Authors: Don Lincoln
The most important fact is that scientists no longer need to speculate about planets around other stars. We’re observing them directly. The Kepler team’s best estimate is that at least 5% of stars include at least one Earth-sized planet and at least 20% of the stars have multiple planets. Given the youth of this field of research, it is highly likely that, as the sensitivity of the equipment improves, we will find the actual numbers are even higher. The Kepler spacecraft is observing about 150,000 of the approximately 300,000,000,000 stars in the galaxy. This is an absolutely fascinating time to be an extrasolar planetary astronomer, and the fun is only beginning.
If there are many stars and many planets, then the next question is how many of those planets harbor life and how many of the planets with life host intelligent life? These numbers are much harder to estimate, but they remain the crux of the question.
In order for Aliens to exist, they need a stable environment. Life on Earth developed several billion years ago. Complex animal life was first preserved in the fossil record about 530 million years ago. Mammals appeared about 210 million years ago, and the first primates originated perhaps 50 million years ago. Finally, the first hominids originated about 17 million years ago and our own species,
Homo sapiens
, is only about 50,000 to 100,000 years old.
It took billions of years for intelligence to develop on Earth. If, at any time during those long eons, the Earth became uninhabitable for life like us, we wouldn’t be here. This doesn’t mean that the climate on the planet must be stable, after all there have been periods during which the entire Earth was frozen over, and huge volcanic eruptions and meteor and comet strikes have killed off vast swathes of species. But there have been no “sterilizing events.”
What might constitute a sterilizing event? Well, one theory of the origin of Earth’s moon is that a Mars-sized planetoid collided with an early version of Earth perhaps 4.5 billion years ago. This collision would have thoroughly melted any crust that had formed by that time. An impact like that would have extinguished life.
Another danger to a planet’s biosphere would be a nearby supernova. If a supernova occurs within a few tens of light-years from the Earth, it could deplete a large fraction of the Earth’s ozone. Since ozone protects the Earth from the sterilizing ultraviolet light from the sun, loss of a large fraction of the Earth’s ozone would be a catastrophic event.
Even more dangerous (although much rarer) is a gamma ray burst. Gamma ray bursts are a special class of supernovae that occur when a very massive, rapidly rotating star explodes. Rather than having the energy expand in a spherical pattern, the energy is blasted into two beams shooting out from the poles of the star. To give a sense of the amount of energy we’re talking about, a gamma ray burst can be observed billions of light-years away. The energy of a typical burst releases in just a few scant seconds as much energy as our sun will release over its entire ten billion year lifetime. The energy release of a gamma ray burst is an astoundingly dangerous occurrence. Luckily, they are rare. One occurs perhaps every 100,000 or 1,000,000 years in a galaxy the size of the Milky Way, and they are a danger only if the beams are pointed directly at us. The nearest candidate for a gamma ray burst is one of the two stars in the binary star system WR 104. It is located about 8,000 light-years away from us in the general direction of the galactic center and its axis appears to be pointed roughly in our general direction. The chances that the burst is pointed exactly at us are rather small, so there is no reason to worry. But if it were, it could badly damage the ozone layer and thus devastate the biosphere.
Dramatic events like supernovae and gamma ray bursts are not required to seriously damage a planet. Little things like the evolution of a star’s output over the course its lifetime can also be the source of destruction of life. Around any star, there is a range of distances in which water can be liquid. Estimates vary, but the current habitable range for our sun is about 0.97 to 1.37 times the Earth’s orbit. Thus the Earth is barely inside the habitable zone. Were the radius of the Earth’s orbit just 10% smaller, Earth would be too hot for life.
It’s actually more complicated than that. The energy output of the sun has evolved over time. Several billion years ago, it is thought that the energy output of the sun was about 80% of what it is now. That suggests that the habitable zone for the solar system would exist for smaller radii than today. Back then, the minimum and maximum habitable distances from the sun was 0.80 and 1.15 times the radius of the Earth’s orbit, respectively.
