Updated on 2024/05/31


*Items subject to periodic update by Rikkyo University (The rest are reprinted from information registered on researchmap.)
Graduate School of Science Master's Program in Physics
Graduate School of Science Doctoral Program in Physics
Specially Appointed Associate Professor
Research Interests
  • Exoterrestrial planet

  • Planetary climate

  • Habitable planet

  • Campus Career*
    • 4 2022 - Present 
      Graduate School of Science   Master's Program in Physics   Specially Appointed Associate Professor
    • 4 2022 - Present 
      Graduate School of Science   Doctoral Program in Physics   Specially Appointed Associate Professor

    Research Areas

    • Natural Science / Space and planetary sciences  / Exoplanetary science

    Research History

    • 4 2022 - Present 
      Rikkyo University

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    • 4 2020 - 3 2022 
      The University of Tokyo   Graduate School of Science

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    • 4 2013 - 3 2020 
      The University of Tokyo   Graduate School of Science   Department of Earth and Planetary Science

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      Country: Japan


    • 4 2009 - 3 2013 
      Hokkaido University   School of Science   Earth Sciences

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    • Numerical Performance of Correlated-k Distribution Method in Atmospheric Escape Simulation

      Yuichi Ito, Tatsuya Yoshida, Akifumi Nakayama

      The Astrophysical Journal962 ( 2 ) 106 - 106   1 2 2024

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      Publishing type:Research paper (scientific journal)   Publisher:American Astronomical Society  


      Atmospheric escape is crucial to understanding the evolution of planets in and out of the solar system and to interpreting atmospheric observations. While hydrodynamic escape simulations have been actively developed incorporating detailed processes such as UV heating, chemical reactions, and radiative cooling, the radiative cooling by molecules has been treated as emission from selected lines or rotational/vibrational bands to reduce its numerical cost. However, ad hoc selections of radiative lines would risk estimating inaccurate cooling rates because important lines or wavelengths for atmospheric cooling depend on emitting conditions such as temperature and optical thickness. In this study, we apply the correlated-k distribution (CKD) method to cooling rate calculations for H<sub>2</sub>-dominant transonic atmospheres containing H<sub>2</sub>O or CO as radiative species, to investigate its numerical performance and the importance of considering all lines of the molecules. Our simulations demonstrate that the sum of weak lines, which provides only 1% of the line emission energy in total at optically thin conditions, can become the primary source of radiative cooling in optically thick regions, especially for H<sub>2</sub>O-containing atmospheres. Also, in our hydrodynamic simulations, the CKD method with a wavelength resolution of 1000 is found to be effective, allowing the calculation of escape rate and temperature profiles with acceptable numerical cost. Our results show the importance of treating all radiative lines and the usefulness of the CKD method in hydrodynamic escape simulations. It is particularly practical for heavy-element-enriched atmospheres considered in small exoplanets, including super-Earths, without any prior selections for effective lines.

      DOI: 10.3847/1538-4357/ad187f


      Other Link: https://iopscience.iop.org/article/10.3847/1538-4357/ad187f/pdf

    • Survival of Terrestrial N2–O2 Atmospheres in Violent XUV Environments through Efficient Atomic Line Radiative Cooling Peer-reviewed

      Akifumi Nakayama, Masahiro Ikoma, Naoki Terada

      The Astrophysical Journal937 ( 2 ) 72 - 72   1 10 2022

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      Publishing type:Research paper (scientific journal)   Publisher:American Astronomical Society  


      Atmospheres play a crucial role in planetary habitability. Around M dwarfs and young Sun-like stars, planets receiving the same insolation as the present-day Earth are exposed to intense stellar X-rays and extreme-ultraviolet (XUV) radiation. This study explores the fundamental question of whether the atmosphere of present-day Earth could survive in such harsh XUV environments. Previous theoretical studies suggest that stellar XUV irradiation is sufficiently intense to remove such atmospheres completely on short timescales. In this study, we develop a new upper-atmospheric model and re-examine the thermal and hydrodynamic responses of the thermospheric structure of an Earth-like N<sub>2</sub>–O<sub>2</sub> atmosphere, on an Earth-mass planet, to an increase in the XUV irradiation. Our model includes the effects of radiative cooling via electronic transitions of atoms and ions, known as atomic line cooling, in addition to the processes accounted for by previous models. We demonstrate that atomic line cooling dominates over the hydrodynamic effect at XUV irradiation levels greater than several times the present level of the Earth. Consequentially, the atmosphere’s structure is kept almost hydrostatic, and its escape remains sluggish even at XUV irradiation levels up to a thousand times that of the Earth at present. Our estimates for the Jeans escape rates of N<sub>2</sub>–O<sub>2</sub> atmospheres suggest that these 1 bar atmospheres survive in early active phases of Sun-like stars. Even around active late M dwarfs, N<sub>2</sub>–O<sub>2</sub> atmospheres could escape significant thermal loss on timescales of gigayears. These results give new insights into the habitability of terrestrial exoplanets and the Earth’s climate history.

