1- 1Institute of Astronomy and Astrophysics, Academia Sinica (Taipei, Taiwan), Taiwan (ytchen@asiaa.sinica.edu.tw)
- *A full list of authors appears at the end of the abstract
The Formation of the Outer Solar System: an Icy Legacy II survey (FOSSIL II) is a wide-field survey of the outer Solar System focused on the discovery and dynamical classification of distant trans-Neptunian objects (TNOs). FOSSIL II emphasizes finding and characterizing TNOs with large perihelion distances and those trapped in Neptunian mean-motion resonances. By expanding the survey’s depth and implementing intensive follow-up, FOSSIL II has substantially increased the census of distant small bodies.
We report on recent observational campaigns and results from Phase II. In particular, a tracking observation on 28 April 2025 successfully extended the observational arcs of most of the newly discovered TNOs to over two years. As a result, we achieved a recovery (re-observation) rate exceeding 90% for a sample of 201 TNOs. The extended arcs dramatically improve orbit determinations — the fractional uncertainty in semi-major axis (Δa/a) is now on the order of 0.001 for these objects. Such precision is sufficient to robustly determine each object’s dynamical class, including separating resonant and non-resonant populations in the Kuiper Belt. These improved orbits provide a clearer picture of the outer Solar System’s structure, delineating the various resonant families and high-perihelion objects at an unprecedented level of precision. Based on these results, this high-precision orbital dataset provides a stronger foundation for future studies of resonant TNO demographics and formation history of the Kuiper Belt. We will outline how the extended tracking baseline and high recovery rate enhance our ability to characterize these distant populations.
Finally, we present a preliminary characterization of “Ammonite” (2023 KQ14), a newly discovered Sedna-like object uncovered by FOSSIL II. Ammonite is a high-perihelion TNO (perihelion ∼66 au). Initial photometric observations obtained on 26 April 2025 using the Magellan telescope include a continuous 7‑hour monitoring window, which revealed no clear periodic lightcurve variations and placed an upper limit on the amplitude of 0.18 mag (1σ), suggesting it does not exhibit a large-amplitude, rotation-induced brightness variation (it may have a relatively uniform shape/albedo or a long rotation period). Additionally, multi-band color measurements indicate that Ammonite’s surface colors are consistent with typical TNOs, with g – r = 0.87 ± 0.18 and r – i = 0.36 ± 0.18, showing a moderate redness comparable to other Sedna-like objects (noting that Ammonite’s diameter is estimated to be ~200–300 km based on its brightness and assumed albedo). The results from FOSSIL II, including the discovery of Ammonite, underscore the value of its unique capabilities: the survey not only builds a broad statistical sample but also uncovers extreme objects that expand our understanding of the Solar System’s outermost reaches.
References:
Chen, Y.-T., et al. (in revision). Discovery and Dynamics of a Sedna-like Object with a Perihelion of 66 au. Nature Astronomy.
