Twelve launch-capable space agencies (having as members about thirty countries) are, among other tasks, looking for alien life inside the solar system. They are exploring the four planets closest to the Sun: Mercury, Venus, Earth and Mars, which have three moons among them, and the five outer planets: Jupiter, Saturn, Uranus, Neptune and Pluto, which have one-hundred-and-sixty-three. With so many moons and planets, one would hope the chance to find life on more than one of them would be good.
Of the 166 moons and nine planets in the solar system, probes have managed to land on only five: Venus, Mars, Jupiter, Earth’s moon, and Titan (a moon of Saturn). Just three moons are located in the Goldilocks zone, where most scientists believe life has the best chance to take hold. Two of them are orbiting Mars at the outermost edge of the habitable zone and are, some scientists think, too cold for life. The third moon is, of course, our own.
Six moons in the solar system are comparable in size to the moon of Earth: Ganymede, Titan, Callisto, Io, Europa and Triton. All the rest are tiny with very little gravity—the force that can hold an atmosphere. No life has been found on any moon—let alone on any planet (except Earth)—thus far. During the next several hundred years, humans will continue to look for life in the solar system, should technology and civilization survive and develop.
The Kuiper Belt—which starts at Neptune and extends past Pluto—is a region of the solar system that is home to an estimated 100,000 bodies of frozen methane, ammonia, and water. [Editors Note: (August 2016) The explorer spacecraft, New Horizons, flew by Pluto on July 14, 2016 and is scheduled to fly by a Kuiper Belt object in January 2019.] Freeman Dyson—physicist, mathematician, and astronomer—says that life might be pervasive in this region of space and be easily detected once spacecraft get there. So we wait and wonder.
The solar system lies within a large disc-shaped galaxy called the Milky Way, which folks can see edge-on in the night sky if they travel out into the countryside away from the well-lit cities, which tend to wash out vision. It may surprise some to learn that no one really knows how many stars are in our galaxy. Credible astronomers believe the number to be somewhere between one-hundred and four-hundred billion—a huge range of uncertainty.
No one knows how many stars are similar to the sun. Certainly, no one knows how many planets there are, or how many moons. Despite a lot of reporting and speculation in the press, humans know almost nothing about the Milky Way. Space is vast, and astronomers have very few telescopes and satellites to accomplish the enormous job of taking it all in and cataloguing what they discover.
This lack of knowledge about the details of our own galaxy helps to explain why it is so difficult to understand the universe as a whole. Astronomers today estimate that between a hundred and two-hundred billion galaxies populate the visible universe (again, a very large uncertainty). [Editor’s Note: On October 1, 2017 CBS News reported that the Hubble space telescope has revealed that the number of galaxies may be as many as two-trillion—ten times more than previous estimates.] These two-trillion galaxies—and the other objects in the universe that lie outside the local area of our own galaxy—are too far away and too fuzzy for astronomers to know almost anything about them.
Our civilization is in the very first stages of placing sensors into space which eventually will help astronomers to learn more. One—the James Webb space telescope—is scheduled to launch in summer 2019. Its purpose—to tear down the 400-million-light-years-after-the-Big-Bang limit of the Hubble telescope. Humans are going to be able to look back to the beginning of time, at long last. Until then, the Drake equation (see illustration at beginning of article) and other speculative tools like it remain not much more than entertaining diversions.
Most articles, television shows, and movies that purport to portray the universe are, to risk overstating it, scams designed to seduce a gullible and curious public. The science community has a vested interest in public funding, so they tend to go-along with some of these fanciful depictions to pander popular support. Any claim that astronomers understand the nature of the universe is ludicrous. They do not. Not really. Not even close.
Three out of four stars in the galaxy are probably red dwarfs. These stars burn essentially forever, but are much cooler than the sun, which makes them impossible to observe without special infra-red detectors. These detectors must be placed outside Earth’s atmosphere in outer-space itself to avoid being overwhelmed by the infrared heat radiating off the earth’s surface. No one knows what percentage of red dwarf stars have planets suitable for life.
The sun, on the other hand, with its estimated life-span of eight to ten billion years, is similar to—who knows?—maybe one in five stars in the galaxy. It’s a guess, based on speculative sampling combined with wishful thinking. And based on sampling, scientists say that our sun is among the larger stars in the Milky Way.
Calculations involving galaxy-motion and gravity suggest that when astronomers look at the cosmos, they aren’t seeing ninety-five percent of what’s out there. Physicists call the missing stuff dark energy and dark matter. Where is it? No one knows.
Many of the galaxies that are visible from Earth are tens-of-thousands of times farther away than the farthest stars in our own galaxy, the Milky Way, which astronomers say is at least 100,000 light years across—a distance of six-hundred-thousand trillion miles. To put this distance into perspective, the latest space probes, which travel at roughly twelve miles-per-second, are not capable of crossing the Milky Way in less than 1.5 billion years.
