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NHK カルチャー講座「惑星科学最前線 ~生命を宿す天体の条件を探る~」(第2回)

NHK カルチャー講座「惑星科学最前線 ~生命を宿す天体の条件を探る~」(第2回)

NHK カルチャー梅田教室にて、生命を宿す天体に関する講座を行ってきました。そのときに用いた講演資料です。

Takanori Sasaki

January 04, 2022
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  1. ✤ ਫ࿭੕ͷ৚݅ͱϋϏλϒϧκʔϯ 
 ɹਫͷ֫ಘɾอ࣋ɾਫྔʹؔ͢Δ੍໿ ✤ ஍ٿ͕ʮੜ໋ͷ࿭੕ʯͱͳͬͨཧ༝ 
 ɹؾީ҆ఆԽϝΧχζϜͱࢎૉେؾͷൃੜ ✤ ଠཅܥ಺ɾଠཅܥ֎ʹੜ໋Λ୳͢

    
 ɹՐ੕ɾΤ΢ϩύɾΤϯέϥυεɾܥ֎࿭੕ ✤ ଟ༷ͳϋϏλϒϧϓϥωοτ 
 ɹ஍ٿͱ͸શ͘ҟͳΔ࿭੕ɾӴ੕Ͱͷੜ໋Մೳੑ ߨ࠲ͷ಺༰
  2. ✤ ਫ࿭੕ͷ৚݅ͱϋϏλϒϧκʔϯ 
 ɹਫͷ֫ಘɾอ࣋ɾਫྔʹؔ͢Δ੍໿ ✤ ஍ٿ͕ʮੜ໋ͷ࿭੕ʯͱͳͬͨཧ༝ 
 ɹؾީ҆ఆԽϝΧχζϜͱࢎૉେؾͷൃੜ ✤ ଠཅܥ಺ɾଠཅܥ֎ʹੜ໋Λ୳͢

    
 ɹՐ੕ɾΤ΢ϩύɾΤϯέϥυεɾܥ֎࿭੕ ✤ ଟ༷ͳϋϏλϒϧϓϥωοτ 
 ɹ஍ٿͱ͸શ͘ҟͳΔ࿭੕ɾӴ੕Ͱͷੜ໋Մೳੑ ߨ࠲ͷ಺༰
  3. ✤ ਫ࿭੕ͷ৚݅ͱϋϏλϒϧκʔϯ 
 ɹਫͷ֫ಘɾอ࣋ɾਫྔʹؔ͢Δ੍໿ ✤ ஍ٿ͕ʮੜ໋ͷ࿭੕ʯͱͳͬͨཧ༝ 
 ɹؾީ҆ఆԽϝΧχζϜͱࢎૉେؾͷൃੜ ✤ ଠཅܥ಺ɾଠཅܥ֎ʹੜ໋Λ୳͢

    
 ɹՐ੕ɾΤ΢ϩύɾΤϯέϥυεɾܥ֎࿭੕ ✤ ଟ༷ͳϋϏλϒϧϓϥωοτ 
 ɹ஍ٿͱ͸શ͘ҟͳΔ࿭੕ɾӴ੕Ͱͷੜ໋Մೳੑ ߨ࠲ͷ಺༰
  4. ࢎԽతͳେؾ ɹʙؾѹ )େؾࢄҳ 4VQFS&BSUI ؐݩతͳେؾ ؐݩతͳେؾ )େؾ͕ ΄ͱΜͲࢄҳ͠ͳ͍ εʔύʔΞʔεͷ৔߹ɿ ɹେྔͷؐݩతͳେؾΛ

    ɹ࣋ͭ࿭੕͕ܗ੒ʁ ඍ࿭੕ ݪ࢝࿭੕ )Ψεʢؐݩతʣ ؐݩతͳେؾ &BSUI ஍ٿͱεʔύʔΞʔεͷେؾͷҧ͍
  5. [Pierrehumbert & Gaidos 2011] Pierrehumbert & Gaidos 0 1 0.1

