There is a new theory called 'panspermia' that could explain how alien life travels across planets and stars in space.
It claims that aliens could have been delivered to Mars.
In a new BBC video, Professor Brian Cox explained that an idea called 'panspermia' is being worked on that may explain how there are some small signs of life on planets throughout the universe.
People have often wondered if there is actually alien life out there.
As of yet, there has been no concrete evidence of any green monster with seven tentacles walking up Oxford Street, or crop circles being caused by anything but an arty farmer in Wiltshire.
While we have no evidence of aliens existing in the form of shape-shifting monsters, it may exist as simple bacteria.
So what is panspermia?
What is panspermia?
Panspermia attempts to explain how life on a planet started and if it can travel across the universe and between planets.
Professor Brian said: "A more interesting theory and one that has more credibility is that of panspermia, that life exists throughout the universe and can be transported through space from one location to another."
Scientists at the prestigious schools of MIT and Harvard in the US are working on the theory that some form of life was delivered to Mars in this way.
"Life is incredibly adaptable. Microorganisms such as bacteria over the course of millions of years of evolution have been able to modify themselves in order to adapt to a vast range of conditions."
The theory believes that there are two ways in which life can be transferred, either as part of a human experiment or when meteorites crash into a planet.
When this happens, tons of debris is then scattered away from the planet as mini-meteorites, containing bacteria.
Some 313 Martian meteorites have been found on Earth.
Do aliens exist?
Aliens have not been found, but at the same time their existence has not been ruled out.
Conspiracy theories of UFO's flying in and out of our airspace are little more than conspiracies.
There is simply no evidence that anyone has ever been abducted and probed, or that ET built the Mayan temples in Mexico.
Much of what we think extraterrestrial life to be about is based on stories and film. Really, it is hard to come up with any idea of what space life is like, if it does exists.
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NASA said: "The more exoplanets we find around other stars, the more we learn about how many different environments could exist for life.
"We can't yet say for sure whether or not aliens exist."
Although, astronomer Carl Sagan said: "The universe is a pretty big place. If it's just us, it seems like an awful waste of space."
What is the wow signal?
For some time, one of the strongest pieces of 'evidence' for life beyond our world was the so-called 'Wow! signal'.
In 1977, experts in Ohio detected a radio signal and were so impressed by the strength of it that an astronomer wrote "Wow!" next to it on the chart, but the reason behind the signal was unclear and it was never found again.
A more recent theory believes that the signal came from a comet.
Is there life on Mars?
There is no life on Mars that we know of.
However scientists have found evidence for the correct criteria of life on Mars existing at one time.
NASA believes that Mars did once look like the Earth with a blue atmosphere, lakes and oceans, but now the red planet is known to be impossible to live on.
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It is usually around -63C and the atmosphere is 95.3% carbon dioxide.
Mars lost its magnetic field and its atmosphere fell away, so any life that may have existed before was likely wiped out.
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