Imagine an active red flare star that has a violent outburst on its surface hurling huge amounts of high energy radiation (X-rays and UV) at a rocky planet orbiting it in its Habitable Zone. And when the radiation hits.... the planet starts to glow...
[video width="938" height="402" mp4="http://carlsaganinstitute.org/wp-content/uploads/2016/08/FluorescentPlanetFlareOMalleyJames.mp4"][/video]
What is really intriguing is that flare stars like Proxima Centauri hurle huge amounts of radiation (high energy X-rays to high energy UV) towards their planets - every 10-30 hours (!) in Proxima Centauri's case. It could strip an atmosphere on a planet around such a star, leaving only a thin atmosphere behind, which lets a lot of the UV flux hit the ground (on Earth we are shielded by our ozone layer and our atmosphere here on Earth). So the question is really how could life survive that?
Life would have to hide deep underground from such UV flares to survive. Or it could develop a strategy to cope with the temporary high UV radiation during flares.
On Earth some corals use bioflourescence to shift harmful UV radiation to harmless visible wavelengths, producing a beautiful glow in a characteristic color while doing that...which we could even glimpse with our telescope. Such a radiant biofluorescence could even uncover hidden worlds through their temporary glow.
Our new paper shows how such a world could look like and be detected.
Article: BiofluorescentWorlds_OMalleyJamesKalteneggerApJ2016 Title: "Biofluorescent Worlds: Biological fluorescence as a temporal biosignature for flare star worlds" Jack O'Malley-James & Lisa Kaltenegger, Carl Sagan Institute, Cornell University, submitted to ApJ.
The animation shows the effect; an artist's impression of a biofluorescent world: the flare hitting the planet and the planet starting to glow due to the biofluorescence (credit Jack O'Malley-James, Carl Sagan Institute, Cornell University).