    1 10 0.1 1 10 100 G star M star Orbital distance (AU) Surface pressure for 280K (bar) H2-He େؾʹΑΔԹࣨޮՌΛߟྀ M, G ܕ੕पΓͷ 3ME ͷεʔύʔΞʔεͰܭࢉ H2 େؾ100barͰ 2.4AU(M), 15AU(G) ·Ͱ H.Z. ෼ް͍ਫૉେؾʹΑΔԹࣨޮՌ 4VQFS&BSUI
  6. IKA Vol. 680 L54 TAJIKA Vol. 680 Fig. 1.—Habitable zone

    (HZ) around main-sequence stars. The region with light gray shading represents the HZ for the Earth-like ocean planet (Kasting et al. 1993; Mischna et al. 2000). The region with dark gray shading (denoted by “minimum HZ”) represents the HZ in the narrow sense (0.84 AU ≤ d ≤ 0.87 for p 1) which is defined here as the region between the runaway L/L 0 greenhouse limit (Kasting 1988; Nakajima et al. 1992) and the orbit at which the effective temperature is 273 K for the planet with no greenhouse gas in the atmosphere (␧ p 1) and A p 0.3. Solid curves represent the orbits at which the surface temperature is 273 K for the planet with some greenhouse gases in the atmosphere (␧ ! 1) and A p 0.3. Filled circle represents the present condition for the Earth. For example, because hot-spot-type volcanism due to a rise of mantle plumes would be the most common type of volcanism on the terrestrial planet (as seen on Venus, Mars, and Earth), there may be terrestrial planets without volcanism due to plate શٿ͕ණʹ෴ΘΕ͍ͯΔ࿭੕Ͱ಺෦೤ݯΛߟྀ ණͷҰ෦༹͕͚಺෦ւΛ࣋ͭՄೳੑ 0.4ME < M ͷ࿭੕Ͱ͸಺෦ւ͕ܗ੒Մೳ [Tajika 2008] ϋϏλϒϧ಺෦ւ࿭੕
  7. Distance from the Central Star 10 1 0.1 Planetary Mass

    (M/M E ) Distance from the Central Star (AU)  0 40 20 ①: Only Ice I ②: Internal Ocean ③: Ocean on the Surface ⑤: High- pressure Ice under Internal Ocean 1, 2, 5, 10 E/E E = 1 M sw /M swE = Internal ocean without high-pressure ice layers The strong constraints on the planetary mass and H 2 O mass ① ② ③  ⑤ த৺੕͔Βͷڑ཭ʢAUʣ ࿭੕ͷ࣭ྔʢME ʣ ༷ʑͳ࿭੕ͷւʢ಺෦ւʣͷߏ଄ ࿭੕ද໘ͷਫͷ࠷ਂ෦ͷѹྗ p (bar) ͸ҎԼͷΑ͏ʹදͤΔɻ p = dwρwg × 10−5 (3.22) ϞσϧΛ؆ུԽ͢ΔͨΊʹɺණ 1-ӷମͷڥքʹ͓͚Δ༥఺ۂઢΛ௚ઢͱΈ ͳ͢ɻ࿭੕ͷݻମ෦෼ͷද໘Թ౓ T ͸ҎԼͷΑ͏ʹਫͷް͞ dw ʹΑܾͬͯ ਤ 3.5: ࿭੕ද໘ʹਫ͕͋Δ৔߹ͷද໘ঢ়ଶͷ෼ྨ: λΠϓ 1,4 ͸ද໘͕ණͷ Έͱͳ͍ͬͯΔ৔߹ͰɺλΠϓ 4 ͸ණͷఈͷ෦෼ʹߴѹණ͕ଘࡏ͢ΔɻλΠ ϓ 2 ͸಺෦ւ͕ଘࡏ͢Δ৔߹ͰϋϏλϒϧͳՄೳੑ͕͋ΔɻλΠϓ 5 ͸಺෦ ւ͕ଘࡏ͢Δ͕಺෦ւͷఈ͕ߴѹණͱͳ͓ͬͯΓɺϋϏλϒϧͱ͸ݴ͑ͳ͍ ͩΖ͏ɻλΠϓ 3,6 ͸࿭੕ද໘Թ౓͕ 273K ΑΓ΋େ͖͘ͳΓɺද໘ʹණ͕ଘ ࡏ͠ͳ͍৔߹Ͱ͋Δ (ւ࿭੕)ɻୠ͠ւͷਂ౓͕େ͖͍৔߹ɺλΠϓ 6 ͷΑ͏ ʹఈʹߴѹණ͕ग़དྷͯ͠·͏ɻ 19 [Ueta & Sasaki 2013] ϋϏλϒϧͳՄೳੑͷ͋Δ಺෦ւ ᶄ Λ΋ͭ৚݅͸ ࿭੕ͷ࣭ྔʹରͯ͠ඇৗʹݫ੍͍͠ݶ͕͋Δ
  8. [Stevenson 1999] © 1999 Macmillan Magazines Ltd mismatching of spectral

