The Kuiper Belt was named in honor of Dutch-American astronomer Gerard Kuiper, who postulated a reservoir of icy bodies beyond Neptune. The first Kuiper Belt object was discovered in 1992. We now know of more than a thousand objects there, and it's estimated it's home to more than 100,000 asteroids and comets there over 62 miles (100 km) across. Credit: JHUAPL

More evidence that the Kuiper Belt is bigger than we thought

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As the New Horizons spacecraft continues its epic journey to explore the Kuiper Belt, it has a study partner here on Earth. The Subaru Telescope on Hawaii’s Big Island is deploying its Hyper Suprime Cam imager to view the Kuiper Belt along the spacecraft’s path. The observations show that the Kuiper Belt extends farther than scientists thought.

The observations support the search for Kuiper Belt objects (KBOs) that New Horizons will explore next. So far, Subaru has found many smaller bodies. None, however, are in the spacecraft’s orbit. To the great surprise of Subaru’s science teams, at least two of those objects orbit farther than 50 astronomical units, which is the Belt’s current supposed “limit.”

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

Implications of an extended or bipartite Kuiper belt

According to Fumi Yoshida, who led the research for the Subaru observation team, the Subaru observations not only extended the boundary of the Kuiper Belt, but also have major implications for our understanding of the solar nebula. “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 extend about 50 AU, is very compact. Based on this comparison, we believe that the primordial solar nebula from which the Solar System formed may have extended farther than the present-day Kuiper Belt,” Yoshida said.

Suppose the primordial disk was quite large. Then it’s possible that undiscovered planetary bodies have clipped the outer edge of the Kuiper Belt. If that’s what happened, it would make sense to search the outer limits of the current belt for such a clipped object. It’s also possible that that clip created a second Kuiper Belt outside the currently known belt. What that looks like is anyone’s guess, though it’s probably dusty and very likely has at least a few larger objects. So even if there’s nothing along New Horizons’ orbit, using Subaru to study the distribution of found objects will help scientists understand the evolution of the solar system.

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

Search for KBOs

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

The next step was for a Japanese team to analyze images from those observations. However, they used a different method than the mission team and found seven new objects in the outer solar system. The scientists then analyzed the HSC data using a Moving Object Detection System developed by JAXA. It normally detects asteroids close to Earth and other space debris. Such bodies move very quickly compared to objects that are farther away. So it was a challenge to search for very faint, distant, slow-moving objects. That’s because the team had to adjust the speed of the Kuiper Belt objects. They then applied an updated image analysis to confirm their findings. Scientists now know the orbits of two of the seven new objects, and they have been given preliminary 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 sign 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 is equal to the distance between the Sun and the Earth). The black dots represent classical Kuiper Belt objects, thought to be a group of icy planetesimals that formed in situ in the early Solar System and are located near the ecliptic plane. The grey dots represent outer Solar System objects with a semimajor axis greater than 30 au. These include objects scattered by Neptune, so they extend far out and many have orbits inclined to the ecliptic plane. The circles and dots in the image 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 sign 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 is the distance between the Sun and the Earth.) The black dots represent classical Kuiper Belt objects, a group of icy planetesimals thought to have formed early in the history of the Solar System. The gray dots represent objects in the outer Solar System with a semimajor axis greater than 30 AU. These include objects scattered by Neptune. They are widely dispersed and many have orbits inclined to the ecliptic plane. The circles and dots in the image represent their positions on June 1, 2024. Credit: JAXA

Continue searching in the Kuiper Belt

The discovery of more KBOs in the outer solar system (along with New Horizons’ continued dust-detection efforts) tells scientists that there is more to the Kuiper Belt than anyone expected. The evidence will come from 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,” Yoshida said. “This research, 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 those papers.”

For more information

A New Horizon for the Kuiper Belt: Wide-field Observations from the Subaru Telescope
An in-depth analysis of New Horizons’s KBO search images
The PI Perspective: Needles in a Cosmic Haystack

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