The faint glow of a directly imaged planet will one day have much to tell us, once we’ve acquired equipment like the next generation of extremely large telescopes (ELTs), with their apertures measuring in the tens of meters. Discovering the makeup of planetary atmospheres is an obvious deep dive for biosignatures, but there is another. Biofluorescence, a kind of reflective glow from life under stress, could be detectable in some conditions at astronomical distances.
New work on the matter is now available from Jack O’Malley-James and Lisa Kaltenegger, at Cornell University’s Carl Sagan Institute. The duo have been on the trail of biofluorescence for some time now, and in fact their paper in Monthly Notices of the Royal Astronomical Society picks up on a 2018 foray into biosignatures involving the phenomenon (citation below). Here the question is detectability in the context of biofluorescence as a protective mechanism, an ‘upshift’ of damaging ultraviolet into longer, safer wavelengths.
“On Earth, there are some undersea corals that use biofluorescence to render the sun’s harmful ultraviolet radiation into harmless visible wavelengths, creating a beautiful radiance,” says Kaltenegger.” Maybe such life forms can exist on other worlds too, leaving us a telltale sign to spot them.”
Image: An example of coral fluorescence. Coral fluorescent proteins absorb near-UV and blue light and re-emit it at longer wavelengths. Credit: Available under Creative Commons CC0 1.0 Universal Public Domain Dedication.
Biofluorescence in vegetation is an effect that is detectable from Earth orbit. Here the effect is comparatively small, accounting for 1-2 percent of the vegetation reflection signal, but of course it is also widespread given the coverage of vegetation over much of Earth’s surface. The phenomenon is also seen in corals, which produce a higher degree of fluorescence. Earth levels of biofluorescence are clearly too small to act as useful biosignatures for exoplanets, but higher levels may well occur elsewhere.
That’s because our early work on the atmospheres of Earth-sized planets will delve into systems around small M-dwarf stars. Such worlds are plentiful and the Transiting Exoplanet Survey Satellite (TESS) is expected to add to our inventory of habitable zone examples nearby. Even now, we have interesting targets: Proxima Centauri b, Ross 128b, TRAPPIST-1e, -f, -g, LHS 1140-b, for example, all of which orbit M-dwarf stars. And M-dwarf stars are known to flare.
This, in fact, is one of the cases originally made against life in such systems, for extreme X-Ray and UV radiation would create a challenging environment for even the simplest lifeforms. M-dwarfs vary in terms of the amount of radiation they produce; indeed, a planet around an inactive M-dwarf receives a lower dose of ultraviolet than Earth. But planets around active stars, particularly given the fact that the habitable zone is so close to the small star, receive much higher flux, and such flaring remains active for longer on M-dwarfs than on stars like the Sun.
Life could conceivably flourish underground on such worlds, or within oceans, but even on Earth, the authors note, we see biological responses like protective pigments and DNA repair pathways that are ways of mitigating radiation damage. The corals mentioned above use biofluorescence to reduce the risk of damage to symbiotic algae, absorbing blue and ultraviolet photons and re-emitting them at longer wavelengths.
Corals that display this phenomenon cover a mere 0.2 percent of the ocean floor, making for only a tiny change in our planet’s visible flux. But the situation could be different in actively flaring M-dwarf systems. To study the matter, O’Malley-James and Kaltenegger begin with a biofluorescent surface biosphere in a shallow, transparent ocean, adjusting the variables to simulate different ocean conditions. They vary fluorescent protein absorption and emission to produce values for reflected and emitted light. The assumption is that life evolving in conditions of extreme UV flux will produce ever more efficient fluorescence, in terms of absorbed vs, emitted photons (at maximum efficiency, all photons are absorbed and re-emitted).
The authors calculate the UV flux for different classes of M-dwarfs and quantify the outgoing emissions of common pigments during fluorescence. The model also includes atmospheric effects with varying cloud coverage, generating spectra and colors for hypothetical planetary conditions. False positives from mineral fluorescence are considered, as are signals produced by surface vegetation, with different fractions of surface coverings and biofluorescent life.
But let’s cut to the chase. A biosphere otherwise hidden from us could be revealed through the temporary glow resulting from the flare of an M-dwarf. From the paper:
Depending on the efficiency of the fluorescence, biofluorescence can increase the visible flux of a planet at the peak emission wavelength by over an order of magnitude during a flare event. For comparison, the change in brightness at peak emission wavelengths caused by biofluorescence could increase the visible flux of an Earth-like planet by two orders of magnitude for a widespread biofluorescent biosphere and clear skies, with low-cloud scenarios being more likely for eroded atmospheres. In an M star system, the reflected visible flux from a planet will be low due to the host star’s low flux at these wavelengths; however, the proposed biofluorescent flux is dependent on the host star’s UV flux, resulting in additional visible flux that is independent of the low stellar flux at visible wavelengths. This suggests that exoplanets in the HZ of active M stars are interesting targets in the search for signs of life beyond Earth.
Video: Lisa Kaltenegger, director of Cornell University’s Carl Sagan Institute, explains why studying bioluminescence on Earth can guide the way humans search for life on other planets.
