The Earth-Moon system has often been characterized as having some characteristics of a “double planet” system. It is demonstrated that while the orbital barycenter of the Earth-Moon pair lies inside the radius of the Earth, the Moon does meet all three requirements of the IAU definition for “planet” and therefore the Moon can correctly be identified as the Solar System’s 9 th planet. In order to avoid confusion by this development it is necessary to add definitions for “double planet”, “double dwarf planet”, “satellite planet”, and “satellite” to complement the International Astronomical Union definitions for “planet” and “dwarf planet”. The Earth-Moon system meets the requirements of a “double planet” system while the Pluto-Charon system meets the requirements of a “double dwarf planet” system. In order to extrapolate sub-stellar taxonomy to exoplanetary systems, general formation mechanisms (star-like gas collapse in molecular clouds vs. proto-planetary disk formation) should be included in the definitions for the various classes of sub-stellar mass bodies.
Baker, R.H. (1959) Astronomy. 7th Edition, D. Van Nostrand Company Inc., New York.
Kaler, J.B. (1994) Astronomy! Harper Collins College Publishers, New York.
McLaughlin, D.B. (1961) Introduction to Astronomy. Houghton Mifflin Company, Boston.
Payne-Gaposchkin, C. (1954) Introduction to Astronomy. Prentice Hall Inc., New York.
Russell, H.N, Dugan, R.S. and Steward, J.Q. (1926) Astronomy I: The Solar System. Ginn and Company, Boston.
Young, C.A. (1889) A Text-Book of General Astronomy. Ginn & Company, Boston.
Young, C.A. (1902) Manual of Astronomy. Ginn & Company, New York.
Stern, S.A. and Levison, H.F. (2002) Regarding the Criteria for Planethood and Proposed Planetary Classification Schemes. In: Rickman, H., Ed., Highlights of Astronomy 12: 205-213, Astronomical Society of the Pacific, San Francisco. https://doi.org/10.1017/S1539299600013289
Lineweaver, C. and Norman, M. (2010) The Potato Radius: A Lower Minimum Size for Dwarf Planets. Preprint, arXiv:1004.1091.
Tancredi, G. (2010) Physical and Dynamical Characteristics of Icy “Dwarf Planets” (Plutoids) Icy Bodies of the Solar System. Proceedings of the International Astronomical Union, IAU Symposium, 263, 173-185.
Weber, R.C., Lin, P.-Y., Garnero, E.J., Williams, Q. and Lognonné, P. (2011) Seismic Detection of the Lunar Core. Science, 331, 309-312. https://doi.org/10.1126/science.1199375
Williams, J.G., Konopliv, A.S., Boggs, D.H., Park, R.S., Yuan, D.-N., Lemoine, F.G., Goossens, S., et al. (2014) Lunar Interior Properties from the GRAIL Mission. Journal of Geophysical Research, 119, 1546-1578. https://doi.org/10.1002/2013JE004559
Soter, S. (2006) What Is a Planet? The Astronomical Journal, 132, 2513-2519. https://doi.org/10.1086/508861
Margot, J.-L. (2015) A Quantitative Criterion for Defining Planets. The Astronomical Journal, 150, 185-192. https://doi.org/10.1088/0004-6256/150/6/185
Asimov, I. (1975) Just Mooning Around. In: Asimov on Astronomy, Anchor Press/ Doubleday, New York.
Legault, R.J. (2013) Moon Loops and Dumbbells—The Most Curious Moon of All. Journal of the Royal Astronomical Society of Canada, 107, 65-68.
Herschel, J. (1836) A Treatise on Astronomy. Carey, Lea, & Blanchard, Philadelphia.
Whipple, F.L. (1941) Earth, Moon and Planets. The Blakiston Company, Philadelphia.
Bless, R.C. (1996) Discovering the Cosmos. University Science Books, Sausilito.
Chaisson, E. and McMillan, S. (2008) Astronomy Today. 6th Edition, Pearson-Addison Wesley, San Francisco.
