Scientists at Cornell University say that life-bearing exoplanets may be detectable by their soft glow. Based on laboratory studies, the team led by Jack O'Malley-James at Cornell's Carl Sagan Institute believes that a mechanism that protects organisms from hard ultraviolet radiation could make worlds beyond the solar system radiate a soft, detectable light.
Anyone who has watched fireflies flitting about in the night sky is familiar with the idea of organisms producing light. Not only can some insects light up, but so can fish, squid, bacteria and many others, for a variety reasons that include attracting mates, camouflage, decoying prey, and marking territory.
But there is another type of luminescence called "photoprotective biofluorescence," which is a protective mechanism found in some species of undersea corals that live at a shallow enough depth for ultraviolet radiation from the Sun to penetrate. Normally, such radiation would be absorbed by the tissues, resulting in a nasty and possibly fatal case of marine sunburn, but these polyps have a trick up their non-existent sleeves.
What happens is that biofluorescent proteins in the coral's tissues absorb the UV radiation, exciting an electron and raising it to an unstable energy state. As the electron returns to its stable state it re-emits radiation in the visible band of the light spectrum. The result is that the UV is rendered harmless and the animal fluoresces.
What occurred to the Cornell teams was that this mechanism could be handy to any extraterrestrial life that happened to evolve in a particularly nasty environment, such as in the habitable zone of M-type stars, where a large number of exoplanets have been found.
M-type stars tend to emit ultraviolet flares, which is bad news for any organisms on planets orbiting them, but if they used biofluorescence to protect themselves, not only would this give them a fighting chance, it would also produce a biosignature that could be detected by telescopes when the flare hits the planet, causing it to temporarily fluoresce.
To test this hypothesis, the team studied the spectral emissions of common fluorescent corals and used this to produce model spectra and colors that could be found on exoplanets orbiting M-type stars. They concluded that the strength of this glow could be enough to be detected by telescopes currently under development.
"These biotic kinds of exoplanets are very good targets in our search for exoplanets, and these luminescent wonders are among our best bets for finding life on exoplanets," says O'Malley-James.
The research was published in the Monthly Notices of the Royal Astronomical Society.
Source: Cornell University