(And there are other reasons why the habitable zone, as traditionally defined, is increasingly viewed as overly simplistic. Other factors must be considered as well, such as the host star's activity and the planet's atmospheric composition. Habitability isn't just about water, however. The eventual goal is to understand how often rocky planets form in the " habitable zone" of their parent stars, which is traditionally defined as the range of orbital distances in which water can exist on a world's surface. "Nearby" is a relative term, however the closest known exoplanet, Proxima b, lies about 4.2 light-years away, which is roughly 25 trillion miles (40 trillion kilometers). Over time, with more advanced telescopes, scientists will be able to probe the chemical compositions of some nearby exoplanets' atmospheres. The sheer number of alien worlds suggests that life may be plentiful throughout the cosmos. There are now more than 4,500 confirmed exoplanets, with more being found every year. And a few years later, the first exoplanet around a sunlike star was confirmed. But in 1992, astronomers spotted worlds circling a superdense stellar corpse known as a pulsar. When Fermi made his famous remark, the only planets scientists knew about were in our own solar system. So alien civilizations have had plenty of time to arise and spread - but they also likely must cross a vast cosmic gulf to get to us. And separate measurements indicate it is about 13.82 billion years old. Data gathered by a variety of telescopes show that the observable universe is about 92 billion light-years wide (and growing faster and faster all the while). And the idea that advanced civilizations may exist beyond Earth has been buoyed by the ongoing exoplanet revolution. Today, the topic of extraterrestrial intelligence is a popular one, with multiple papers appearing every year from different research groups. He also wrote in the 1980 paper that those who believe in extraterrestrial intelligence are similar to UFO (unidentified flying object) enthusiasts, because both camps believe "we are going to be saved from ourselves by some miraculous interstellar intervention." Since evidence of such advanced machinery has never been found on Earth, Tipler argued that we are likely the only intelligence out there. The bulk of his paper dealt with how to get resources for interstellar travel, which he suggested could be achieved by having some kind of self-replicating artificial intelligence move from star system to star system, creating copies of itself as it traveled.
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