"The quest for planets outside our solar system stretches back over millenia. Buddhist texts described a “thousandfold world systems with thousands of moons, thousands of planets orbiting countless suns…”
In more recent historical times the Italian Philosopher Giordano Bruno (1548-1600) asserted that the Milky Way was filled with millions of inhabited worlds, thus defying Papal Decree. For this impiety he was burnt at the stake. The sun has 8 planets including the Earth and millions of asteroids and comets which may themselves be fragments of larger bodies. Life was most probably brought to Earth by a comet impacting our planet nearly 4000 million years ago.
The modern interest in searching for planets outside our solar system stems from the fact that planets are the natural homes for life. Finding alien planets like Earth increases greatly the chances that life is not confined to our planet and that we are not alone in the incredible vastness of space. A team of scientists led by Professor Chandra Wickramasinghe has shown, in a paper published in the journal Astrophysics and Space Science this week, that the grand total of Earth-sized planets in the Milky Way numbers a few hundred thousand billion....."
The scientists have proposed that these life-bearing planets originated in the early Universe within a few million years of the Big Bang, and that they make up most of the so-called "missing mass" of galaxies. The scientists calculate that such a planetary body would cross the inner solar system every 25 million years on the average and during each transit, zodiacal dust, including a component of the solar system's living cells, becomes implanted at its surface. The free-floating planets would then have the added property of mixing the products of local biological evolution on a galaxy-wide scale.
Since 1995, when the first extrasolar planet was reported, interest in searching for planets has reached a feverish pitch. The 750 or so detections of exoplanets are all of planets orbiting stars, and very few, if any, have been deemed potential candidates for life. The possibility of a much larger number of planets was first suggested in earlier studies where the effects of gravitational lensing of distant quasars by intervening planet-sized bodies were measured. Recently several groups of investigators have suggested that a few billion such objects could exist in the galaxy. Wickramasinghe and team have increased this grand total of planets to a few hundred thousand billion (a few thousand for every Milky Way star) - each one harbouring the legacy of cosmic primordial life.
Professor Wickramasinghe said, "The biggest challenge for science is to understand how life began. Within a few million years of the Big Bang, the conditions within trillions of primordial planets were better suited for life to start than at any later epoch. The transition from non-life to life most probably occurred at this stage in the Universe within the warm interiors of these planets. When our solar system was formed, such a life-bearing planet was shattered to generate hundreds of billions of cometary bodies. One such body striking the Earth brought the first life to our planet."