DATE2026.09.11 #Press Releases

The asteroid Ryugu is a remnant of an early-generation celestial body in the solar system

Disclaimer: machine translated by DeepL which may contain errors.

— The celestial body that formed the basis of Ryugu was born more than 4.565 billion years ago —

Summary

Associate Professor Noriyuki Kawasaki and Emeritus Professor Hisayoshi Yurimoto of the Graduate School of Science at Hokkaido University; Associate Professor Naoya Sakamoto of the University’s Institute of Engineering Innovation;Associate Professor Wataru Fujitani of the Division of Basic Natural Sciences at the Institute of Academic Research, Ibaraki University; Researcher Sota Arakawa of the Japan Agency for Marine-Earth Science and Technology (JAMSTEC); Professor Tetsuya Yokoyama of the Faculty of Science at Tokyo University of Science; and Professor Shogo Tachibana of the UTokyo Organization for Planetary and Space Science (UTOPS) at the Graduate School of Science, The University of Tokyo, have investigated the age and temperature at which the carbonate minerals contained in samples collected from the asteroid “Ryugu” by the Japan Aerospace Exploration Agency’s (JAXA) asteroid probe “Hayabusa2” were formed,revealing that the celestial body from which “Ryugu” originated formed more than 4.565 billion years ago.

The asteroid “Ryugu” is a carbonaceous asteroid rich in water-bearing minerals and organic matter, and is believed to preserve material from the time of the solar system’s formation.Previous research had shown that the main constituents of “Ryugu” were formed through the reaction between an aqueous solution—produced by the melting of ice within the progenitor body—and rock. However, it had not yet been determined when these constituents, or the progenitor body itself, were formed.

In this study, measurements using an isotope microscope at Hokkaido University revealed the age and temperature at which the carbonate minerals contained in the “Ryugu” samples formed.Furthermore, we compared the obtained ages and temperatures with numerical simulations that reproduce temperature changes within the celestial body. As a result, we found that the celestial body from which “Ryugu” originated formed more than 4.565 billion years ago—that is, within approximately 2 million years of the start of the solar system’s formation.Since the formation ages of many other known carbonaceous bodies are not as old as 4.565 billion years, this body is thought to have been an early-generation carbonaceous planetesimal that formed before many other carbonaceous bodies.The present-day asteroid “Ryugu” is a body formed when this object was shattered and its fragments coalesced—in other words, it is the remnant of an early-generation carbonaceous planetesimal.These research findings suggest that carbonaceous planetesimals may have formed across multiple generations during the first few million years of the solar system’s formation.

These research findings were published online in the journal *Science* on Friday, September 11, 2026 (Japan Standard Time).

The celestial body that gave rise to “Ryugu” formed more than 4.565 billion years ago. It is believed that this body subsequently shattered, and its fragments coalesced to form the current asteroid “Ryugu.”

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