Seven Earth-like planets orbiting a small star in our Galaxy called Trappist-1, revealed Wednesday, are the most recent -- and arguably the most spectacular -- in a string of exoplanet discoveries going back 20 years.

Herewith a backgrounder on the search for life beyond the horizon of our Solar System.

Exoplanet is any planet outside our Solar System. | Source: AFP

What conditions support life?

That depends on what the meaning of "life" is!
For life as we know it, liquid water is an essential ingredient. Of the exoplanets found to date, however, only a handful are in a "temperate" zone in relation to their star: not so hot that water evaporates, not so cold that it freezes rock solid.


Life on Earth is also unimaginable without an atmosphere, containing in our case the oxygen we need to survive. An atmosphere also protects animal species in particular from damaging high-energy radiation from the Sun -- ultraviolet and X-rays.


But as we do not even know yet how life emerged on Earth, it is also possible that living creatures elsewhere in the Universe could survive and thrive in conditions that would be lethal for our tender-footed species.


But as we do not even know yet how life emerged on Earth, it is also possible that living creatures elsewhere in the Universe could survive and thrive in conditions that would be lethal for our tender-footed species. | Source: Reuters