Alone in the Cosmos

Since the mid-twentieth century it's been the accepted assumption that the universe must be teeming with life. So many stars out there. So many stars like our sun. So many planets must be orbiting them. There must be billions of planets with living things many of which are biologically and technologically advanced.

But if so, where are they?

As time went on and more and more discoveries were made about the geo-physical properties that must obtain for a planet to produce and sustain life, the first seeds of doubt began to germinate. Books like Rare Earth by Ward and Brownlee and Privileged Planet by Gonzalez and Richards began to feed those doubts.

Cosmologists are loath to conclude that earth-bound life is unique in all the cosmos, but the chances of another planet meeting all the criteria that a planet must meet in order to sustain life are so vanishingly slim that some of them are admitting that it may be so. We may be all alone.

Ethan Seigel at Forbes tentatively suggests the formerly unthinkable in a recent essay. He writes:

When it comes to the question of extraterrestrial life, humans optimistically assume the Universe is prolific. After all, there doesn't appear to be anything particularly special about Earth, and life not only took hold here on our world, but evolved, thrived, became complex and differentiated, and then intelligent and technologically advanced. If the same ingredients are everywhere and the same rules are at play, wouldn't it be an awful waste of space if we're alone?

But this is not a question that can be answered by appeals to either logic or emotion, but by data and observation alone. While our investigations have revealed the existence of an enormous number of candidate planets for life, we have yet to find one where intelligent aliens, complex life, or even simple life is known to exist. In all the Universe, humanity may truly be alone.

He goes on to state that,

• somewhere between 80%-100% of stars have planets or planetary systems associated with them,
• approximately 20%-25% of those systems have a planet in their star's "habitable zone," or the right location for liquid water to form on their surface,
• and approximately 10%-20% of those planets are Earth-like in size and mass.

A substantial fraction of stars out there (around 20%) are either K-, G-, or F-class stars, too: Sun-like in mass, luminosity, and lifetime. Putting all these numbers together, there are around 1022 potentially Earth-like planets out there in the Universe, with the right conditions for life on them.

Unfortunately, the assertion that there's nothing special about earth is very misleading. There are many more conditions that must be met for a planet to be suitable for life than the ones he lists. Here are just a relative few to give an idea of the complexities involved:

In order to support life in its solar system a star must be located within a fairly narrow region in the galaxy. It can't be too close to the center, where radiation would be intense, nor too far away where it would revolve at dangerous speeds around the galactic pinwheel.

The star has to be rich in heavier elements, and has to be fairly remote from other stars in the galaxy. It has to be a middle-aged star of relatively constant luminosity, not too big and not too small, not too old and not too young.

In other words, stars suitable for sustaining life are relatively unusual in our galaxy, but this is just the beginning. The star has to possess a planetary satellite capable of generating and sustaining life and this means that planet has to have perhaps hundreds of precisely-tuned properties.

It has to be just the right distance from its star which means it has to revolve around the star at just the right speed. It has to have a nearly circular orbit and the right tilt to its axis. It has to be just the right mass so that its gravity will hold oxygen in the atmosphere but not hold slightly lighter noxious gases like ammonia. It has to rotate on its axis at the right speed, lest the temperature differences between day and night be too great, and it must possess a shifting crust.

It must also have ample water and carbon, among other things, and also a large moon which has to be at just the right distance from the planet to stabilize its wobble. It must also be in a solar system where it's protected from meteorites by large gravitational vacuum sweepers like Uranus, Neptune, Jupiter and Saturn, and so on.

As the number of parameters that must be just right in order for a planet to be able to support life increases the chances of such planets existing in great numbers in our galaxy decrease.

Seigel seems to feel the weight of all this and concludes with this paragraph:

But how did life arise to begin with, and how likely is a planet to develop life from non-life? If life does arise, how likely is it to become complex, differentiated, and intelligent? And if life achieves all of those milestones, how likely is it that it becomes spacefaring or otherwise technologically advanced, and how long does such life survive if it arises?

The answers may be out there, but we must remember the most conservative possibility of all. In all the Universe, until we have evidence to the contrary, the only example of life might be us.

And if we are in fact the only conscious, sentient beings in the physical universe, does that suggest that we are in some sense special? In what sense? Could it be that we were intended?