and typical of what's out there across the vastness of space?
It took some 413 years after Bruno's (most unfortunate) death for us to have an answer, even if partial, to this question. In four centuries, we moved from mere speculation about the existence of other Earths to the observation of other planets that, if not like ours, are — or could be — very similar. Today, astronomy has a new branch known as "comparative planetology," where properties of diverse planets are contrasted and studied in detail.
Even if still in its infancy as a discipline, we have learned much: that the majority of stars have planets circling around them; that life is only possible in those that obey a series of regularities in their astronomical properties and that have a relatively narrow chemical composition; that simple, bacterial life may be widespread, but that complex life may be rare.
But note that when scientists talk about life they are refereeing to life as we know it, that is, carbon-based and operating in an aqueous medium. Other types (such as silicon-based and using ammonia as solvent), even if interesting, are probably more fiction than reality, although some speculations are quite unsettling: intelligent life could have evolved so beyond what we can conceive that it reached a stage where it could leave behind its carbon envelope, existing in some sort of ethereal digital web sustained by electromagnetic fields, that is, a life without a narrow spatial confine or a biochemical metabolism. Who knows?
Even if we are still like toddlers in our cosmic exploration, we can at least rejoice in what we have learned so far: that there are hundreds of billions of other planets in our galaxy alone (and their moons, making for trillions of other worlds); that the same plurality is true in hundreds of billions of other galaxies across space; that some of these planets will have properties similar to Earth; that life, if present in these worlds, will be very particular to each of them, dependent on the planet's detailed and unique history; and that, for this reason, we are unique in the Universe, a product of the Earth and its very unique history.
The history of life on a planet mirrors the planet's life history.
Other space telescopes are planned to continue Kepler's mission. But what we have learned already demonstrates our uniqueness as a life form.
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