    peaks and resolu- tion1,4,5, but we believe that our analysis of a complete and high-resolution record using a powerful spectral technique provides strong evidence of 33-Myr periodicity. It is not yet clear what drives this period- icity and there is no simple relationship with mass extinction or impact cratering. However, the existence of remarkable spec- tral power stability and the statistical relia- bility of our results support the authenticity of this cycle and provide a stimulus for fur- ther research into the coupling of bio-geo- chemical cycles. R. K. Tiwari, K. N. N. Rao Theoretical Geophysics Group, National Geophysical Research Institute, Hyderabad 500 007, India e-mail: [email protected] 1. Raup. D. M. & Sepkoski, J. J. Science 231, 833–836 (1986). 2. Stigler, S. M. & Wagner, M. J. Science 238, 940–945 (1987). 3. Raup, D. M. & Sepkoski, J. J. Science 241, 94–96 (1988). 4. Rampino, M. R. & Caldeira, K. Earth Planet. Sci. Lett. 114, 215–227 (1993). 5. Negi, J. G., Tiwari, R. K. & Rao, K. N. N. Mar. Geol. 133, 113–121 (1996). 6. Fischer, A. G. & Arthur, M. A. in Deep Water Carbonate Environments (eds Cook, H. E. &. Enos, P.) 19–50 (Soc. Econ. Paleont. Mineral., Tulsa, 1977). 7. Follmi, K. B. Earth Sci. Rev. 40, 55–124 (1996). 8. Harland, W. B. et al. A Geological Time Scale (Cambridge Univ. Press, 1989). 9. Filippelli, G. M. & Delancy, M. L. Palaeoceanography 9, 643–652 (1994). 10.Van Cappellen, P. & Ingall, E. D. Palaeoceanography 9, 677–692 (1994). 11.Thomson, D. J. Proc. IEEE 70, 1055–1096 (1982). 12.Mann, M. E. & Lees, J. M. Clim. Change 33, 409–445 (1996).Ã nebula is still present1. These bodies form an envelope of nebula gas bound together by gravity. In the outer Solar System, they may continue to accrete gas, forming the giant planets, while the high ultraviolet out- put of the early Sun causes those bodies closer to it to lose their gaseous envelope. In one development of these ideas2, runaway accretion causes many embryos to form quickly, some of which may merge, while others may be scattered into escape trajec- tories by proto-Jupiter or proto-Saturn. The formation of planets may be quite ineffi- cient in the sense that more solid material is ejected than retained. There are uncertain- ties and alternatives3–5 but, because solar systems may have been formed in diverse ways, the possibility of bodies of roughly Earth mass in interstellar space should be taken seriously. The amount of nebular gas accumulated and retained depends strongly on planet mass, nebula temperature, opacity assump- tions and accretion timescale. An Earth- mass body eliminating its energy of formation in a million years and with only pressure-induced opacity of hydrogen6 develops an atmosphere with Matm /MLJ0.01, where M is planet mass and Matm is atmos- pheric mass. More opaque models7 yield atmospheric masses with Matm /MLJ0.001, in agreement with detailed models1. The retention of a major part of this atmosphere is difficult at Earth orbit once most of the nebula has cleared, but becomes increasingly likely at greater distances, espe- cially once the atmosphere has cooled (so that the photosphere is no longer large compared with the solid body). The atmos- pheric escape time can be as short as a mil- lion years at one astronomical unit early in the Solar System1, but longer than the age of the Solar System in the interstellar medium. Sputtering (collision with interstellar mol- ecular or atomic hydrogen at tens to hun- dreds of kilometres per second) can be important if denser interstellar regions are encountered, but the column density of hydrogen in the case of Matm /MLJ0.001 to 0.01 is so large that removing such an atmosphere would correspond to much more mass being sputtered per unit area than the total mass per unit area of a comet in the Oort cloud. At the present epoch (assumed to be around 4.6 Gyr after formation), an inter- stellar planet would have a luminosity derived from long-lived radionuclides of around 4ǂ1020 ȡ erg sǁ1 if it is like Earth8, where ȡ is the planet mass in units of Earth masses. Assuming a thin atmosphere and an Earth-like density, the effective tempera- ture Te of the planet is given by Te LJ34 ȡ1/12 K. From hydrostatic equilibrium, the sur- face pressure Ps LJ106ǂMatm /M bars. How- ever, optical-depth