Let’s continue exploring the case where life is hard to originate, meaning it starts itself almost nowhere, but is easy to evolve, meaning once you start it, it just doesn’t stop. If an alien civilization realizes this is the case, and decides they want to do something about it, they can undertake seeding on all nearby planets which can support the life they begin there. So, after they have had enough time to seed all the planets within the range capability, what would there be?
If you looked at a three-dimensional map of the galaxy, with
red dots for planets with life and blue dots for planets without it, you would
see a large disk with a central bulge, all blue, and somewhere in the disk
there would be a little red bubble, the bubble of life. Somewhere near the center of the bubble would
be the home planet of the alien civilization.
Seeded planets take billions of years to evolve from simple seed cells
to new civilizations of intelligent aliens, so for some billions of years, the
seeded planets wouldn’t be capable of sparking new bubbles. During those billions of years, the galaxy
would be rotating and shearing, so the bubble would not stay round, and proper
motions of the stars involved would make it enlarge itself and become less
distinct. The alien civilization would
likely be long gone, and their home planet would have reverted to just one more
planet with life.
Suppose Earth was nearby the bubble, and was a bit younger
that the seeder’s planet, so that Earth blossomed into an advanced civilization
after the seeders had done their work
and proceeded to become extinct. This,
of course, is some time in our future, if we are lucky and don’t make too many
emistakes. What would we see as we
examined our surroundings? If we were a
half-billion of so years later than the seeders, we would see planets with
oxygen atmospheres, or other signatures of life, in something like a bubble
around some central point. This pattern
is almost necessarily solid evidence of a civilization that decided to seed
life wherever it could. Furthermore, it is not just evidence of life in the
galaxy, but of a long-past alien civilization with space travel capability.
There doesn’t seem to be other
causes for a bubble of life. If life
could originate easily, instead of a bubble of life, the whole galactic disk
would have specimens. It is the
localized nature that gives rise to the idea of a difficulty in origination of
life, and the possibility of a civilization seeding multiple other planets. It’s also hard to imagine something an
asteroid striking a planet with life, somehow bouncing off after adsorbing some
living cells, which stay alive until the asteroid is somehow propelled out of
its home solar system and travels to another, and then has another impact on a
planet that can support life, and the impact doesn’t kill the cells, but leaves
them in some place where they are viable.
Nor could a nearby supernova blast living cells from one planet to one
in another solar system.
One way to look at this example
of seeding is a gift to civilizations that come into existence later. A later civilization near the bubble of life
would have a myriad of planets to colonize, if this were possible and they were
motivated to do so. Colonization in a
galaxy barren of life can only lead to a harsh life, probably under the surface
of some mineral-rich moon or planet, with no hope of surviving long enough to
transform the moon or planet into something like their home world, with the
right atmosphere, vegetation and animal life.
What about someone inheriting the
mantle of the original seeders? The
oldest stars in the galaxy are a bit better than 13 billion years old, but that
doesn’t mean the whole galaxy came into existence that quickly. The time to form depends on what preceded it,
but let’s just say 2 billion years were necessary. Then the disk of stars might have formed,
along with the bulge and the other details.
If a star formed then, and had a planet or a few, one of which originated
life, we might be up to 4 billion years.
If it took 4 billion more years to evolve to a space-faring alien
civilization, that might be 8 billion.
Then the alien civilization seeded planets, and it is another 4 billion
for the second generation of life to reach civilization level. There could have been a hundred or so seeded
planets, and if one of them started seeding a second round, we, at 13 billion,
might see a second bubble of life, seemingly growing out the side of the first
one. Since we don’t know the variability
in the timing of the evolution of life, or even what it depends on, it could be
13 billion years from the oldest star’s birth is not enough, or if the timing
could be shorter, the second round of seeding might be more or less complete,
right up to the generation of an observable oxygen atmosphere. The oxygen atmosphere on Earth came into existence
in a geologically short time, so that signal is a good early indicator of a
planet with life. Seeing a double bubble
would dramatically confirm our observations of other life in the galaxy, and
give us something toward a date of the first generation.
Suppose we can find no bubble of
life, no matter how far out we get our giant telescopes to search for oxygen or
some other signature of life. Then we
are faced with a decision. Perhaps we
are the only life form that is going to originate in the galaxy. Should we let it all disappear? Or should we make it the planetary goal to
figure out how to seed other planets, capable of growing life, with some seed
cells. That would be a purpose that
might unite mankind, and even carry over into any AI entities that come into
existence. Or we could just figure out
how to have a good time until the sun burns out.