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 a study partner back here on Earth. The Subaru Telescope on the Big Island of Hawaii is deploying its Hyper Suprime-Cam imager to look at the Kuiper Belt along the spacecraft’s trajectory. Its observations show that the Kuiper Belt extends farther than scientists thought.

The observations support the search for Kuiper Belt objects (KBO) for New Horizons to explore next. So far, Subaru has found many smaller bodies out there. However, none of them are along the spacecraft’s trajectory. In a big surprise to the science teams at Subaru, at least two of those objects orbit beyond 50 astronomical units, which is the current assumed “limit” of the Belt.

If observers continue to find more such objects outside that 50 AU “limit”, it means the Kuiper Belt is bigger than everybody thought. Or it could exist in two parts—a sort of inner and outer Kuiper Belt. Scientists already know that the belt is much dustier than expected, thanks to observations taken with the dust counter onboard New Horizons.

Implications of an Expanded or Two-part Kuiper Belt

Beyond simply expanding the limit of the Kuiper Belt, the Subaru observations have profound implications for our understanding of the solar nebula, according to Fumi Yoshida, who led the research for the Subaru observation team. “Looking outside of 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 extend about 50 AU, is very compact. Based on this comparison, we think that the primordial solar nebula, from which the Solar System was born, may have extended further out than the present-day Kuiper Belt,” said Yoshida.

Let’s say the primordial disk was quite large. Then it’s possible that undiscovered planetary bodies clipped the outer edge of the Kuiper Belt. If that happened, then it makes sense to search the outer limits of the current Belt to find such a cut-off object. It’s also possible that perhaps that truncation created a second Kuiper Belt beyond the currently known belt. What it’s like is anybody’s guess, although it’s probably dusty and very likely has at least a few larger objects. So, even if there’s nothing along the New Horizons trajectory, using Subaru to study the distribution of objects it has found will help scientists to understand the evolution of the Solar System.

The Hyper Suprime-Cam at the Subaru Telescope in Hawai'i is part of the search for New Horizons flyby targets. It is outfitted with a special filter to aid in the search. Credit: Subaru Telescope.
The Hyper Suprime-Cam at the Subaru Telescope in Hawai’i is part of the search for New Horizons flyby targets. It has a special filter to aid in the search. Credit: Subaru Telescope.

Searching for KBOs

Subaru Telescope’s has been searching for more KBOs to explore ever since New Horizons flew past Arrokoth in 2019. The idea is to find additional KBOs along the path of flight. The search focused two Hyper Suprime-Cam fields along the spacecraft’s trajectory through the Belt. The New Horizons team spent about 30 half-nights to find more than 240 outer Solar System objects.

The next step was for a Japanese team to analyze images from those observations. However, they used a different method than the mission team did and found seven new outer Solar System objects. The scientists then analyzed the HSC data with a Moving Object Detection System developed by JAXA. Normally it detects near-Earth asteroids and other space debris. Those types of bodies move very fast, compared to more distant ones. So, looking for very dim, faraway, slow-moving objects was a challenge. That’s because the team had to adjust for the speed of the Kuiper Belt objects. Then they applied some updated image analysis to confirm their findings. Scientists now know the orbits of two of the seven new objects and they’ve been assigned provisional designations by the Minor Planet Center (MPC.

Schematic diagram showing the orbits of the two discovered objects (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, which are thought to be a group of icy planetesimals that formed in situ in the early Solar System and are distributed near the ecliptic plane. The gray dots represent outer Solar System objects with a semi-major axis greater than 30 au. These include objects scattered by Neptune, so they extend far out, and many have orbits inclined with respect to the ecliptic plane. The circles and dots in the figure represent their positions on June 1, 2024. Credit: JAXA
 Schematic diagram showing the orbits of the two discovered objects (red: 2020 KJ60, purple: 2020 KK60). The plus symbol represents the Sun; 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. These are thought to be a group of icy planetesimals that formed early in Solar System history. The gray dots represent outer Solar System objects with a semi-major axis greater than 30 au. These include objects scattered by Neptune. They extend far out, and many have orbits inclined with respect to the ecliptic plane. The circles and dots in the figure represent their positions on June 1, 2024. Credit: JAXA

Continuing to Search the Kuiper Belt

The discovery of more KBOs in the outer Solar System (along with New Horizons’ continued dust detection activities) tells scientists that there’s more to the Kuiper Belt than anyone expected. The proof will be in continued Subaru observations to detect 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 will be published in the future, mainly by the North American group,” said Yoshida. “This research, the discovery of sources with the potential to expand the Kuiper Belt region using a method developed in Japan and led by Japanese researchers, serves as a precursor to those publications.”

For More Information

A New Horizon for the Kuiper Belt: Subaru Telescope’s Wide-Field Observations
A Deep Analysis of New Horizons’s KBO Search Images
The PI’s Perspective: Needles in the Cosmic Haystack

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