The Milky Way

The Milky Way, a barred spiral galaxy, is just one of the billions of galaxies that make up the observable universe. From Earth it appears as a band of misty light arching across the night sky brightest when looking toward the galactic center. A hundred thousand light years (30 kiloparsecs) in diameter and averaging a thousand light years (0.3 kiloparsecs) thick it is home to between 200 and 400 billion stars
the Sun, our star, lies within the inner rim of the Orion Arm a small stub of gas and dust located between the much larger Perseus and Sagittarius Arms. The Milky Way is approximately thirteen and a half billion years old the Sun four and a half billion.

The stars with planets capable of supporting life are thought to be confined to what is known as the galactic habitable zone, [1] an area within which our Solar System fortunately resides. Earth itself, also fortunately, resides within what is known as the Solar System’s circumstellar habitable zone. [2] It is possible that life can arise only on planets that meet both these criteria, but even so given the age of the galaxy and the sheer number of stars, thousands if not millions of extraterrestrial civilizations have to exist or must they (the Fermi paradox). What is behind the Great Silence?

The Drake equation a formula devised in 1960 by university professor Frank Drake, is an attempt to quantify the number of extraterrestrial civilizations existent in our galaxy and the factors which influence their rise, type, lifespan and decline.

The equation states:

The number of civilizations in our galaxy with which we might perhaps communicate is equal to the average rate at which stars are formed, times the percentage of stars having planets, times the average number of potentially life supporting planets, [3] times the number that actually develop life, times the number on which intelligent life actually evolves, times those that develop technological civilizations and signal their presence, times the length of time in which they continue to do so. [4]

Obviously, according to the equation, the number of civilizations with which we can converse is dependent upon the numbers affixed to the variables. Carl Sagan at one time placed it at one million, others, more pessimistically, have placed it much lower from a handful to none.

We search the Galaxy for life with an ever increasing technological sophistication, the devices in our arsenal ranging from ground based radio and optical telescopes to telescopes in orbit the information they gather sifted and correlated by super-computers operating at prodigious speeds, and yet so far nothing.

A number of theorems attempt to explain our apparently lifeless universe:

Earth exists as a zoo or wilderness area isolated and shielded from the more civilized galaxy.

Aliens are here living among us, studying us, their technology for the most part shielding them from observation. After all, purported sightings of strange beings and their craft reach back into prehistory.

A civilization is invisible to us because it isn‘t technological, perhaps its environment makes that impossible, a water world inhabited by beings similar to whales or dolphins for example.

Civilizations may come into being exist for a cosmologically short time and then disappear. It’s possible that civilizations have existed in the Galaxy prior to us and many more may exist after we are gone but that at present we are alone.

Humanity hasn’t been searching long enough, though we have existed as a species for about 200,000 years we have been civilized for but a few thousand. Technologically we are still infants our ability to detect and comprehend a message from an extraterrestrial civilization measured in mere decades.

We have existed for too short a time to have been found ourselves and this may be a good thing. It's possible that other civilizations exist but are themselves hiding from some malevolent entity or super race that views all others much as we view annoying insects and will destroy us as soon as we are discovered. Perhaps we too should be hiding. [5]

And lastly and perhaps the most frightening, it may be that technological civilizations invariably destroy themselves shortly after achieving technological status utilizing any number of means including but not limited to, biological warfare, an irreversible environmental disaster or nuclear war.

[1] A zone circling our Galaxy's core at a distance and within which a sufficiently high level of heavy elements exist to favor the formation of rocky planets and provide for the building blocks of life but sufficiently distant in that close encounters between stars are infrequent and the lethal radiation from supernova and the supermassive black hole at the Galaxy’s center is diminished. In the Milky Way the GHZ is located twenty five to thirty thousand light years from the Galaxy’s core (though estimates vary) and contains stars four to eight billion years old.

[2] A zone circling a star at a distance and within which the amount of stellar radiation is conducive to life, liquid water can exist on a planets surface and the chance of large asteroid impacts are at a minimum. In our solar system the habitable zone (sometimes referred to as the "Goldilocks Zone" because it's neither too hot nor too cold but just right) extends from just outside the orbit of Venus to roughly the orbit of Mars.

[3] 51 Pegasi b, a hot Jupiter, was discovered October 6, 1995, the first of the hundreds of extrasolar planets that have since been found orbiting Sun-like stars (51 Pegasi is a yellow dwarf and early discoveries were limited to gas giants with orbits tight to their primary). Today, however, new technology and refinements in technique have made it possible to identify more Earth-like planetary bodies such as Kepler-20e and Kepler-20f which with diameters 0.87 and 1.03 that of Earth, respectively, are the smallest exoplanets orbiting a normal star discovered thus far though apparently not the most hospitable; presently that title falls to super-Earths (planets larger than Earth but smaller than ten Earth masses) such as Gliese 581g (if it exists), Gliese 581d, Gliese 370 b or the extrasolar planets Kepler-442b, Kepler-283c, Kepler-296e and Kepler-186f, some of the super-Earths discovered by and deemed to be orbiting within the habitable zones of their prospective star systems by the Kepler mission probe.