What matters is the “continuously habitable zone,” which is the biggest minimum and the smallest maximum radius over the course of the sun’s lifetime
within which life can exist. So far, the continuously habitable zone is the very narrow region of about 0.97 to 1.15 times the radius of the Earth’s orbit. A planet outside that small region would not remain habitable long enough for intelligent life to develop.
You should be aware that the numbers offered here are actually quite contentious within the astrobiology community. Different experts have come up with different estimates. Considerations like the chemical makeup of the atmosphere and effects arising from plate tectonics can change the outcome of the calculations. Further, the discussion here is concerned with energy from the central star. There are other energy sources, like the tidal flexing of moons in close orbit to a big planet. This is the reason that moons like Jupiter’s Europa are considered to be candidates for places where life might have arisen.
But the general idea is still largely valid. For life like us, which depend on the warmth of our sun to survive, there is a “Goldilocks Zone” around the central star—not too warm and not too cold. Other stars are hotter or colder and the details of the habitable zone will adjust accordingly. But still, the star’s energy output will have to be stable enough so that a planet on which life begins will continue to be a hospitable spot for life to evolve.
Even if a star is very stable, it is equally imperative that the orbit of the planet be stable and pretty circular. A highly elliptical orbit will bring the planet alternatively too close and too far from the central star. While the orbits of the planets of the solar system are elliptical, they are close enough to being circular that the difference is indistinguishable to the naked eye. A slightly elliptical orbit is allowed, as long as the orbit doesn’t leave the habitable zone.
Further, it’s not enough that the orbit of the planet hosting life be nearly circular. If another planet in the planetary system has an eccentric orbit, the gravity of the uncooperative planet could eject the hospitable planet either into the star or out into the coldness of interstellar space.
There is a very long list of things that have to go right to have a planet that can (1) have life begin on it and (2) allow life to persist long enough to evolve an inquisitive intelligence like our own. It may well be that this is an extremely rare situation.
Wrap Up
If you look at the simplest of facts, for instance Carl Sagan’s oft-quoted estimate of there being “billions and billions” of stars in the universe, including that there are some 300 billion stars in our galaxy alone, it seems unfathomable
that there is not life elsewhere in the universe. If not, to repeat another commonly made statement, it sure seems like an awful waste of space.
The history of science has been an unremitting onslaught of the mediocrity principle. While humanity once thought that the Earth occupied a special spot on the solar system and then the cosmos, we now know that in many respects, the Earth is a small planet around an undistinguished star, orbiting in an undistinguished location in an undistinguished galaxy. Mankind was once thought to be a species of an entirely different kind, given dominion over every living being that moved on the Earth. We now know that mankind is instead a single species, with a genetic heritage shared by all other organisms on the planet.
If Earth and mankind are, indeed, entirely ordinary, it seems inevitable that there must be life on other planets; that we are one day fated to encounter species like us in many ways, driven by the instincts to reproduce and survive, and no doubt utterly alien in form and thinking.
Yet our initial searches for stellar neighbors have come up short. Despite diligent and imaginative attempts to listen in on conversations of our fellow travelers, we have no evidence that there is anyone out there.
This is an interesting time in the field of astrobiology. While the funding for direct SETI searches is less stable than it should be, the planet finders are going like gangbusters. Planets are being found every day. The technology and techniques have improved to the point of being able to find Earth-like planets within the next couple of years. Techniques for looking at the atmospheres of extrasolar planets are conceivable. It may well be that the next few years or decades will answer the question “Are we alone?” once and for all.
EPILOGUE
THE VISITORS
Kindly take us to your President.
Alex Graham, in a March 21, 1953,
New Yorker
cartoon in which two Aliens are talking to a horse.
