Red dwarf stars (M type stars) are the most common type of star in the galaxy and most of the habitable planets we know about (to date) have red dwarf suns. So understanding how life would work in these systems is important for guiding our first biosignature-hunting missions. However, these stars, though cooler and fainter than the Sun, provide some interesting challenges for life as we know it in the form of large stellar flares and harsh ultraviolet radiation. Looking at how life on Earth copes under similar pressures can tell us a lot about what we may find on these alien worlds.
Compared to a star like the Sun, M stars are very active, flaring regularly. These flares send streams of biologically harmful charged particles and ultraviolet (UV) radiation towards that star's planets. To make matters worse, because M stars are much cooler than the Sun, a planet needs to be much closer to an M star to be warm enough to support life. This makes these planets more vulnerable to the full force of their host star's flares. Regular flaring can erode planetary atmospheres, stripping them of protective ozone and exposing the planet's surface to dangerously high levels of UV radiation. This has resulted in a lot of debate about how suitable these kind of star systems are for life. However, the outlook may not be as bleak as it first seems.
By modelling the expected UV radiation levels on known habitable planets in nearby M star systems, these papers demonstrate how even in the worst cases conditions are never as bad as they were on the young Earth. In its youth, Earth had little oxygen and ozone in the atmosphere and is likely to have been exposed to what we would consider deadly quantities of UV radiation. We know Earth was habitable and inhabited at this time, which means planets in M star systems should not yet be dismissed in our search for life beyond Earth.
Even if life were to exist in the harsh UV environments expected on planets in M star systems, there is still debate over our chances of detecting that life. Biosignature gases in an atmosphere that would give away the presence of life on a planet could be continually destroyed by flare activity before they have a chance to build up to detectable levels. Meanwhile, any biological surface features we could hope to detect may be obscured by water, rock or sand, because being underground, or underwater is an easy way for organisms to protect themselves from UV radiation. But what if high levels of UV radiation could actually produce a detectable biosignature?
The Biofluorescent Worlds papers explore the idea of UV-protecting biofluorescence that downshifts harmful UV light to harmless visible light creating just such a detectable signature on highly UV-irradiated worlds.