Outer Solar System More Populated Than Previously Thought, Study Finds

Observations using the Subaru Telescope’s Ultra-Wide Fixed-Focus Camera have revealed that there may be a population of small bodies further out in the Kuiper Belt just waiting to be discovered.

These results, important for understanding the formation of the solar system, were obtained thanks to an international collaboration between the Subaru telescope and the New Horizons spacecraft traveling through the outer solar system.

NASA’s New Horizons spacecraft was launched in 2006 with the primary mission of closely observing the surfaces of the outer bodies of the solar system for the first time in human history; it successfully completed a flyby of the Pluto system in 2015, and in 2019 it flew by one of the Kuiper Belt objects, (486958) Arrokoth.

Five spacecraft have flown to the outer solar system (including New Horizons), but New Horizons is the only spacecraft to have flown through the Kuiper Belt while observing Kuiper Belt objects.

When we observe Kuiper Belt objects from Earth, we can only observe them at small solar phase angles (the angle between the Sun, the object, and the observer). In contrast, when we observe a Kuiper Belt object from a spacecraft in the Kuiper Belt, the same object can be observed at different phase angles, and its reflection characteristics can be used to estimate the surface properties of the object. This is something that only New Horizons can do.

However, the spacecraft’s camera has a narrow field of view and cannot detect Kuiper Belt objects on its own. That’s where the Subaru telescope comes in. The Subaru telescope uses its wide-field camera to find many Kuiper Belt objects and then narrows down the list of objects that the spacecraft can fly by and observe. This collaboration between New Horizons and the Subaru telescope began in 2004.

For observations made between 2004 and 2005 with the Subaru Telescope Prime Focus Camera (Suprime-Cam), due to the orbital relationship between Pluto and the spacecraft, an area near the center of the Milky Way galaxy was considered in the background of the Kuiper Belt Object search area.

Although it was extremely difficult to search for solar system objects with many stars in the background, the search team was able to find 24 Kuiper Belt objects.

Unfortunately, the Kuiper Belt objects discovered so far during this observation require too much fuel for the spacecraft to fly by, but new objects at great distances could be within range of New Horizons’ available fuel. In 2020, more extensive observations began with Hyper Suprime-Cam (HSC) on the Subaru telescope, and by 2023, 239 Kuiper Belt objects had been discovered.

“The most exciting part of the HSC observations was the discovery of 11 objects at distances beyond the known Kuiper Belt,” says team member Dr. Fumi Yoshida of the University of Occupational and Environmental Health Sciences and the Planetary Exploration Research Center at the Chiba Institute of Technology.

Many of the objects discovered by the HSC are located at distances of 30 to 55 astronomical units (AU) from the Sun (1 AU is the distance between the Sun and Earth) and are thought to lie in the known Kuiper belt.

In contrast, the team did not expect what appears to be a cluster of objects in the 70–90 AU region and a valley between 55 AU and 70 AU (where only a small number of objects are distributed, see figure below). Such a valley had not been reported in other observations.

There may be a new population of Kuiper Belt objects at 70–90 AU. “If confirmed, this would be a major discovery. The primordial solar nebula was much larger than previously thought, and this could have implications for studying the planet formation process in our solar system,” says Dr. Yoshida.

Dr Alan Stern, Principal Investigator of the New Horizons mission, said: “This is a groundbreaking discovery that reveals something unexpected, new and exciting in the most distant regions of the Solar System. This discovery would likely not have been possible without the world-class capabilities of the Subaru Observatory.”

To determine the exact orbits of the objects discovered in this study, the research team is continuing their observations with the HSC. “I think that discovering distant objects and determining their orbital distribution is important for understanding the history of the formation of the solar system, comparing it to exoplanetary systems, and understanding the formation of universal planets,” says Dr. Yoshida about the significance of this study.

New Horizons is currently traveling further out, to 60 AU from the Sun. There must be many more distant objects that we have not yet discovered. The research team is eager to see what the Subaru telescope and the New Horizons spacecraft will discover beyond the Kuiper Belt.

These results will be published in two articles in the Journal of Planetary Sciences. They are currently available on the arXiv preprint server.