Alien life may not be like life on Earth, so trying to find evidence of their existence requires some creativity.

Scientists know of only one example of life in the universe, and it exists on Earth. But what if life can form in other ways? How do you search for aliens when you don’t know what they might look like?

It’s thought that microbes are likely the most common form of extraterrestrial life, since they can arise more easily than larger organisms.

But it’s possible that an advanced alien civilization exists somewhere in space. In either case, these alien species may not resemble anything scientists know about life, Live Science reports.

Since the discovery of the first planet outside our solar system in 1995, scientists have discovered more than 5,000 planets orbiting other stars. Many of these planets are small and rocky, like Earth, and reside in the habitable zone of their stars.

This is the region of space around a star where conditions allow liquid water to exist on the planet and thus support life as we know it. There is a good chance that many planets that are not yet known to us have the right conditions for life to emerge.

But scientists disagree on how exactly to define “life” when it comes to living organisms. NASA defines life as “a self-sustaining chemical reaction capable of Darwinian evolution.” That is, organisms with complex chemical systems that evolve to adapt to their environment.

Life on Earth has evolved over billions of years, from single-celled organisms to large animals and other species, including humans.

Using spectroscopy, a technique for detecting chemical traces of life, scientists are looking for signatures of atmospheric oxygen on other planets that were created by microbes, or signatures of chlorophyll that indicate plant life.

But is Darwinian evolution universal? What chemical reactions could lead to life beyond Earth?