unity at relevant infrared wavelengths (about 100 Ȗm) is achieved in such an atmosphere at a pres- sure of around 1 bar (refs 6,9) and liquefac- tion at this pressure occurs at a temperature of around 22 K, below the actual atmos- pheric temperature. A convective gas adia- bat must form at all greater depths (at a pressure between 1 bar and Ps ), even when the heat flow is very low. An adequate esti- mate for this adiabat turns out to be TʷP0.36, which does not intercept the con- densation curve for hydrogen. It follows that the surface temperature is given by Ts LJ425ȡ1/12((Matm /M)/0.001)0.36 K. The melting point of water is typically exceeded for basal pressures of the order of one kilobar. The atmosphere will have sev- eral cloud layers (methane, ammonia and perhaps water, like Uranus), but this has lit- tle influence on the temperature estimate. It seems, then, that bodies with water oceans are possible in interstellar space. The ideal conditions are plausibly at an Earth mass or slightly less, similar to the expected masses of embryos ejected during the for- mation of giant planets. Bodies with Earth- like water reservoirs may have an ocean underlain with a rock core. Either way, these bodies are expected to have volcanism in the rocky component and a dynamo- generated magnetic field leading to a well developed (very large) magnetosphere. Despite thermal radiation at microwave fre- quencies that corresponds to the tempera- tures deep within their atmospheres (analogous to Uranus9), and despite the possibility of non-thermal radio emission, they will be very difficult to detect. If life can develop and be sustained without sunlight (but with other energy sources, plausibly volcanism or lightning in this instance), these bodies may provide a long-lived, stable environment for life (albeit one where the temperatures slowly decline on a billion-year timescale). The complexity and biomass may be low because the energy source will be small, but it is conceivable that these are the most common sites of life in the Universe. Details of the above results are available from the author. David J. Stevenson Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, California 91125, USA e-mail: [email protected] 1. Hayashi, C., Nakazawa, K. & Nakagawa, Y. in Protostars and Planets II (eds Black, D. C. & Matthews, M. S.) 1100–1153 (Univ. Arizona Press, Tucson, 1985). 2. Lissauer, J. J. Icarus 69, 249–265 (1987). 3. Levison, H. F., Lissauer, J. J. & Duncan, M. J. Astron. J. 116, 1998–2014 (1998). 4. Cameron, A. G. W. Meteoritics 30, 133–161 (1995). 5. Boss, A. P. Science 276, 1836–1839 (1997). 6. Birnbaum, G., Borysow, A. & Orton, G. S. Icarus 123, 4–22 (1996). 7. Stevenson, D. J. Planet. Space Sci. 30, 755–764 (1982). 8. Stacey, F. D. Physics of the Earth 3rd edn (Brookfield, Brisbane, 1992). 9. Conrath, B. J. et al. in Uranus (eds Bergstrahl, J. T., Miner, E. T. & Matthews, M. S.) 204–252 (Univ. Arizona Press, Tucson, 1991). scientific correspondence 32 NATURE | VOL 400 | 1 JULY 1999 | www.nature.com Life-sustaining planets in interstellar space? During planet formation, rock and ice embryos of the order of Earth’s mass may be formed, some of which may be ejected from the Solar System as they scatter gravitation- ally from proto-giant planets. These bodies can retain atmospheres rich in molecular hydrogen which, upon cooling, can have basal pressures of 102 to 104 bars. Pressure- induced far-infrared opacity of H2 may pre- vent these bodies from eliminating internal radioactive heat except by developing an extensive adiabatic (with no loss or gain of heat) convective atmosphere. This means that, although the effective temperature of the body is around 30 K, its surface temper- ature can exceed the melting point of water. Such bodies may therefore have water oceans whose surface pressure and tempera- ture are like those found at the base of Earth’s oceans. Such potential homes for life will be difficult to detect. Planet formation is imperfectly under- stood, but many models involve the accu- mulation of solid bodies of up to several Earth masses while the hydrogen-rich solar ԁ൫ΨεΛେྔʹ·ͱ͍ ܥ֎ʹඈͼग़ͨ͠ු༡࿭੕ ೤ݯɿ์ࣹੑ֩छ อԹɿೱ͍ H2 େؾ ! = 425 ! !! ! !" !!"# 0.001! !.!" ![K] ϋϏλϒϧු༡࿭੕ͷଘࡏՄೳੑ
  9. ༷ʑͳ࣭ྔͷ੕ͷण໋ ੕ͷ࣭ྔʦଠཅ࣭ྔʧ ੕ͷण໋ʦ೥ʧ 100 2.7×106 50 5.9×106 10 2.6×107 5

