More evidence that the Kuiper belt is bigger than we thought

As the New Horizons spacecraft continues its epic journey to explore the Kuiper Belt, it has an exploration partner here on Earth. The Subaru Telescope on Hawaii’s Big Island is deploying its Hyper Suprime-Cam to observe the Kuiper Belt along the spacecraft’s trajectory. His observations show that the Kuiper belt extends further than scientists thought.

The observations support the search for Kuiper Belt Objects (KBOs) for New Horizons to explore next. So far, Subaru has found many smaller bodies there. However, none of them are in the trajectory of the spacecraft. Much to the surprise of the science teams at Subaru, at least two of these objects orbit above 50 AU, which is the current assumed “boundary” of the Belt.

If observers continue to find more such objects beyond that 50 AU “boundary,” it means the Kuiper belt is bigger than everyone thought. Or it could exist in two parts – something like an inner and outer Kuiper belt. Scientists now know that the belt is much dustier than expected, thanks to observations made with the dust counter aboard New Horizons.

Consequences of an extended belt or two-part Kuiper belt

Beyond simply expanding the boundary of the Kuiper Belt, Subaru’s observations have profound implications for our understanding of the solar nebula, according to Fumi Yoshida, who led the study for the Subaru Observing Team. “Viewed from outside the Solar System, a typical planetary disk extends about 100 AU from the host star (100 times the distance between the Earth and the Sun), and the Kuiper belt, which is estimated to be about 50 AU, is very compact. Based on this comparison, we believe that the primordial solar nebula from which the solar system was born may have extended further than today’s Kuiper belt,” Yoshida said.

Let’s say the primary disk was quite large. Then it is possible that undiscovered planetary bodies have clipped the outer edge of the Kuiper belt. If this happens, then it makes sense to search the outer boundaries of the current belt to find such a clipped object. It is also possible that this truncation created a second Kuiper belt beyond the currently known belt. It’s anyone’s guess what it is, though it’s probably dusty and very likely has at least a few larger objects. So, even if there is nothing in New Horizons’ trajectory, using Subaru to study the distribution of objects discovered will help scientists understand the evolution of the Solar System.

The Hyper Suprime-Cam at the Subaru Telescope in Hawaii is part of the New Horizons search for flying targets. There is a special filter to help you search. Credit: Subaru Telescope.

Search for KBO

The Subaru Telescope has been searching for more KBOs to study since New Horizons flew past Arrokoth in 2019. The idea is to find additional KBOs along the flyby. The search focused two Hyper Suprime-Cam fields on the spacecraft’s trajectory through the belt. The New Horizons team spent about 30 midnights to discover more than 240 objects outside the solar system.

The next step was for a Japanese team to analyze images from these sightings. However, they used a different method than the mission team and discovered seven new outer solar system objects. The scientists then analyzed the HSC data with a moving object detection system developed by JAXA. It usually detects near-Earth asteroids and other space debris. These types of bodies move very quickly compared to more distant ones. So looking for very dark, distant, slow-moving objects was a challenge. That’s because the team had to adjust to the speed of the Kuiper belt objects. They then applied updated image analysis to confirm their findings. Scientists already know the orbits of two of the seven new objects and have been given provisional designations by the Minor Planet Center (MPC.

Schematic diagram showing the orbits of the two objects discovered (red: 2020 KJ60, purple: 2020 KK60). The plus symbol represents the Sun, and the green lines represent the orbits of Jupiter, Saturn, Uranus, and Neptune from the inside out. The numbers on the vertical and horizontal axes represent the distance from the Sun in astronomical units (au, one au corresponds to the distance between the Sun and the Earth). The black dots represent classical Kuiper belt objects, believed to be a group of icy planetesimals that formed in situ in the early Solar System and are distributed near the ecliptic plane. Gray dots represent outer Solar System objects with a semimajor axis greater than 30 au. These include objects scattered from Neptune, so they extend far and many have orbits inclined with respect to the plane of the ecliptic. The circles and dots in the figure represent their positions on June 1, 2024 Credit: JAXA — Schematic diagram showing the orbits of the two objects discovered (red: 2020 KJ60, purple: 2020 KK60). The plus symbol represents the Sun; the green lines are the orbits of Jupiter, Saturn, Uranus and Neptune. The numbers on the vertical and horizontal axes represent the distance from the Sun in astronomical units. (1 AU corresponds to the distance between the Sun and the Earth). The black dots represent classical Kuiper belt objects. They are believed to be a group of icy planetesimals that formed early in the history of the Solar System. Gray dots represent outer Solar System objects with a semimajor axis greater than 30 au. These include objects scattered by Neptune. They extend far and many have orbits inclined to the plane of the ecliptic. The circles and dots in the figure represent their positions on June 1, 2024 Credit: JAXA

Continuing the search for the Kuiper Belt

The discovery of more KBOs in the outer Solar System (along with New Horizons’ ongoing dust detection activities) tells scientists that there is more to the Kuiper Belt than anyone expected. The proof will be in Subaru’s continued observations to discover and confirm more objects “out there”.

“The mission team’s search for Kuiper belt objects using Hyper Suprime-Cam continues to this day, and a series of papers, mainly from the North American group, will be published in the future,” Yoshida said. “This study, the discovery of sources with the potential to extend the Kuiper Belt region using a method developed in Japan and led by Japanese researchers, serves as a precursor to these publications.”

For more information

A New Horizon for the Kuiper Belt: Wide Range Observations by the Subaru Telescope
In-depth analysis of New Horizons’ KBO search images
The PI’s Point of View: Needles in the Cosmic Haystack