      DOI: 10.3847/1538-4357/ac86ca


      Other Link: https://iopscience.iop.org/article/10.3847/1538-4357/ac86ca/pdf

    • Two Bright M Dwarfs Hosting Ultra-Short-Period Super-Earths with Earth-like Compositions* Peer-reviewed

      Teruyuki Hirano, John H. Livingston, Akihiko Fukui, Norio Narita, Hiroki Harakawa, Hiroyuki Tako Ishikawa, Kohei Miyakawa, Tadahiro Kimura, Akifumi Nakayama, Naho Fujita, Yasunori Hori, Keivan G. Stassun, Allyson Bieryla, Charles Cadieux, David R. Ciardi, Karen A. Collins, Masahiro Ikoma, Andrew Vanderburg, Thomas Barclay, C. E. Brasseur, Jerome P. de Leon, John P. Doty, René Doyon, Emma Esparza-Borges, Gilbert A. Esquerdo, Elise Furlan, Eric Gaidos, Erica J. Gonzales, Klaus Hodapp, Steve B. Howell, Keisuke Isogai, Shane Jacobson, Jon M. Jenkins, Eric L. N. Jensen, Kiyoe Kawauchi, Takayuki Kotani, Tomoyuki Kudo, Seiya Kurita, Takashi Kurokawa, Nobuhiko Kusakabe, Masayuki Kuzuhara, David Lafrenière, David W. Latham, Bob Massey, Mayuko Mori, Felipe Murgas, Jun Nishikawa, Taku Nishiumi, Masashi Omiya, Martin Paegert, Enric Palle, Hannu Parviainen, Samuel N. Quinn, George R. Ricker, Richard P. Schwarz, Sara Seager, Motohide Tamura, Peter Tenenbaum, Yuka Terada, Roland K. Vanderspek, Sébastien Vievard, Noriharu Watanabe, Joshua N. Winn

      The Astronomical Journal162 ( 4 ) 161 - 161   1 10 2021

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      Publishing type:Research paper (scientific journal)   Publisher:American Astronomical Society  

      DOI: 10.3847/1538-3881/ac0fdc


      Other Link: https://iopscience.iop.org/article/10.3847/1538-3881/ac0fdc/pdf

    • Runaway climate cooling of ocean planets in the habitable zone: a consequence of seafloor weathering enhanced by melting of high-pressure ice Peer-reviewed

      Akifumi Nakayama, Takanori Kodama, Masahiro Ikoma, Yutaka Abe

      Monthly Notices of the Royal Astronomical Society488 ( 2 ) 1580 - 1596   1 7 2019

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      Publishing type:Research paper (scientific journal)   Publisher:Oxford University Press (OUP)  

      Terrestrial planets covered globally with thick oceans (termed ocean planets)
      in the habitable zone were previously inferred to have extremely hot climates
      in most cases. This is because ${\rm H_2O}$ high-pressure (HP) ice on the
      seafloor prevents chemical weathering and, thus, removal of atmospheric CO$_2$.
      Previous studies, however, ignored melting of the HP ice and horizontal
      variation in heat flux from oceanic crusts. Here we examine whether high heat
      fluxes near the mid-ocean ridge melts the HP ice and thereby removes
      atmospheric ${\rm CO_2}$. We develop integrated climate models of an Earth-size
      ocean planet with plate tectonics for different ocean masses, which include the
      effects of HP ice melting, seafloor weathering, and the carbonate-silicate
      geochemical carbon cycle. We find that the heat flux near the mid-ocean ridge
      is high enough to melt the ice, enabling seafloor weathering. In contrast to
      the previous theoretical prediction, we show that climates of terrestrial
      planets with massive oceans lapse into extremely cold ones (or snowball states)
      with CO$_2$-poor atmospheres. Such extremely cold climates are achieved mainly
      because the HP ice melting fixes seafloor temperature at the melting
      temperature, thereby keeping a high weathering flux regardless of surface
      temperature. We estimate that ocean planets with oceans several tens of the
      Earth's ocean mass no longer maintain temperate climates. These results suggest
      that terrestrial planets with extremely cold climates exist even in the
      habitable zone beyond the solar system, given the frequency of water-rich
      planets predicted by planet formation theories.

      DOI: 10.1093/mnras/stz1812


      Other Link: http://arxiv.org/pdf/1907.00827v3

    Professional Memberships

    Research Projects

    • 低温度星周りの地球型惑星が温暖環境を保持する条件の解明

      日本学術振興会  科学研究費助成事業 

      中山 陽史

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      4 2023 - 3 2026

      Grant number:23K13161

      Grant amount:\4550000 ( Direct Cost: \3500000 、 Indirect Cost:\1050000 )