Until scientists know more—and it could be decades or even centuries from now—prudence and the scientific method advise odds-makers to use the most conservative estimates, not the most optimistic, to speculate about intelligent life in the cosmos. Until evidence accumulates that is more compelling than what is available today, plugging conservative numbers into the Drake equation, or any other speculative tool, always seems to give the same discouraging result—a number so small it might as well be zero.
No intelligent life that can communicate across space should exist in our galaxy or anywhere else in the universe, for that matter. None. Zippo. Nada. Yet, here we are. It’s kind of mysterious, at least to me.
Anyway, Earth is fortunate to orbit a star that is located in a sparsely populated region of space. The sun lies in a nearly empty region between two densely populated spiral arms in the outer reaches of the galaxy. Life-destroying cosmic events are rare in this region. Earth’s relative safety has enabled the evolution of life to progress to intelligence, civilization, and space travel during the past four-and-a-half billion years.
The earth has a number of unusual features which make it a good candidate for highly evolved life. One important feature is its nearly circular orbit around the sun, which helps Earth avoid the catastrophic temperature variations characteristic of the more egg-shaped (elliptical) paths of some of the other planets. Only the orbits of Venus and Neptune are more round than Earth’s. Mars is five times less circular. Of all the solar objects, only Neptune’s moon, Triton, is known to have, for all practical purposes, a perfectly circular orbit.
Another advantage for Earth is its 300-mile thick atmosphere of nitrogen and oxygen, eighty percent of which lies within ten miles of its surface. Nitrogen and oxygen make up 99% of the earth’s atmosphere. The atmosphere is opaque to non-electrically-charged, high-frequency light. Nitrogen molecules block high-frequency ultra-violet light, while oxygen molecules, slightly smaller in size, block higher-frequency (shorter wave-length) x-rays and gamma-rays, which can be lethal to living organisms.
In the distant past, the earth’s atmosphere held fifty-percent more oxygen than it does now, which provided more shade against damaging high-energy light. Dinosaurs and large insects—like dragonflies with three-foot wing-spans—thrived in the highly-oxygenated air they breathed.
It is one of the wonderful ironies of our planet that the oxygen which empowers the biology of life also defends it against the physics of life-destroying high-energy light and cosmic rays that are always raining down from outer space.
In contrast to nitrogen and oxygen, which block high-frequency light from reaching the earth’s surface, carbon-dioxide, methane, and water vapor in the atmosphere trap low-frequency light (infra-red light, or heat) and prevent it from radiating (or escaping) into space.
These green-house gases work like a blanket to help keep the earth at a constant temperature. Without atmospheric moisture, methane, and carbon dioxide, the temperature of the earth would average one-hundred degrees Fahrenheit below zero and vary widely between day and night, like it does on the moon.
Although water vapor and carbon dioxide make a tiny percentage of the atmosphere, they have a significant impact on the planet’s ability to retain heat when their concentrations increase in the upper atmosphere. Exhaust from commercial jet aircraft, believe it or not, contributes greatly to the carbon dioxide and water vapor concentrations in the upper atmosphere.
After the terrorist attack on 911, the government suspended all flights over the United States—including those by commercial aircraft—for four days. The skies over America cleared themselves of clouds and turned a deep blue color. Temperatures dropped. I was amazed to observe these changes develop so quickly after all flying was suspended. It took about two weeks for aviation to return to pre-attack intensity. With the return of aviation, familiar weather patterns followed.
The planet Venus, unlike Earth, has so much carbon dioxide that its surface broils with heat. An explorer would have to hover thirty-seven miles above its surface to experience atmospheric pressures and temperatures like those on Earth. By contrast, the atmosphere of Mars, though almost entirely carbon dioxide, is thin—only 1% as thick as Earth’s—so it remains cold and sheds heat, even as it endures bombardment by dangerous-to-life high-frequency light and cosmic particles.
Another asset that gives Earth an advantage for life is its large moon, whose gravitational field acts like a vacuum cleaner to suck up cosmic-debris like asteroids and comets that might threaten to strike. Only Jupiter, Saturn and Neptune are similarly equipped.
Another life-enhancing feature of the earth is its large, open, ice-free, salt-water oceans. Most scientists believe salt-water oceans provide safe habitat for evolving life. Earth’s oceans make up three-fourths of the planet’s surface. In addition to providing a vast incubator for life, oceans reduce the probability that space-debris will fall onto land.
Odds are that debris will fall into the oceans, where it is rapidly cooled and rendered harmless. Should debris strike land and throw up clouds of dust and ash to block the sun, the oceans provide a safety-blanket of thermal protection.
Besides Earth, only Titan—one of Saturn’s many moons—has open oceans (of liquid methane and ethane) on its surface. These oceans are more like shallow seas, estimated to be about five-hundred feet deep. Scientists think Titan has a very salty sub-surface water ocean, as well.
