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64 A Mirror Image. P.E. Cleator, one of the founders of the British Inter- planetary Society, pointed out in 1934 that, from an extraterrestrial point of view, conditions on the Earth may be deemed unsuitable for life. Arthur Clarke later developed this idea in his “Report on Planet Three,” a fictional document prepared by Martian astronomers. They concluded that the prospects of life on Earth appeared to be extremely poor because of Earth’s poisonous atmosphere, strong gravity, and other factors. Although some form of vegetation might be possible on the third planet, the Martians resigned themselves to the idea that they were the only rational beings in the solar system.” Scientists have suggested that some moons of giant planets might be suitable for life, expanding the range of potential life into systems with no Earth-like planets. Shapley broadened the search space for discovery, spec- ulating that there may be many planet-sized objects that are not gravita- tionally bound to stars. Some could generate enough internal heat to support biological evolution; perhaps thousands of these self-heating planets would be suitable for life. “Many billions of dark Earths could be roaming the Galaxy,” David Darling wrote 40 years later, “cut adrift from the star systems in which they were made.” The discovery of young, isolated planetary mass objects in star-forming regions suggests that a majority of large planets, ranging from 3 times to 80 times Jupiter’s mass, may not be associated with suns. Such free-floating objects may exceed, by orders of magnitude, the total number of stars; given their numbers, they could constitute the largest environment for life in the galaxy. One or more interstellar planets could be closer to our solar system than the nearest stars. Astronomy continues to reveal significant bodies that had been hidden from us, challenging our assumptions about our galactic envi- ronment. We have learned that our own solar system extends much farther into interstellar space than we had thought. Instead of ending at Pluto, the Sun’s family is now known to reach out at least three times abate fn that far. In 2003, astronomers discovered a planetoid called Sedna that ranges as much as 84 billion miles away from our star. Sedna’s elliptical orbit may carry it into the inner section of the long hypothesized but previously undetected Oort Cloud, an array of icy objects. There could be more than a million bodies larger than 100 kilometers (62.5 miles) in diameter in the Sun’s extended scattered disk. Observers already have found more than a thousand trans-Neptunian objects, includ- ing one bigger than Pluto. Astronomer Alan Stern speculated that some would be as large as the Earth.*° Other solar systems also may extend far beyond the orbits of the planets we can detect. There is much more stuff out there than Newton would have predicted. Probabilities: The Astronomical Factors