Although this search is now in its fifth decade (on again/off again/on again, in various incarnations), to date, we have found - exactly nothing. However, considering the practically infinite number of stars in the visible universe, we've just begun to explore in this sense, and what has been explored is only a tiny sample; merely a fraction of a percent of all known stars.
Moreover, it has become altogether clear to us - by the sheer number of new planetary systems that have been discovered, using a highly specialized piece of orbiting, electronic equipment: the Kepler Orbiting Observatory*, that, life as we know it, can exist – and, possibly, even flourish to the point of the production of large, macrobiotic organisms, like ourselves, and, possibly intelligent organisms on planets, far beyond our own Solar System.
Additionally, our own “Solar system” is just one of many, many other planetary systems that are now known to exist, circling nearly every star (and, technically, making those stars, “suns”, in their own right). How many planets? - in how many new planetary systems? - more than 4,500, recently discovered planets, hundreds of which have been confirmed to be Earth-like, in size, existing in planetary systems currently numbering over 2,000 - that's how many! Astonishingly, the current rate of planetary discovery, is, roughly: one planet, every 30-40----days! (And, I'm happy to report, that, I have been a part of that search for the last few years, and have contributed to at least one planetary discovery!) These new planets are called exoplanets; the prefix, 'exo', denotes that, they are planets, which are beyond - and are not a part of - our own Solar system.
The “Solar system”, proper - our planetary system of planets is, of course, called the Solar system for the simple reason, that, the sun's proper name is, “Sol” (pronounced, "sole"), which takes its name from the Roman god of the Sun, hence, the "Sol-ar" system, or, the system of planets belonging to the star – our star - "Sol".**
We used to believe that planets orbiting stars, other than the sun, were a rarity until we were able to analyze data gathered by the Kepler space telescope - a powerful, Earth-orbiting satellite whose job it was to detect certainly, ‘signature’ anomalies, in the stars of our galaxy. When we did, we found that planets were very much in abundance, in orbit around these stars; typically, from one, up to, at present - eight; in fact - we now know that there are more planets in our Milky Way galaxy - than there are stars - far more! Naturally, this extends to countless other galaxies, as well.
We know that life has arisen at least one time in the universe - right here on our own planet – and, that this life is composed of the very elements with which the rest of the universe is made. This information was gleaned when astronomers learned that they could analyze the light of a star, using an optical instrument similar in principle to a prism called the spectroscope. That instrument divides star light up into a visible spectrum of color, interspersed with the dark, tell-tale, absorption lines of various elements, e.g., hydrogen and helium, and, with that, astronomers found that every element in our bodies, and all of the elements found in the Earth, are also scattered throughout the rest of the universe. This tells us that the universe, is, essentially, and fundamentally, the same everywhere.
Today, we understand that the elemental composition of our bodies was, literally, forged under the tremendous heat and pressures within the interiors of a certain type of star called a supernova - which is what prompted the astronomer and exobiologist, Carl Sagan, to make the surreptitious, and, somewhat deceptively accurate statement: "We are all, star-stuff".
And, so it is true. We are what the stars are; indeed - star-stuff. But we are also living star-stuff.
We know that the elements of the universe, ultimately, came together to produce life, here on our planet, by way of the “kick-starting” of amino acids - the very building blocks of all living things. Amino acids seem to be so prevalent in the cosmos that they are even found within the dense, frozen cores of certain types of meteorites, known as, carbonaceous chondrites: a kind of meteorite formed from the “leftover”, constituent, primordial elements from the Solar System's formation.
Life on the Earth comes in many varieties and adaptations, culminating in the million+ known species of living organisms on our planet; and there isn't a single biome that is lacking for life forms to inhabit them, including, deep into the Antarctic's mile-deep ice sheets.
Now, let’s consider that most of the exoplanets, now known to exist, were discovered in just a tiny portion of the sky – a swath, about the size of an adult's, closed fist held at arm's length. Now, imagine this tiny area, just a few degrees across, containing all those stars, closely examined by the Kepler telescope - with their planets, in orbit around them - and multiplying that out, to cover the rest of the sky. It would seem that, everywhere we look, we are going to find other worlds – many of them, as inhabited as Earth. The universe is likely littered with them! And, using the principle of mediocrity, if we can use ourselves as an example, then, we are one instance of a "living" planet.
How many more life-bearing planets must there be out there? What kind of life these worlds may host, is entirely unknown - be it, intelligent, or otherwise. But before we came to know, what we know, do know, we had believed that planets – and, especially, planets like the Earth - must be a very rare occurrence in the Cosmos. Now, however, we know this not to be the case; more than 750, Earth-sized planets, have now been confirmed to exist.
Data from the Kepler Space Telescope is still being downloaded to NASA's Jet Propulsion Laboratory, in Pasadena, California, and it will take several more years to sift through all of it. How many more exoplanets will be, 'plucked from the sky', as we continue with our rapid progress in the space sciences? In my opinion, I think we will find, that: life in the universe - will be the rule, rather than the exception. Moreover, rather than seeing our Universe, as a barren, lonely desert, with just, one, unique, life-bearing planet – we may come to look upon it, as a potential, veritable, 'life-building machine'.
*(a large, orbiting space telescope, akin to the Hubble Space Telescope)
**Exo-planetary systems take on the name of their 'host', or, 'parent' star, so that, one can imagine a "Vegan System", or, "Rigellian System".
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