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
stars with planets capable of supporting life are thought to be
confined to what is known as the galactic habitable zone,  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. 
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
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.
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,  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. 
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.
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
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.
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.
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.
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
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. 
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.
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.
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.
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.
Methods for detecting extrasolar planets vary and include: astrometry,
radial velocity, pulsar timing, transit method, gravitational
microlensing, circumstellar disks and direct imaging.
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.
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 Centauri—the
distance a manageable 4.2 light-years, close enough for a future visit
and possible colonization. A real shocker times seven—seven Earth-sized planets orbiting a single star less than 40 light
years away—three in the habitable zone. The star named TRAPPIST-1 is small, barely the size of Jupiter. The orbits of the planets are short—just 1½ to 20 days.
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).
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).
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.
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.
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.
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."
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.
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
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:
cast this message into the cosmos . . . Of the 200 billion stars in the
Milky Way galaxy, some—perhaps many—may 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
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