One million solar systems. One hundred billion known galaxies. One hundred quintillion planets.* Only one planet with life. How can this be? The universe is so big and unexplored, which gives us reason to believe that aliens could be out there.

The first believed sighting of extraterrestrial life was in 1561, when the people of the city of Nuremberg, Germany, reported seeing a “celestial phenomenon that has been interpreted by some UFO authors as an aerial battle of extraterrestrial spacecraft.”

Since then, UFOs have been spotted all over the world.

Even NASA’s scientists believe alien life could be possible.

“I think we’re going to have strong indications of life beyond Earth within a decade,” said NASA chief scientist Ellen Stofan, who spoke to Iflscience.com. “[It’s possible] we could have definitive evidence within 20 to 30 years.”

Although sightings and theories have some facts backing them up, there still isn’t any actual evidence of alien life somewhere in space. Why is that?

It could be because we don’t have the technology to explore deep space yet. We can only send spacecrafts so far out without running out of fuel or data connection to space centers. Aliens could be too far out of reach for our technology to find.

We haven’t been searching for extraterrestrial life long enough yet — we’ve only been actively searching for aliens since 1957.

Maybe we aren’t being visited. Earth is in a very lonely place. It could be possible aliens don’t know about our solar system or galaxy because no other life has been discovered yet, so they would have no reason to broadcast into what they believe a lifeless part of space.

A last theory, from Ross Pomeroy at Space.com, is that aliens are already on Earth and walk among humans.

Could it be that aliens are collecting information about Earth without our knowledge? Could they be on the planet disguised as humans or other species of life? Is the government hiding any information about aliens visiting Earth?

It may be hard to believe that aliens exist, but when you look at all the possibilities and research people have done, extraterrestrial life is very probable and could be discovered in the near future.

*Scienceblogs.com