We tend to assume that life began here on Earth.
But it’s possible that life originated on an alien world and in some form or another
was brought here.
This notion is called panspermia.
Panspermia is the hypothesis that life permeates throughout the universe, being sent from one
place to another by asteroids, comets and also by spacecraft, whether intended or not.
Panspermia assumes that life exists at least in one place in every galaxy, because if life
didn’t begin somewhere then it couldn't be distributed to any other planets.
Panspermia also proposes that microbes are able to survive the vacuum of space for long
periods of time, which isn’t at all a far-fetched proposal since we’ve seen microbes that
can do this.
Tardigrades can survive extreme conditions, including the vacuum of space, as well as
extreme temperatures, both hot and cold, and can live for more than 30 years without any
food or water.
Panspermia implies one of two origins for life.
Either microbial life began on a planet and rocks containing microbes reached into space
or life began on an asteroid or comet.
Now what’s important to note is that panspermia doesn’t necessarily imply intelligent life,
all that is needed are simple life forms such as microbes.
So long as one planet has simple life, it’s theoretically possible that that life could
populate planets in the same solar system or even in solar systems hundreds of light
years away.
There’s quite a big debate surrounding the possibility of simple life being common in
the universe.
In the Fermi paradox, the question that asks “Where are all the aliens?”, one of the
most common camps is the simple notion that planets like Earth are exceptionally rare.
So rare that there may only be one such planet per galaxy or less.
This is called the Rare Earth hypothesis and raises the idea that we are alone in our galaxy,
possibly even in the universe.
A common question that gets asked with panspermia is how the microbes get to space from a planet.
One way is from asteroid impacts, that send rock filled microbes into space.
This is seen with the most likely explanation for the formation of the Moon.
Theoretically the Moon most likely was formed when a Mars-sized object, called Theia, hit
Earth, sending a lot of material in orbit around Earth that collectively formed the
Moon.
It’s possible, in the right circumstances, that microbes could form and thrive on an
asteroid or comet.
The only thing they have to survive to colonize a planet is the exceptionally long journey
through space and the impact.
Panspermia can be said to be either interplanetary or interstellar but, either way, quite a lot
has to go in your favour in order to have life colonize a planet.
The chances of microbes getting from one planet to another are pretty low.
However the transfer of rock from one planet to another has been observed, since Martian
rocks have been found here on Earth.
Asteroids aren’t the only way of distributing the microbes though, it could be done unintentionally
by an intelligent civilization, such as ourselves, through spacecrafts.
An extreme possibly are von Neumann probes.
Von Neumann probes are self-replicating spacecrafts that could travel to a nearby solar system,
grab natural resources in order to build some more probes and send them to other solar systems,
and repeat.
This scenario could be dangerously risky because the probes are travelling to thousands of
exoplanets while most likely carrying microbes, and it’s especially risky if the probes
are designed to target Earth like exoplanets that have good chances of having and sustaining
life.
This is why agencies like NASA go to a large extent to remove spacecrafts of as much bacteria
as possible.
Although unlikely, some scientists have hypothesised that aliens could have planted us here on
purpose.
This is called directed panspermia, where an advanced civilization deliberately plants
microbes or life on other planets.
Another popular variation of panspermia is lithopanspermia, we discussed this earlier.
Lithopanspermia is transferring microbes in rocks ejected from one planet to another,
either to a planet in the same solar system or to one in a different solar system.
It’s certainly possible since, as I mentioned, Martian rocks have been found here on Earth.
It does help with a lower gravity like Mars as as it’s easier for rocks to reach space.
This is the first hurdle, managing to get ejected off the planet, while having the microbes
survive.
The next hurdle is surviving the vacuum of space.
Firstly, the microbes must be able to survive the effects of space.
The biggest threat would then be the constant cosmic rays.
The microbes would certainly benefit from being shielded as much as possible by being
buried in the rock.
Another factor that comes into play is how long the microbes can last for.
Interstellar travel takes a very long time, especially for speeds that asteroids tend
to go.
Even for a neighbouring planet, it can still take as long as a year.
The last obstacle is surviving the impact on the planet, which in itself is pretty difficult.
However if the microbes aren’t able to survive the trip, dead microbes are still sufficient
to spawn life on a planet.
This is another variation of panspermia, called necropanspermia.
So if microbial life is making a trip across space and all dies, it’s thought that the
corpses can still spawn life or act as a template for life.
A more exotic variation is accidental panspermia.
Accidental panspermia is the hypothesis that life on Earth could have begun due to an advanced
civilization dropping off their waste long ago.
Comment down below your thoughts on panspermia.
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