Chan-Kao Chang¹, Takashi Ito²,³, Fumi Yoshida⁴,², Shiang-Yu Wang¹, Mike Alexandersen⁵, Edward Ashton¹, Michele T. Bannister⁶, Young-Jun Choi⁷, A. Paula Granados Contreras¹, Wesley C. Fraser⁸,⁹,¹, Brett Gladman¹⁰, Yukun Huang³, Jianghui Ji¹¹, Youngmin JeongAhn⁷, JJ Kavelaars⁸,⁹,¹⁰, Jian Li¹²,¹³, Zhong-Yi Lin¹⁴, Matthew J. Lehner¹,¹⁵, Samantha M. Lawler¹⁶, Myung-Jin Kim⁷, Patryk Sofia Lykawka¹⁷, Marco Muñoz-Gutiérrez¹⁸, Surhud More¹⁹,²⁰, Keiji Ohtsuki²¹, Lowell Peltier⁹, Rosemary E. Pike⁵, Tsuyoshi Terai²², Seitaro Urakawa²³, Hui Zhang²⁴, Haibin Zhao¹¹, Ji-Lin Zhou¹³ ¹ Institute of Astronomy and Astrophysics, Academia Sinica, No. 1 Sec. 4 Roosevelt Rd, Taipei 106319, Taiwan ² Planetary Exploration Research Center, Chiba Institute of Technology, 2-17-1 Tsudanuma, Narashino, Chiba 275-0016, Japan ³ Center for Computational Astrophysics, National Astronomical Observatory of Japan, Osawa 2-21-1, Mitaka, Tokyo 181-8588, Japan ⁴ University of Occupational and Environmental Health, 1-1 Iseigaoka, Yahata, Kitakyushu 807-8555, Japan ⁵ Center for Astrophysics | Harvard & Smithsonian, 60 Garden Street, Cambridge, MA 02138, USA ⁶ School of Physical and Chemical Sciences—Te Kura Matū, University of Canterbury, Private Bag 4800, Christchurch 8140, Aotearoa New Zealand ⁷ Korea Astronomy and Space Science Institute, 776 Daedeok-daero, Yuseong-gu, Daejeon 34055, Republic of Korea ⁸ Herzberg Astronomy and Astrophysics Research Centre, National Research Council of Canada, 5071 West Saanich Road, Victoria, BC V9E 2E7, Canada ⁹ Department of Physics and Astronomy, University of Victoria, Victoria, BC V8W 2Y2, Canada ¹⁰ Department of Physics and Astronomy, University of British Columbia, 6224 Agricultural Road, Vancouver, BC V6T 1Z1, Canada ¹¹ CAS Key Laboratory of Planetary Sciences, Purple Mountain Observatory, Chinese Academy of Sciences, Nanjing 210023, China ¹² School of Astronomy and Space Science, Nanjing University, 163 Xianlin Avenue, Nanjing 210023, China ¹³ Key Laboratory of Modern Astronomy and Astrophysics (Nanjing University), Nanjing 210023, China ¹⁴ Institute of Astronomy, National Central University, 300 Zhongda Rd., Zhongli, Taoyuan City 32001, Taiwan ¹⁵ Department of Physics and Astronomy, University of Pennsylvania, 209 S. 33rd Street, Philadelphia, PA 19125, USA ¹⁶ Campion College and Department of Physics, University of Regina, 3737 Wascana Parkway, Regina, SK S4S 0A2, Canada ¹⁷ Kindai University, Shinkamikosaka 228-3, Higashiosaka, Osaka 577-0813, Japan ¹⁸ Instituto de Astronomía y Ciencias Planetarias, Universidad de Atacama, Copayapu 485, Copiapó, Chile ¹⁹ Inter-University Centre for Astronomy and Astrophysics, Ganeshkhind, Pune 411007, India ²⁰ Kavli Institute for the Physics and Mathematics of the Universe (WPI), 5-1-5 Kashiwanoha, Kashiwa, Chiba 277-8583, Japan ²¹ Department of Planetology, Kobe University, Kobe 657-8501, Japan ²² Subaru Telescope, National Astronomical Observatory of Japan, 650 North A‘ohoku Place, Hilo, HI 96720, USA ²³ Japan Spaceguard Association, Bisei Spaceguard Center, 1716-3 Okura, Bisei, Ibara, Okayama 714-1411, Japan ²⁴ Shanghai Astronomical Observatory, Chinese Academy of Sciences, 80 Nandan Road, Shanghai 200030, China
How to cite: Chen, Y.-T. and the FOSSIL II Team: Formation of the Outer Solar System: An Icy Legacy (FOSSIL II) – Phase II Status and Updated Characterization of the Sedna-like Object 2023 KQ14, EPSC-DPS Joint Meeting 2025, Helsinki, Finland, 7–12 Sep 2025, EPSC-DPS2025-1230, https://doi.org/10.5194/epsc-dps2025-1230, 2025.
