X-Ray image of Supernova Cassiopeia A in the constellation Cassiopeia.
From explosions like this all of the matter in our bodies and our planet was created.
Photo from Chandra X-Ray Observatory in orbit above Earth. Courtesy Harvard University.

Why Not Visited Yet


Our galaxy, the Milky Way, contains 40 billion stars, of which ours is one. The probability of a civilization visiting our solar system is 1 in 40 billion.

If an explorer civilization in the Milky Way visited one star in the Milky Way each year and began these visits when our sun was born (5 billion years ago), today they would still have 35 billion stars in the Milky Way to visit; the odds of our star being visited would be 1 in 35 billion.

Our star is located on an outer arm of the Milky Way galaxy, far from the galaxy's central, bulging disk of stars. If the galactic center were a city, we would be a tiny town out in the sticks, not far from the abyss of the galaxy's edge.

If our sun were near the center of our galaxy, our chances of being visited would be much better. There are far more stars in the center of the galaxy than in our part of the galaxy and they are packed much closer together. The likelihood of any unit volume of the galaxy supporting life increases with the number of stars within that volume. Because the density of stars per unit volume is highest near the center of the galaxy, there is a much higher chance for civilizations to exist near the galactic center than near us.

An exploring civilization would focus its efforts on the stars and planetary systems closest to them, working outwards as technology allows. For an exploring civilization located in the galactic center, our solar system would be "way way way down the list" as a visitation priority. Due to its density of stars, the galactic center contains multitudes of exploration targets. All of these targets are much easier to reach than our solar system. It is unlikely an exploring civilization near the galactic center would consider a journey to our solar system until all more proximate targets had already been explored.

The thick cloud of dust and gas surrounding the galactic center skews the exploration efforts of a civilization near the galactic center. Because we cannot see into the galactic center at the frequency of visible light, civilizations there cannot see us. Except via radio and x-rays, our neighborhood of stars on the edge of the galactic disk is invisible to civilizations near the galactic center. A civilization would not launch an exploration to our neighborhood of stars unless they had evidence it contained interesting objects. The light obscuring dust and gas around the galactic center makes it very difficult for civilizations there to be aware our star and solar system even exist.

The density of explorer civilizations in the Milky Way Galaxy is highest where the density of stars is highest and lowest where the density of stars is lowest. This creates a triple whammy for us. Our section of the galaxy has a low density of stars. Our section of the galaxy is very far from those areas with a high density of stars. And our section of the galaxy is invisible to center of the galaxy and the section of the galactic disk opposite the center from us. It is ironic that we on Earth have a much more complete view of the Andromeda Galaxy (1.7 million light years away) than we do of the center of our own galaxy and the galactic disk opposite the center from us.

Why would someone visit us?


The composition of our solar system is not unique. Spectroscopic analyses have yet to find any elements in outer space different than those which exist on Earth. All objects thus far observed in our universe consist of varying proportions of the same 92 elements known on Earth and their sub-atomic particles. Even complex organic molecules such as amino acids have been detected in outer space.

This means our planet and solar system has no special value as a source of materials for explorer civilizations. Any explorer civilization which requires elements such as gold, platinum, cobalt, niobium, tantalum or uranium can easily find them much closer to home than in our solar system. If you need a hammer, you don't fly 7,000 miles to Siberia; you go to the local hardware store. Analyses show our galaxy is well stocked with all 92 of the elements found on Earth. For this reason, "resource extraction" is an unlikely motivation for civilizations to visit us.

The only somewhat unique thing our solar system could offer an explorer civilization is life.

It seems doubtful an explorer civilization would come all the way to Earth to enslave us and make us work for them; they could likely find species much closer to home to conquer and enslave.

If explorer civilizations are similar to us, a motivation to visit could be religious evangelism. Like religious pamphleteers banging on our doors, an explorer civilization might visit to convert us to their 12-dimensional religion. Let's pray not.

An explorer civilization could visit because they would like to see our passenger pigeons, great auks, Carolina parakeets and ivory billed woodpeckers, but we would have to tell them we killed them all.

An explorer civilization could visit us out of scientific curiosity. If so, the explorers may conceal themselves so they can observe us "in the wild." A civilization with the technological capability to travel to our solar system could easily make themselves invisible.

Remote Observation


Humans have gleaned enormous amounts of information about our solar system, local star neighborhood, galaxy and universe by remote means. We have devised technologies to gather much of this information without our bodies leaving Earth. The monstrous expense, risk and difficulty of sending manned missions into the solar system has forced us to develop robotics and other devices to gather information for us; ie. taking pictures from the surface of Mars, sampling its soil, etc. For extremely hostile environments such as the surface of Venus or Titan, robots and probes are the only way to gain access to these places.

It is likely explorer civilizations use remote observation. Civilizations more advanced than ours have likely developed remote observation techniques which make ours the equivalent of Galileo without a telescope. Any explorer civilization with a biology similar to ours would encounter similar challenges when sending people into space instead of robots, probes or remote observations from home. This would help explain the absence of visitors at our doorstep.

Our Echoes in Space


Since the invention of radio transmission 100 years ago, an expanding globe of manmade radio transmissions has radiated at light speed outwards from Earth in all directions. The earliest of these transmissions has now travelled 100 light years into space, a distance much greater than humans may ever travel.

Civilizations 50 light years from Earth can now listen in real time to our radio broadcasts from the 1940s and eavesdrop on encrypted messages sent from all of the armies and governments during World War II. They can now detect bursts of gamma radiation emanating into space from the detonation of the atomic bombs at Hiroshima and Nagasaki in 1945.

With the invention of the hydrogen bomb in the late 1940s, the fear of nuclear annihilation sparked a frenzy of reports of UFO sightings which has now fallen to nearly zero. Many ascribed these sightings as visits from civilizations trying to prevent us from destroying our blue and green planet.

We will never know if these UFO visits actually occurred, since no conclusive evidence has ever come to light. However, any civilization within 50 light years from us which can decode our radio transmissions knows how close we have come to destroying our planet with nuclear weapons. Like us, all they have to do is listen to the news.

If any civilization within 50 light years has translated our radio transmissions since the 1950s and observed the bursts of gamma radiation from our nuclear explosions, they might send a message of reply. The gamma radiation from Hiroshima and Nagasaki are only now reaching civilizations 50 light years from us. If these civilizations respond, their message will not reach us until 2050. If these civilizations can translate and understand our radio transmissions, they will send their reply in a form and at wavelength identical to ours, so they can be sure we will be able to identify the message and understand it.

Children of Bacteria


The Second Law of Thermodynamics makes our bodies dissolve at death. After death our bodies disorganize and disaggregate. Bacteria and other life use our dead bodies as fuel to organize and aggregate their own bodies so they can live.

On the moon, our bodies do not rot. They remain perfectly preserved and lifeless. Our bodies remain perfectly preserved on the moon because no bacteria exists there to use our bodies as fuel to live. Only on Earth do our bodies rot and dissolve when we die. If bacteria did not consume our bodies we could not live. If Earth could not support bacteria, we would not exist.

A Note in a Bottle in an Endless Sea


We on Earth are like a note in a tiny bottle floating thousands of miles offshore in the sea, wondering why no one has picked us up.

We are not the writer of the note. We are not the bottle which holds the note. We are not the yellowed paper upon which the note is written. We are the note itself.