What organisms can survive on Proxima Centauri b under the flashes of a hard ultraviolet from a red dwarf

On August 24, world media spread the news about the discovery of an exoplanet near the star Proxima Centauri near us at a distance of only 1.295 parsec (4.22 light years) from the solar system.

Formally, the planet Proxima Centauri b really corresponds to the definition of the “Earth twin”, that is, in terms of size, mass and temperature regime roughly corresponds to the Earth, it can potentially be suitable for life and allows for the existence of liquid water. The minimum mass of the planet is 1.27 Earth, radius ≥1,1 ± 0,3 R⊕, it is located in the habitable zone of the red dwarf of class M5, the orbital period around the star is 11.186 days.
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As always, the devil is in the details. If you look at the real climatic conditions of Proxima b, then the optimism of the mass media seems slightly redundant. The fact is that Proxima Centauri is a highly unstable star that flashes every 10-30 hours, including in the ultraviolet and x-ray ranges. “It will be difficult for a person to survive in such conditions, to put it mildly,” commented Leonid Ksanfomaliti, head of the laboratory of photometry and IR radiometry at the Department of Physics of Planets and Small Bodies of the Solar System of the Space Research Institute of the Russian Academy of Sciences.

Ultraviolet flashes of such power have enough energy to break bonds in the DNA molecule. Especially destructive effect has a radiation with a shorter wavelength. For complete destruction of microorganisms, it is sufficient to irradiate them with a UV wavelength of about 260 nm for a certain period of time.


Proxima Centauri b is very close to its star

The intensity of UV radiation during flares of a star near the surface of Proxima Centauri b is several orders of magnitude higher than the intensity of solar radiation at the surface of the Earth. The situation is aggravated by the fact that due to proximity to the star, the magnetic field of the planet can shrink and worse protect the atmosphere of Proxima Centauri b.


The UV activity on the surface of the atmosphere of a hypothetical planet that is 1 AU near the active star AD Leo of class M3.5 in the usual state (gray) and during flares (blue) compared to solar UV radiation at the entrance to the earth's atmosphere (gray dotted line)

But there is hope. Theoretically, on Proxima Centauri b, life is still possible, even with such powerful flashes of ultraviolet radiation, American astrobiologists Jack O. Malley-James and Lisa Kaltenegger from the Institute for them believe . Carl Sagan at Cornell University.

According to them, some living creatures are able to hide from the deadly UV radiation, using natural shelters. For example, they may exist underground or deep under water.

In addition, astrobiologists pay attention to another potentially possible form of life, which is able to convert the received energy into biofluorescent radiation with a different wavelength. This process involves special protective proteins that absorb harmful UV radiation.

Astrophysics cite as an example the earthly form of life - coral polyps. Some types of coral polyps contain fluorescent proteins, which are photoactivated when exposed to ultraviolet radiation in the long-wavelength range A at wavelengths of 315-400 nm and in blue regions of the spectrum 420-700 nm, transforming it into radiation with a longer wavelength. Perhaps in coral polyps, fluorescence is a defensive response to eliminate harmful UV.


An example of coral biofluorescence

The table lists the four most common fluorescent protin in corral polyps.

Peak Emission (nm)	Excitation range (nm)	Fluorescence efficiency (%)
486	350-475	3-5
515	400-525	10-12
575	350-575	8-10
685	350-650	1-2

The efficiency of fluorescence increases with increasing intensity of UV radiation. That is, on Proxima Centauri b, the efficiency should be much higher than that of coral polyps.

For more information on biofluorescence in coral polyps, see Maxim Gorbunov’s scientific papers with colleagues " Photosynthesis and photoprotection in symbiotic corals " (Gorbunov, MY, Kolber, ZS, Lesser, MP & Falkowski, PG 2001. Limnol. Oceanogr., 46: 75 - 85), as well as the article " Contribution of fluorescence to the spectral signature and perceived color of corals " (Mazel, CH, \ & Fuchs, E. 2003 Limnol. Oceanog., 48, 390).

If living organisms at Proxima Centauri b have mastered this method of energy conversion, then this planet can glow very strongly at certain wavelengths, perhaps even in the visible range. Scientists suggest that such a powerful glow can be seen from Earth. In their scientific work, they proposed several models of fluorescence indicating the wavelengths on which to look for the glow of alien life forms.

The work of scientists is also important because the red dwarfs of the spectral class M are not only Proxima Centauri. In fact, stars of this type make up 75% of all nearby stars to the solar system. Preliminary observations indicate the existence of three terrestrial planets in a similar star TRAPPIST-1 of class M8, which is also located not far from the Solar System. Some of the possible orbits of the farthest of these planets TRAPPIST-1d are in the habitable zone.


The nearest environment of the solar system. Data source: NASA. Graphics: A. Cuadra / Science

In 2017, NASA plans to launch the TESS (Transiting Exoplanet Survey Satellite) space telescope, which will carefully examine the orbits of transit exoplanets near the nearest bright stars to the Solar System. It is assumed that TESS will find hundreds of planets with a radius of 1.25-2 from Earth and dozens of Earth-sized planets. It is also assumed that up to 20 of these planets will be in the habitable zone near their cold stars. So Proxima Centauri b is just the first of dozens of similar worlds that we will open in the very near future.


Proximate Centauri Proximate Landscape b. Render: ESO / M. Kornmesser

The scientific work " Biofluorescent Worlds: Biological Fluorescence for a World Stars " was published in the free access site arXiv.org.