[b] Methods for detecting extrasolar planets vary and include: astrometry, radial velocity, pulsar timing, transit method, gravitational microlensing, circumstellar disks and direct imaging.

[c] The number of cataloged extrasolar planets (exoplanets) has experienced a dramatic upswing following the successful launch from Cape Canaveral Air Force Station of NASA's Kepler mission aboard a Delta ll rocket March 7, 2009. At present the probe, searching for Earth-like planets orbiting within the habitable zones of 150,000 stars located in the northern constellations of Cygnus, Lyra and Draco, has identified thousands of candidates with hundreds confirmed.

The latest in the search for an Earth 2.0: A real shocker, it's possibly right next door. Officially named Proxima Centauri b, it orbits our nearest stellar neighbor Proxima Centaurithe distance a manageable 4.2 light-years, close enough for a future visit and possible colonization. A real shocker times sevenseven Earth-sized planets orbiting a single star less than 40 light years awaythree in the habitable zone. The star named TRAPPIST-1 is small, barely the size of Jupiter. The orbits of the planets are short—just 1 to 20 days.

[4] SETI (Search for Extraterrestrial Intelligence) continues to scan the skies for radio emissions using vast banks of radio telescopes notwithstanding the fact that technological species may use this method only for short lengths of time or not at all (the human race has only been communicating via electromagnetic emissions for slightly more than a century while our ability to detect such emissions from space only goes back to 1937 and the invention of the radio telescope).

[5] In 1972 and 1973 identical gold-anodized aluminum plaques were attached to the Pioneer 10 and 11 probes destined for interstellar space: amongst other things the small plates showed the nude figures of a man and woman, the spacecraft's outline and a graphical representation of our home star system as it was perceived at the time of launch (a sun and nine planets).

[b] In 1974 the 305 meter Arecibo radio telescope (the world's largest curved focusing dish) beamed a message toward M13 a high density globular star cluster located over 25,000 light years from Earth: amongst other things the signal contained information about humanity (e.g. the number of nucleotides in the human genome and how many of us there are), our home star system, Earth's position within it and even the basic biochemical makeup of indigenous life.

[c] On February 4, 2008, the National Aeronautics and Space Administration (NASA), celebrating its 50th anniversary and the Beatles' earliest beginnings, beamed the groups' song "Across The Universe" into space in the direction of Polaris, the North star, a yellow supergiant located some 431 light years from Earth. Declared to be "Across the Universe Day" by Beatles' fans, February 4, 2008, being the 40th anniversary of the recording, people were encouraged to join in and play the song simultaneous with NASA's transmission.

[d] On October 9, 2008, members of the website "Bebo" beamed "A Message From Earth" toward Gliese 581 c, a super-Earth, using Ukraine's RT-70 radio telescope. The message is due to arrive early 2029 and a response, assuming there is one, could be received by 2049. At 20.3 light years Gliese 581 is the 87th closest star system to the Sun.

[e] Voyager 1 launched by NASA on September 5, 1977, and now the farthest from Earth of any man-made object, is according to the space agency in uncharted territory on the boundary of a region known as the heliosheath, the outermost layer of the heliosphere a bubble of charged particles that envelopes our solar system on the other side of which is interstellar space. With the outer edge (heliopause) an unknown Voyager project scientist Ed Stone, at Caltech, put it this way "Voyager is showing that what is outside is pushing back. We shouldn't have long to wait to find out what the space between stars is really like."

On September 12, 2013, a long awaited announcement, the wait was over, verification that Voyager 1 had exited the heliosphere and was now in interstellar space (the generally accepted date of the event August 25, 2012).

Across 12 billion miles (19 billion kilometers) sounds from interstellar space courtesy Voyager 1.

In the waning days of 2018, Voyager 2, launched August 20, 1977, followed its twin into interstellar space having been delayed by a different trajectory that allowed for a "Grand Tour" of the outer planets.

Voyager 1 and 2 each have golden phonograph records attached. More comprehensive than the Pioneer plaques they contain information selected to portray the diversity of Earth's life and culture along with printed messages from U.S. President Jimmy Carter and U.N. Secretary-General Kurt Waldheim. Below is an excerpt from President Carter's official statement:

"We cast this message into the cosmos . . . Of the 200 billion stars in the Milky Way galaxy, some
perhaps manymay have inhabited planets and space faring civilizations. If one such civilization intercepts Voyager and can understand these recorded contents, here is our message: We are trying to survive our time so we may live into yours. We hope some day, having solved the problems we face, to join a community of Galactic Civilizations. This record represents our hope and our determination and our goodwill in a vast and awesome universe."   

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