"Everything in moderation" might be a good rule of thumb for the search for life in the universe. Exoplanets can’t be too hot or too cold, too big or too small. Researchers also think that ultraviolet (UV) light plays a part, but, again, too much or too little could be a deal breaker.

According to a new study from researchers at Harvard, life requires a certain amount of UV rays to start. UV helps to break down basic common molecules that tend interact to form more complex ones. Ribonucleic acid (RNA), which is used by every life form, seems to form only when a certain amount of UV light is present.

This research, published in the Astrophysical Journal, might spell more bad news for exoplanets orbiting red dwarfs, like TRAPPIST-1 or Proxima Centauri. These stars might not have emitted enough UV light to kick start biological processes, even if they might have water on them.

"It would be like having a pile of wood and kindling and wanting to light a fire, but not having a match," lead author Sukrit Ranjan, said in a statement. "Our research shows that the right amount of UV light might be one of the matches that gets life as we know it to ignite."

Using computer simulations, the team has estimated that exoplanets in the habitable zone around red dwarfs would experience between 100 to 1,000 times less UV radiation than what primordial Earth used to get.

"It may be a matter of finding the sweet spot," added co-author Robin Wordsworth. "There needs to be enough ultraviolet light to trigger the formation of life, but not so much that it erodes and removes the planet's atmosphere."

This finding complicates the already messy volume of hypotheses about life around red dwarfs. Red dwarfs can be extremely active, producing powerful UV flares. These have been assumed to be too damaging to both a planet’s atmosphere and potential life on it. But maybe those flares can balance out the usual low levels of UV and help life.

"We still have a lot of work to do in the laboratory and elsewhere to determine how factors, including UV, play into the question of life," stated co-author Dimitar Sasselov. "Also, we need to determine whether life can form at much lower UV levels than we experience here on Earth."

This whole research field is extremely exciting. Scientists worldwide are trying to work out what exactly is needed for life not only to form but also to thrive. The answer probably won’t come easy – or anytime soon – but it’s fascinating seeing how people are approaching this important question.