Life in the universe
The concept of life in the universe represents a number of ideas that have long stoked the fires of human imagination. These ideas include life on Earth traveling out into the universe—that is, to outer space; life from Earth, especially humans, living elsewhere in the solar system or the universe; and life not from Earth—that is, extraterrestrial life—and our search for it. Although people have pondered these ideas since time immemorial, it has only been recently—within a human lifetime—that humanity has made significant
strides in these endeavors. Since the 1950s, we have sent rockets and satellites into space. Since the 1960s, we have sent humans into space and returned them safely to Earth. Since the 1970s, we have sent humans to live in space for weeks, months, and even years at a time. Since the 1980s, scientifically significant searches for extraterrestrial life have been made. And since the 1990s, astronomers have discovered hundreds of planets orbiting stars other than the Sun.
Life “as we know it”
Ironically, though astronomers search for life beyond Earth, biologists here on Earth have still not conclusively determined what constitutes life as we know it. The basic definition of a living thing is something that begins an active existence (is born), changes and matures over time (grows), replicates itself through a wellordered process (reproduces), and then ends its existence (dies). On Earth, all things that go through these steps achieve them through the complex interactions of very large molecules such as ribonucleic acid (RNA) and deoxyribonucleic acid (DNA). Some terrestrial things, however, go through all four of those steps, at least by some definitions, but may or may not be alive; certain viruses, for example, defy
easy classification. In the cosmic context, the line between living and non-living things may be even blurrier; stars, after all, are born, grow, reproduce, and die—all after a fashion, at least. So, are stars alive?
It is still beyond current technology to look for individual living things. When astronomers search for extraterrestrial life, therefore, the targets are ecosystems— environments on other worlds that could harbor life. As far as we know it, life requires three basic ingredients: liquid water, a consistent source of gentle warmth, and a small set of basic chemical elements such as carbon, nitrogen, sulfur, and phosphorus. (Liquid water provides hydrogen and oxygen, as well.) If all three of these requirements are found together anywhere on Earth, life is always present; extrapolating to the universe, environments with these requirements may well also harbor life as we know it.
The study of the universe—perhaps especially the study of life in the universe— often depends on a key assumption called the Copernican Principle. This principle, named after the Polish astronomer who proposed that Earth was not the center of the universe, posits that the same laws of nature hold true everywhere in the universe. Earth is not an exception to that rule; in other words, “we are nothing special.” This means that, if life formed on Earth because it had certain characteristics, then any other planet matching those characteristics will have the same chance of eventually supporting life, too. The main question is, which characteristics are the important ones? Scientists think that the keys to life on Earth are liquid water, the right chemicals, and a steady energy supply. It is not certain, though, that these are indeed the correct necessities for life, nor is it certain what kinds of life these conditions could support.
The discovery of exoplanets in very strange orbits—for example, gas giants orbiting their host stars at distances much closer than Mercury is from our Sun—has shown that planets often migrate from the orbits where they formed. That means, in turn, that planets can often be flung out of their planetary systems by gravitational interactions with other migrating planets, as if they were in an interplanetary game of billiards. Although this has not happened in our own solar system for billions of years, the Copernican Principle suggests that, someday, our solar system could undergo such an upheaval as well. If this planet-flinging scenario proves to be true, then there could be billions of rogue planets flying through interstellar space, free from the gravitational wells of the stars that birthed them. If such a planet had a thick crust and an underground liquid ocean, then tidal or geothermal processes deep in those planetary cores may be warming that ocean, creating a teeming ecosystem that is hurtling unfettered through the galaxy. Could such a planet fly through our own solar system someday?
The odds are slim to none, but it is not impossible. Since the definitive discoveries of the first exoplanets in the 1990s, hundreds of exoplanets (or “extrasolar planet”) have been discovered. Of these detections, dozens have been shown to exist in exoplanetary systems that contain more than one planet orbiting a single star. These discoveries completely changed the way scientists thought about extraterrestrial life. On the one hand, if so many planets exist, and so many planetary systems with multiple planets exist, then surely solar systems like ours exist—and with them, the possibility that they harbor life as we know it. On the other hand, the remarkable variety among planetary systems discovered so far suggests that scientists may have thought too narrowly in the past about environments where life might thrive. Instead of basing the search for life solely on solar systems like our own, or planets like those orbiting the Sun, astronomers are now thinking much more broadly and creatively about ways to find life in the universe.
Habitable zones—where the heat from a star would keep water on a planet’s surface liquid—exist around most of the stars where exoplanets have been discovered so far. Almost none of their exoplanets, however, are orbiting in those stars’ habitable zones. In November 2007, however, a planet was discovered around the star 55 Cancri  that appears to be orbiting in the habitable zone. This planet is almost certainly a gas giant planet and not a terrestrial one—its minimum mass is about twice the mass of Neptune. But like the gas giants in our solar system, it may have moons orbiting it that may have rocky or metallic crusts and mantles. Those moons, if they exist, could harbor liquid water. Thus, with its host star providing just the right amount of heat and light, such a moon could harbor life as well.
Of all the areas of research in astronomy today, the study of exoplanets is one of the most intriguing. The search for extraterrestrial life, which was previously relegated to the realm of science fiction, has only recently become a viable topic for legitimate scientific study. The combination of searching for exoplanets and for the life forms that might inhabit them is only in its infancy. Scientists are steadily inventing new
ways to go about this task, which has no precedent. Ultimately, this new adventure embodies everything that is exciting, inspiring, and just plain fun about astronomy.
In the end, the questions we will have to answer will be ones we have yet to ask.

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