If biofluorescence can evolve on the planets of active M-dwarf stars, it may turn out that high ultraviolet flux could be the key to reveal its existence. We need to quantify such effects because our ground- and space-based assets in the hunt for biosignatures are going to be homing in on the most readily studied planets first, and that means nearby worlds around red dwarfs. What O’Malley-James and Kaltenegger are doing is charting one possible signature that we may or may not find, but one which we now know to include in our investigative toolkit.
The paper is O’Malley-James and Kaltenegger, “Biofluorescent Worlds – II. Biological fluorescence induced by stellar UV flares, a new temporal biosignature,” Monthly Notices of the Royal Astronomical Society 13 August 2019 (abstract/full text). The earlier paper on this issue is O’Malley-James and Kaltenegger, “Biofluorescent Worlds: Biological fluorescence as a temporal biosignature for flare stars worlds,” accepted at MNRAS (preprint).
Biofluorescent coral still need to use photosynthesis, so we should still have an atmosphere that contains oxygen in its spectral signature in addition to the Biofluorescence. H2O or water would indicate an ocean. Of course, the jury is till out on the idea that life could evolve on an Earth sized exoplanet around an M dwarf star. It could go either way since we don’t yet have any spectral signatures or Earth sized planets.
I think this is exactly why JWST’s transit spectroscopic/photometric assessment of the TRAPPIST-1 planets is pivotal . As stated in the previous article , though not capable of identifying a fully terrestrial atmosphere, it is capable of discrimating between the presence and absence of atmospheres . It is also sensitive enough to go beyond think envelope H2 atmospheres and reach down to high molecular weight atmospheres – including out gassed non dessicated O2 variants containing H2O & CO2 (and thus not the product of photodissociation) . This is well on the way to constraining a fully terrestrial atmosphere if not there completely.
TRAPPIST-1 is a fairly representative M dwarf in terms of its intermediate photospheric activity and flaring ( as opposed to inactive LHS 1140, or active Proxima ). More than enough activity according to many current theories to erode away atmospheres over billion year time frames , but equally to allow intra flare recovery times . So if JWST can demonstrate significant atmospheres of any type , around any , some or all of the TRAPPISt-1 planets, it would be reasonable to assume that many M dwarf planets can indeed retain atmospheres over extended periods. A negative result wouldn’t necessarily be the end, but a positive one would surely keep all ( atmospheric ) options on the table . Including the type of terrestrial atmosphere that might allow some form of photosynthesis .( assuming there are suitable photo pigments that can harness low energy infra red radiation)
It is obvious that Aurora’s shine is much more probable case on exoplanets than biofluorescence.
I can suppose also, that during M-star flares (than causes Aurora!) the visible light emission from Aurora will overcome biofluorescence emission level…
Wandering how they plan to mask Aurora’s shine ?
It is an interesting idea but I would caution against the idea that life solves all problems using an extrapolation of terrestrial mechanisms under more extreme conditions. Evolution selects for the most efficient mechanisms that are available on a fitness landscape. To avoid UV damage life can transform the UV via fluorescence, hide under UV blocking material, reduce effective UV intensity by living at greater depths in water during the day, etc. etc. There should be no assumption that fluorescence is likely to be common. This should be obvious as land organisms have little to no fluorescence to transform the much harsher UV experienced compared to aquatic environments. The weak fluorescence of plants may just be a side effect of the mechanism pigments use to trap sunlight. If it was useful to shield against UV, we might expect land organisms to look like those on Pandora. We don’t. What we see instead is the evolution of repair mechanisms for DNA and programmed cell death in animals to remove damaged cells. We see protective other layers – epidermis, shells, and furs to block damaging radiation. Plants have less capability to handle damaging radiation, but this might simply result in a limitation of their habitat to aquatic and near aquatic environments.
That is a very good point. All too often, proposals to look for particular signatures of life start by constructing a very specific scenario hiding all kinds of assumptions, simply to have a hypothesis that results in a potentially detectable signature. We have no justification for limiting our hypotheses to a narrow number of possibilities that *might* be detectable with our upcoming instruments.
The same bias happens with SETI proposals. There, the skew is towards alien civilizations that are either trying to attract our attention in order to communicate with known technologies like radio, or are at Kardashev type II or above and deploy technologies like Dyson swarms that make signatures we could detect whether or not that civilization wishes to communicate. The problem with all these ideas is that not finding those signals only proves that we have not found any civilizations that correspond with our beginning assumptions.
This is not to say that we should not search for such signatures. At the very least, we can narrow down the list of possibilities. But we should not forget that for every carefully-constructed scenario that might leave a particular detectable signature, there plenty of other likely possibilities that are far more difficult to rule in or out.
This is an absolutely fascinating possibility. Before reading this, I knew little about biofluorescence, and I had no idea that this phenomena could be a potentially detectable life-signature with the upcoming generations of ELTs.
At this point, I can only wonder, what would the surface of such a world look like? An increase of visible light output of “over an order of magnitude” should be visible from orbit, and I can only imagine the spectacle of colors a visitor on the surface of such a planet would see during a flare. That said, the visible light from the flaring star might make the biofluorescence difficult to see.