NASA reported this year that another moon of Saturn, tiny Enceladus (310 miles in diameter), holds a six mile deep subsurface ocean—confirmed from Cassini fly-bys. Its over one-hundred geysers are what is populating Saturn’s E-ring.
Of the moons of Jupiter, only Europa, Ganymede, and Calisto are thought to harbor salt-water oceans.
Europa is known to have a salt-water ocean, but it is covered by miles-thick ice.
Ganymede, is believed to have several sub-surface salt-water oceans stacked one on top of the other and separated by ice layers. It’s surface is thought to be a rock and ice mixture.
Scientists suspect that Callisto has a salt-water ocean, but it might be sandwiched between ice layers sixty or more miles beneath its surface.
Only the oceans of Earth are open, un-frozen, and deep enough (averaging three miles) to protect the earth against most encounters with meteors and other space-debris.
Fortunately for Earth, the solar system itself contains a massive structure which helps to protect and shield it from danger. It is Jupiter, the large and strongly gravitational planet that, like the moon, pulls away space-debris that might otherwise zoom in to harm all life.
And Earth has, geologists say, a molten iron-core that emits a strong magnetic field to deflect life-destroying, electrically-charged cosmic particles, some which have energies approaching that of baseballs traveling sixty miles-per-hour.
The particles that do manage to blast through Earth’s magnetic-shield (magnetosphere) are often scattered and rendered harmless—fortunately—by collisions with the oxygen molecules in the earth’s abundant atmosphere. Muons, though, are a byproduct of collisions, which in high concentrations are lethal down to hundreds of yards beneath the earth’s land surfaces and its oceans. Muons are similar to electrons except that they are 207 times heavier and shorter lived.
The magnetosphere is strong enough to deflect the solar wind, which can strip away all or part of the atmosphere of any planet that lacks one (like Mars). The magnetosphere is effective and strong, because it is huge and surrounds the earth out to five Earth-diameters on the side facing the sun; and one-hundred Earth-diameters on the side opposite. In any small area of space, though, a simple bar-magnet is fifty times stronger.
Absent the magnetic field, life could evolve in relative safety only in the deep oceans or far below the surface of the earth. Stated differently: a strong, protective magnetosphere is essential for the survival of surface life on any planet. Large solar flares are known to have enough energy to kill exposed astronauts. It’s the main reason NASA is reluctant to send people to Mars. Mars lacks a magnetosphere.
All planets have magnetic fields of various strengths except Venus and Mars. The iron in their cores is believed to have frozen solid hundreds of millions of years ago, which caused their protective magnetic fields to collapse. It’s a shame, because both planets have other attributes which might otherwise make them good candidates for life.
The only moon known to have a magnetic field is Jupiter’s Ganymede. Jupiter itself harbors a field fourteen times more powerful than Earth’s. The giant planet’s four largest moons orbit inside it, where they are protected from the solar-wind and low frequency (low-energy) cosmic particles. By contrast, Mercury’s magnetic field is one-hundred times less powerful than Earth’s.
Despite these several advantages for sustained evolution of life, the earth has the apparent disadvantage of a volatile climate which, scientists believe, has turned cold and icy during several extended periods. I mention this volatility to remind people that the circumstances that have enabled life to advance to the technological civilization of today are complex and not obvious.
Until scientists are able to tease out of history what is actually important and significant for the development of advanced life, no one can really know what the rest of the universe may have in store—unless we travel out into space and explore it.
Here’s the problem. The closest star to the sun is over twenty-five trillion miles away. The fastest spacecraft of the future that we can build—based on the best technology anyone has thus far envisioned—can’t get us to the nearest stars, Proxima Centauri, or the binary star system, Alpha Centauri, in less than 25,000 years.
How are we going to explore the universe? How are we going to answer the questions about our place in the cosmos, when we can’t travel to the nearest stars?
There are trillions of stars, most of them many millions of times farther away than these, our closest neighbors. It seems hopeless that anyone will ever know the answers to the basic questions about the universe that so many are asking.
Still, in my heart of hearts, I want to believe we will find the way.
Editors Note: November 2017. NASA announced this fall that the latest count of galaxies is as high as two trillion. The velocity required by spacecraft to escape the Milky Way galaxy from Earth (it’s 25,000 light years from the galaxy center) is 210 miles per second. At this velocity the nearest galaxy—Andromeda—is a flight of 2.28 billion years. It doesn’t really matter.
The Parker Solar Probe is scheduled to launch during the summer of 2018. It will use seven gravity assists from Venus over six years to reach a velocity of 120 miles per second before it embarks on a 2024 suicide mission into the outer atmosphere of the sun.
Venus and the sun combined can’t accelerate the Parker Solar Probe to galaxy-escape velocity. Minus gravity assists, the fastest vehicles in development today by space-flight engineers will cruise at speeds less than 40 miles per second. Humans are trapped inside the Milky Way. They can’t leave, at least not yet; most likely, not ever