Do philosophers believe in aliens? On Wednesday a team of Nasa scientists reported their findings on UFOs for the first time in history. Some say it raises big questions.
Nasa reveals what it knows about UFOs
Searching for answers: The US Defence Department receives 50 to 100 UFO reports every month. Glossary
USA - The United States of America is a country in North America. It has a population of 329 million.
Nasa - The National Aeronautics and Space Administration, responsible for the US space programme.
Philosophers - People who ask big questions about the meaning of life and how we should live.
Species - A set of animals or plants with similar features. Members of the same species can breed with each other.
Universe - Everything. Including all of time and space and its contents.
Intelligent - Having a great ability to reason and understand things.