Schneider, S.E. and Arny, T.T. (2015) Pathways to Astronomy. 5th Edition, McGraw-Hill Education, New York.
Runyon, K.D., Stern, S.A., Lauer, T.R., Grundy, W., Summers, M.E. and Singer, K.N. (2017) A Geophysical Planet Definition. 48th Lunar and Planetary Science Conference, LPI Contribution No. 1964, Article ID: 1448.
Lucas, P.W. and Roche, P.F. (2000) A Population of Very Young Brown Dwarfs and Free-Floating Planets in Orion. Monthly Notices of the Royal Astronomical Society, 314, 858-864. https://doi.org/10.1046/j.1365-8711.2000.03515.x
Kumar, S. (2003) The Bottom of the Main Sequence and Beyond: Speculations, Calculations, Observations, and Discoveries (1958-2002). IAUS, 211, 3-12. https://doi.org/10.1017/S0074180900210176
Chabrier, G., Johansen, A., Janson, M. and Rafikov, R. (2014) Giant Planet and Brown Dwarf Formation. In: Planets, V.I., Beuther, H., Klessen, R., Dullemond, C. and Henning, T. (Eds.), Protostars, University of Arizona Press, Tucson, 619-642. https://doi.org/10.2458/azu_uapress_9780816531240-ch027
Lucas, P., Weights, D., Roche, P. and Riddick, F. (2006) Spectroscopy of Planetary Mass Brown Dwarfs in Orion. Monthly Notices of the Royal Astronomical Society: Letters, 373, L60-L64. https://doi.org/10.1111/j.1745-3933.2006.00244.x
Bihain, G., Rebolo, R., Zapatero Osorio, M.R., Béjar, V.J.S., Villó-Pérez, I., Díaz-Sánchez, A., Pérez-Garrido, A., Caballero, J.A., Bailer-Jones, C.A.L., Barrado y Navascués, D., Eislöffel, J., Forveille, T., Goldman, B., Henning, T., Martín, E.L. and Mundt, R. (2009) Candidate Free-Floating Super-Jupiters in the Young Sigma Orionis Open Cluster. Astronomy & Astrophysics, 506, 1169-1182. https://doi.org/10.1051/0004-6361/200912210
Caballero, J.A., Béjar, V.J.S., Rebolo, R., Eislöffel, J., Zapatero Osorio, M.R., Mundt, R., Barrado Y Navascués, D., Bihain, G., Bailer-Jones, C.A.L., Forveille, T. and Martín, E.L. (2007) The Substellar Mass Function in Sigma Orionis. II. Optical, Near-Infrared and IRAC/Spitzer Photometry of Young Cluster Brown Dwarfs and Planetary-Mass Objects. Astronomy & Astrophysics, 470, 903-918. https://doi.org/10.1051/0004-6361:20066993
Ma, B. and Ge, J. (2014) Statistical Properties of Brown Dwarf Companions: Implications for Different Formation Mechanisms. Monthly Notices of the Royal Astronomical Society, 439, 2781-2789. https://doi.org/10.1093/mnras/stu134
Mulders, G.D., Paardekooper, S.-J., Panic, O., Dominik, C., van Boekel, R. and Ratzka, T. (2013) Planet or Brown Dwarf? Inferring the Companion Mass in HD 100546 from the Wall Shape using Mid-Infrared Interferometry. Astronomy & Astrophysics, 557, 68-78. https://doi.org/10.1051/0004-6361/201220930
Hatzes, A. and Rauer, H. (2015) A Definition for Giant Planets Based on the Mass-Density Relationship. The Astrophysical Journal Letters, 810, L25-L29. https://doi.org/10.1088/2041-8205/810/2/L25
Chen, G. and Kipping, D. (2017) Probabilistic Forecasting of the Masses and Radii of Other Worlds. The Astrophysical Journal, 834, 17-30. https://doi.org/10.3847/1538-4357/834/1/17
Kumar, S. (1963) The Structure of Stars of Very Low Mass. The Astrophysical Journal, 137, 1121-1125. https://doi.org/10.1086/147589