    1.0×108 2 1.3×109 1 1.0×1010 0.7 4.9×1010 0.5 1.7×1011 ࣭ྔͷେ͖ͳ੕΄Ͳण໋͕୹͍
  10. Worlds without Moons: Exomoon Constraints for Compact Planetary Systems S.

    R. Kane, ApJL 839:L19 (2017) ࿭੕͕Ӵ੕Λอ࣋͢ΔͨΊͷඞཁ৚݅ɿ ɾӴ੕ͷيಓ͕ϩγϡݶք൒ܘҎ্ ɾӴ੕ͷيಓ͕ώϧ൒ܘҎԼ ϩγϡݶք൒ܘΑΓ ಺ଆͰ͸Ӵ੕͸ഁյ ώϧݶք൒ܘΑΓ ֎ଆͰ͸Ӵ੕͸ࢄҳ ɹˠ53"11*45ܥͰ͸͜ͷ৚݅Λຬͨ͞ͳ͍  ɹɹͭ·ΓͲͷ࿭੕΋Ӵ੕Λ͍࣋ͬͯͳ͍Մೳੑ
  11. No Snowball on Habitable Tidally Locked Planets J. Checlair et

    al., arXiv:1705.08094 يಓ͕ைࣚϩοΫ͞Εͯ ͍Δ৔߹ ਖ਼ͷϑΟʔυ όοΫ͕ޮ͔ͳ͍ ˠશٿౚ݁ঢ়ଶͱ ɹ෦෼ౚ݁ঢ়ଶͷؒΛ ɹ࿈ଓతʹߦ͖དྷ͢Δ
  12. ଟ༷ͳϋϏλϒϧఱମͷෆརͳ఺ ᶃ εʔύʔΞʔε͸ॏྗ͕ڧ͗͢Δ ᶄ εʔύʔΞʔε͸ւ࿭੕ʹͳΓ΍͍͢ ᶅ εʔύʔΞʔε͸ࢎԽతͳେؾΛ࣋ͯͳ͍ ᶆ ॏ͍಺෦ւ࿭੕Ͱ͸ߴѹණ͕ੜ͡Δ ᶇ

    ු༡࿭੕ͷੑ࣭ʹ͸ෆఆੑ͕େ͖͍ ᶈ Mܕ੕पΓͷ࿭੕͸Ӵ੕͕࣋ͯͳ͍ ᶉ Mܕ੕पΓͷ࿭੕͸ைࣚϩοΫ͞Ε͍ͯΔ ᶊ Mܕ੕पΓͷ࿭੕͸దྔͷਫΛ֫ಘ͠ͳ͍ ᶋ Mܕ੕͸ܹ͍͠ϑϨΞ׆ಈΛى͜͢