This reminds me of the difficulties mineralogists faced during the early days of the study of fluorescent minerals. In the 1920’s, the main source of UV light was a machine called an “iron spark”, which was a type of arc lamp. It gave off plenty of ultraviolet light, but also gave off lots of visible light. This made it quite difficult to tell whether a mineral was fluorescing. Unless the fluorescence was bright enough to compete with the visible output, the effect could be easily washed out.
TRAPPIST- 1 has just 0.00000373 the luminosity of the Sun in the visual spectrum. Any erstwhile terrestrial sized planet ( “e” ?), if it has anything like Earth’s vegetation coverage – and if fluorescence is ubiquitous – should be obvious. The more so for being emissive rather than dependent on reflecting incident starlight as with ocean “glint”.
I think the JWST has a chronograph which will block out the light. There might have to be a reflection of an exoplanet ocean with a lot of coral florescence.
JWST does have coronagraphs, but these well predate the “direct imaging” revolution spurred by WFIRST ( in space anyway ) . They consequently have nothing like the necessary contrast , inner working angle and wavefront control performance ( space adaptive optics ) for precision exoplanet imaging , even for large bright gas giants in distant orbits. Let alone small close in rocky M dwarf planets. Biofluresnce might be within the capability of the E-ELT when it gets its EPIC visual wavelength imager installed as a second stage instrument ( first stage METIS only images in the near to shorter mid IR) . Even then the TRAPPIST-1 planets are likely too remote to resolve , at 40 light years distance – tight around a very dim M8 star. Proxima b will be fine but with its active flare cycle I suspect biofluoresence won’t be enough. Even with all of the other very realistic radiation protection adaptations you suggest . For the observable surface any way.
This might proove an important Shortcut to finding biosignatures … if it becomes possible to screen a large number of waterworlds suffering from UV exposure , it would sem logic that somekind of lifeform on one of them would have developed a similar defence mekanism to earth´s corals …..and the best part is that this could happen i just afew years ..
What about the star shade option for JWST? One does not need a sensitive chronograph for that.
Fluorescent glow may reveal hidden life in the cosmos
By Blaine Friedlander
August 13, 2019
Astronomers seeking life on distant planets may want to go for the glow.
Harsh ultraviolet radiation flares from red suns, once thought to destroy surface life on planets, might help uncover hidden biospheres. Their radiation could trigger a protective glow from life on exoplanets called biofluorescence, according to new Cornell research.
“Biofluorescent Worlds II: Biological Fluorescence Induced by Stellar UV Flares, a New Temporal Biosignature” was published Aug. 13 in Monthly Notices of the Royal Astronomical Society.
“This is a completely novel way to search for life in the universe. Just imagine an alien world glowing softly in a powerful telescope,” said lead author Jack O’Malley-James, a researcher at Cornell’s Carl Sagan Institute.
“On Earth, there are some undersea coral that use biofluorescence to render the sun’s harmful ultraviolet radiation into harmless visible wavelengths, creating a beautiful radiance. Maybe such life forms can exist on other worlds too, leaving us a telltale sign to spot them,” said co-author Lisa Kaltenegger, associate professor of astronomy and director of the Carl Sagan Institute.
The Earth has habitable zones where advanced life can exist – places that exclude that such as deserts, arctic and high altitudes are a large percentage of the land area and the oceans are largely habitable. The tidally locked worlds around M dwarfs that exist in the habitable zone of the stars should also have a habitable zone on those planets. Probably the most common earth like worlds in the universe should have an area near their 45 to 90 degrees from high noon that avanced life forms would find habitable. Could this extend up into the K dwarfs? That’s a good question, at what level would tidal locking cease in the habitable zone that exist around warmer stars? Besides having more atmosphere to block the hard radiation these areas may have a much higher shaded area for ET to roam in. This may be easier to spot biofluorescence from these zones and also areas closer to the solar zenith as these would be well placed at different angles as the planet orbits the M dwarf. This could be called the Tidally Locked Habitable Zone or TLHZ for the those areas on these planets.
Mario Damasso (INAF – Torino) – A Low-mass Planet Candidate Orbiting Proxima Centauri at a Distance of 1.5 AU
By analyzing ~17 years of radial velocity data of Proxima Cen collected with the UVES and HARPS spectrographs, we detected a signal of period P~5 years that could be explained by the presence of a second planet, Proxima c, with minimum mass m sin i~6. Earth masses. Together with the low-mass temperate planet Proxima b, this candidate planet would make Proxima the closest multi-planet system to the Sun. We will present the properties of the new RV signal and investigate the likelihood that it is related to a magnetic activity cycle of the star. We will discuss how the existence of Proxima c can be confirmed, and its true mass determined with high accuracy, by combining Gaia astrometry and radial velocities. Proxima c would be a prime target for follow-up and characterization measurements, especially with next generation direct imaging instrumentation due to the large maximum angular separation of ?1 arcsecond from the parent star. Since the orbit would be beyond the original location of the snowline, Proxima c would challenge the models of the formation of super-Earths. Presently, this study is under review by Science Advances.