Life is pretty easy to recognize.
It moves, it grows, it eats, it excretes, it reproduces.
In biology, researchers often use the acronym MRSGREN to describe it.
It stands for movement, respiration, sensitivity, growth, reproduction, excretion, and nutrition.
How could that be possible?
For example, a 3D printer can reproduce itself, but we wouldnt call it alive.
On the other hand, a mule is famously sterile, but we would never say it doesnt live.
As nobody can agree, there are more than100 definitionsof what life is.
When we think about aliens, we often picture a humanoid creature.
But theintelligent lifewe are searching fordoesnt have to be humanoid.
Furthermore,she wonders: Will they be like you and me, made up of carbon and nitrogen?
Its possible theyre here right now and we simply cant see them.
Such life would exist in a shadow biosphere.
By that, I dont mean a ghost realm, but undiscovered creatures probably with a different biochemistry.
This means we cant study or even notice them because they are outside of our comprehension.
Assuming it exists, such a shadow biosphere would probably be microscopic.
So why havent we found it?
This may mean that there could indeed be many lifeforms we havent yet spotted.
If we find such a biosphere, however, it is unclear whether we should call it alien.
That depends on whether we mean of extraterrestrial origin or simply unfamiliar.
Silicon-based life
A popular suggestion for alternative biochemistry is one based on silicon rather than carbon.
It makes sense, even from a geocentric point of view.
It struggles to create strong bonds, so long-chain molecules are much less stable.
Life on Earth is fundamentally different from the bulk composition of the Earth.
Another argument against a silicon-based shadow biosphere is that too much silicon is locked up in rocks.
So if there were viable silicon lifeforms here, they may have evolved elsewhere.
That said, there are arguments in favor of silicon-based life on Earth.
There are plenty of experiments testing out these alternative biochemistries, such as the one from Caltech.
The Earthsupportsthe only known life in the universe.
So could aliens be among us?
