Question: how do we know that there is more life in space

Stephen Rhatigan answered on 15 Nov 2017:

We don’t know but it is extremely unlikely that we are alone in the universe. The Milky Way contains between 200 and 400 billion stars and at least 100 billion planets. And current estimates are that there are at least 2 trillion galaxies in the observable universe.

Only recently have we developed the technology to search for exoplanets: planets outside our solar system. For an exoplanet to be habitable (capable of supporting life), there are a number of criteria:
1. The exoplanet must be similar in size to Earth.
2. Must orbit a Sun-like star or red dwarf star.
3. Must be the correct distance from their star.
4. There are others…..
These criteria are based on our experience of the conditions necessary for life (which is limited to Earth as far as we know).
Already scientists estimate that there are 40 billion exoplanets which satisfy these criteria in the Milky Way. 11 billion of these may orbit Sun-like stars.
You can see a list of potentially habitable exoplanets here:
https://en.wikipedia.org/wiki/List_of_potentially_habitable_exoplanets
A lot of things had to go very right for Earth to support life as we know it and we are balanced on a razor’s edge. But the universe is so vast (infinite?) that these conditions (or at least similar conditions) could be replicated elsewhere.