11:05 〜 11:20
[MIS06-07] Possible Photosynthetic Fluorescence from Earth-like Planets Around Cool stars
キーワード：太陽系外惑星、バイオマーカー、光合成
Recent remote sensing of the Earth has revealed that photosynthesis is traceable as fluorescence [1] as well as the vegetation red edge (VRE), the steep rise in the reflection around 750 nm. It is under discussion how and where the VRE might present as a sign of the presence of life (biosignature) in the spectra of exoplanets, especially around stars other than the Sun [2]. Around M dwarf stars, cooler than the Sun, any VRE signal will likely be more observable at longer wavelengths, where there are available stellar photons for the planet to reflect [3]. However, evolution of photosynthesis in water may drive a preference for using visible light, even after organisms colonize land surfaces [4]. Instead, biofluorescence may provide an alternative signal to VRE for the identification of photosynthesis on an exoplanet.
In this research, the detectability of photosynthetic fluorescence on extrasolar planets is presented at the first time. We examined how signals of photosynthetic fluorescence emerge in spectra of Earth-like planets around M dwarfs (TRAPPIST-1 and GJ667C) and the Sun using simulated atmospheric transmittance of planets assuming several geological epochs of Earth’s history [5]. Fluorescence, light absorption and reflectance of photosynthetic organism are consistently modeled using the spectra of vegetation with chlorophylls a and b (Chl) and bacteriochlorophyll b-bearing purple bacteria (BChl). We found that the fluorescence from BChl is ideal for the detection because it is not blended with the VRE feature and is located in a region of the spectrum free from water vapor absorption, while the fluorescence emission from Chl is overlapped with the VRE feature and is difficult to be extracted. A simulated observation of an Earth-Sun twin at 10 pc from a LUVOIR-A like large space telescope shows that it requires an unrealistically long to detect the biofluorescence even in the optimistic conditions. However, biofluorescence produces a large apparent enhancement in the reflectance of the model planet around TRAPPIST-1, which is made more detectable by the favorable planet/star flux ratio due to low stellar flux by large stellar absorption, which is a typical feature in ultrared cool dwarf. Biological fluorescence around late type M stars may therefore be a promising biosignature using high-dispersion spectrometers on upcoming ground-based telescopes, although an exploration of the detectability of this feature is still needed. Based on our knowledge on photosynthesis, we also discuss physiological conditions that could enhance fluorescence emission, and how using the non-linearity of emission to incoming photons can help avoid false positive/negative detection of biofluorescence.
[1] Mohammed, G. H., et al.: 2019, Remote Sens Environ, 231,111177.
[2] Kiang N. Y., et al.: 2007, Astrobiology, 7, 252.
[3] Tinetti, G., et al.: 2006, Astrobiology, 6, 881.
[4] Takizawa, K., et al.: 2017, Sci. Rep., 7, 7561.
[5] Rugheimer, S. & Kaltenegger, L.: 2018, ApJ, 854, 19.
In this research, the detectability of photosynthetic fluorescence on extrasolar planets is presented at the first time. We examined how signals of photosynthetic fluorescence emerge in spectra of Earth-like planets around M dwarfs (TRAPPIST-1 and GJ667C) and the Sun using simulated atmospheric transmittance of planets assuming several geological epochs of Earth’s history [5]. Fluorescence, light absorption and reflectance of photosynthetic organism are consistently modeled using the spectra of vegetation with chlorophylls a and b (Chl) and bacteriochlorophyll b-bearing purple bacteria (BChl). We found that the fluorescence from BChl is ideal for the detection because it is not blended with the VRE feature and is located in a region of the spectrum free from water vapor absorption, while the fluorescence emission from Chl is overlapped with the VRE feature and is difficult to be extracted. A simulated observation of an Earth-Sun twin at 10 pc from a LUVOIR-A like large space telescope shows that it requires an unrealistically long to detect the biofluorescence even in the optimistic conditions. However, biofluorescence produces a large apparent enhancement in the reflectance of the model planet around TRAPPIST-1, which is made more detectable by the favorable planet/star flux ratio due to low stellar flux by large stellar absorption, which is a typical feature in ultrared cool dwarf. Biological fluorescence around late type M stars may therefore be a promising biosignature using high-dispersion spectrometers on upcoming ground-based telescopes, although an exploration of the detectability of this feature is still needed. Based on our knowledge on photosynthesis, we also discuss physiological conditions that could enhance fluorescence emission, and how using the non-linearity of emission to incoming photons can help avoid false positive/negative detection of biofluorescence.
[1] Mohammed, G. H., et al.: 2019, Remote Sens Environ, 231,111177.
[2] Kiang N. Y., et al.: 2007, Astrobiology, 7, 252.
[3] Tinetti, G., et al.: 2006, Astrobiology, 6, 881.
[4] Takizawa, K., et al.: 2017, Sci. Rep., 7, 7561.
[5] Rugheimer, S. & Kaltenegger, L.: 2018, ApJ, 854, 19.