Imagine someday that an amateur astronomer is taking pictures of the sky and finds that one of the thousands of dots on the screen has moved. After he checks the online ephemerides, he reports that a new comet has been found. The professionals turn their bigger telescopes to the comet candidate, and they project its path and find that it will come close enough to the Earth to make them nervous. Around the planet, world leaders are notified of a comet that will have a near Earth encounter. Concerned leaders are reassured that space is large and the Earth is small. The precision of the predictions for the comet’s path isn’t great and it will probably be a near miss; a spectacular lightshow to be sure, but a miss nonetheless.
Subsequent study reveals that the comet’s path really does seem to be intersecting the Earth’s. Perhaps the Earth really does have a big bulls-eye painted on it. Discussions in the halls of power revolve around whether the public should be notified or whether quiet preparations should be made to
ensure the survival of civilization if, as happened once 65 million years ago, a comet or even a piece of the comet plummeted out of the sky, striking the Earth and causing enough damage to drive species to extinction.
This being the twenty-first century, secrets are impossible to hide and a Facebook or a blog post is made, the press gets wind of the story, and people are told. Like the Y2K case, the press will write hysterical stories. Survivalist and religious organizations get a spike in membership, while some people discount the situation as typical media hype.
Astronomers across the world keep round-the-clock watch on the comet, firming up the predictions of the trajectory. There is no longer any doubt. The comet is aimed directly at Earth. There’s only one problem. As it gets near, the object appears to be slowing. This can’t be explained by orbital dynamics and the fact perplexes physicists. To the UFO community, the message is clear. The comet isn’t a comet at all, it’s an Alien spaceship. While the claim seems pretty ludicrous, scientists admit that the claim would explain things. By now the comet (or spacecraft) has come under observation by amateur telescopes and its size is known. It’s pretty big, whatever it is.
As astronomers watch, the slowing object misses the Earth and settles into a high orbit. This behavior answers the question. It’s not a comet or an asteroid, but rather some phenomenon under intelligent control.
As the radar watches, a small object detaches itself from the bigger one and approaches the Earth, descending slowly. The object enters the atmosphere and appears to be headed to Washington, D.C. The combat air patrol permanently stationed over the city since September of 2001 heads to intercept, while fighters scramble from nearby airfields to lend support. The commander of the air force asks the president for orders. She’s a tough old bird, so she orders the vice president onto Air Force 2, tells the air force to hold their fire and waits in the Oval Office. As the fighters converge on the descending object, they see it is egg-shaped, with the wide end appearing to be the front.
Surrounded by dozens of air force fighters, the unknown craft descends onto the Mall in front of the White House, landing on the Ellipse. The military had earlier dispatched quick response troops, which now surround the egg. Secret Service agents on the top of the White House aim Stinger missiles at it, while the president watches pensively through the windows below. Overhead, the sky is relatively quiet, criss-crossed by jet contrails that have warned off the helicopters for the local television stations. And everyone waits. An extraterrestrial craft has landed on Earth. If this were a movie, a diminutive gray being would exit it and say, “Take me to your leader.”
But this isn’t a movie. It isn’t a book. It’s real. With all eyes on the craft, a glowing crack appears on its flawless surface, revealing what is clearly a door and a ramp. The crowd holds its collective breath and sees emerging a …
… a what? That’s really the question behind this book. What will we see when we encounter our first example of intelligent extraterrestrial life? Will it be a humanoid Gray? Will it be Adamski’s Space Brothers? Will it be Jabba the Hutt, ET, or Spock?
Of course it won’t. Nor can I tell you what it will be. It will be Alien, for sure. To remind us of Haldane’s famed quote, it will not only be weirder than we imagine, it may well be weirder than we
can
imagine. But we’ll try.
The Alien will be intelligent. It will have technology that surpasses our own. It will have limbs to manipulate the world around it and it won’t be a water breather. It will almost certainly be able to see light in the spectrum of its parent star. It won’t be able to breed with humans, nor will it likely be able to eat and digest Earth food. It will be a curious being and most likely it will be one that consumes resources, perhaps resources found here on Earth.