Okay so here's my analysis.
Earth formed 4.562 Billion years ago. First 1000 million years ago..it was just a rotating ball of fire. Then some years later(Astronomically) it cooled, got most of its dense matter into its core. I think we all know this. They say first life form was just a micro-organism in water.. Where did this water come from? Where did the microbe come from? Its impossible to say that it just formed naturally..Forming such accurate chains of Carbon is impossible..So where did we come from? Where did we originate?..I guess we may never find an answer to this question..
We live in a galaxy named Milky-Way, a barred spiral Galaxy. It takes 10,000 years to travel through milky-way, if we travel at the speed of light. In this distance, there war 400 Billion stars..More than the population of Earth! So now, atleast 1 in 1000 stars will have a planetary system. That gives us 0.4 BILLION PLANETS.
Well that increases the chances of life Exponentially!. Thats why i believe Aliens exist.
Alien---> A living creature originated on a different planet, having different Genetics altogether.
Considering that such creatures exist, even we are Aliens to those creatures.
They may/ may not be more intelligent than us. It is also possible that they are our hosts.
In our universe, there are 1 Billion Galaxies...Which means more stars..which means more planets..That increases possibility of finding life Stupendously!
Stephen Hawking is one of the leading scientist of this generation, who has done a marvel in the field of "Black Holes" (My personal Favorite Topic), who thinks Aliens exist. Albert Einstein was another amazing scientist who gave his special theory of relativity..So basically, you cant compare both of them.
In the end, it comes to what you believe, leaving behind all the facts.. Do they exist..or they dont?