Bennu Asteroid Reveals Mixed Origins in Early Solar System, Challenging Formation Theories

Samples returned from NASA's OSIRIS-REx mission to asteroid Bennu indicate the asteroid is a "hybrid" of materials originating from both the inner and outer Solar System, a finding that challenges prior beliefs that such asteroids formed exclusively in outer regions[4][8][9]. This discovery suggests that Bennu, along with asteroid Ryugu and CI chondrites, formed in a transitional zone approximately 4.5 billion years ago[8][10].

This finding changes the understanding of how celestial bodies formed in the early Solar System, offering new clues about the raw materials that built terrestrial planets like Earth[10][21]. The detailed chemical composition of Bennu provides a pristine record of the primordial elements available during the Solar System's infancy[8][10].

What the records show

Researchers at ETH Zurich conducted geochemical analyses on samples collected by NASA's OSIRIS-REx mission from asteroid Bennu[8][9]. These analyses focused on iron, titanium, and chromium isotopes, revealing that Bennu's composition blends characteristics of both inner and outer Solar System materials[4][8][9][20][21]. Professor Maria Schönbächler at ETH Zurich stated, "Bennu is a hybrid: the material does not clearly match either the inner or the outer solar system"[4][9][13]. The studies propose Bennu's parent body emerged in a transition zone near the water-ice line, where temperatures allowed water to freeze[8][9][12][18]. Scientists attribute this material mixing to Jupiter's early formation, which acted as a "bridge pillar" or "filter" within the protoplanetary disk[4][11][12][13][17][19][21]. This filtering mechanism blocked most coarse material but allowed finer dust particles from various regions to mix evenly near the water-ice line[4][11][12][13][15][17][18][19][20][21]. This explains why Bennu, asteroid Ryugu, and CI chondrites consist mainly of tiny particles and share a similar isotopic fingerprint, chemically resembling the early Sun and rocky inner planets[4][10][15][16][17][20][21]. Bennu's material is considered "very primordial," dating back about 4.5 billion years, retaining a record of the early Solar System's building blocks[8][10].

The timeline

The OSIRIS-REx spacecraft launched on September 8, 2016, with its primary objective to collect samples from asteroid Bennu[6]. In October 2020, the spacecraft successfully scooped up rock and dust from Bennu[3]. These precious samples made their journey back to Earth, parachuting down into the Utah desert on September 24, 2023[3]. Although the sample return portion concluded, the OSIRIS-REx mission officially ended on September 30, 2023, with the spacecraft being redesignated OSIRIS-APEX for a new mission to asteroid Apophis[3][14]. Portions of the 4.3 ounces of returned material have since been distributed to various research institutions, including the University of Arizona, for further study[3]. The new chemical analyses of these samples were published in late September and early October 2026, leading to the conclusions about Bennu's surprising origin[17][12][18][20][21][11][13].

What changed and for whom

Previously, many scientists believed that asteroids like Bennu, which are rich in volatile materials, formed exclusively in the outer regions of the Solar System[4][8][9]. The new analyses of the OSIRIS-REx samples challenge this long-held assumption by demonstrating that Bennu's parent body incorporated materials from both the inner and outer Solar System[4][9][13]. This revised understanding impacts how scientists model the formation and evolution of primitive asteroids and the distribution of fundamental materials within the early protoplanetary disk, fundamentally altering perspectives on Solar System origins[3][4][10].

What the subject has said publicly

Professor Maria Schönbächler of ETH Zurich, a lead researcher in the analysis, publicly stated the significance of the findings, explaining, "Bennu is a hybrid: the material does not clearly match either the inner or the outer solar system"[4][9][13]. Schönbächler further highlighted Bennu's importance, noting, "Bennu may offer our best glimpse of the original mix of chemical elements from which the terrestrial planets were ultimately built"[10]. The University of Arizona, which led the OSIRIS-REx mission, described its impact as having "revolutionized what we know – or thought we knew – about asteroids and the history of our solar system"[3].

What we could not confirm

PrimaryBrief could not confirm the precise chemical mechanisms that facilitated the even mixing of fine dust from disparate regions near the water-ice line. Specific details on the exact proportions of inner versus outer Solar System material incorporated into Bennu's overall composition remain unconfirmed. Whether Jupiter's filtering mechanism was a singular event or an ongoing process over an extended period in the early Solar System is also unconfirmed.