Recent observations using the ultra-widefield prime focus camera of the Subaru Telescope have shown that there may be a population of small astronomical objects further out in the Kuiper Belt still waiting to be discovered.
According to the experts, the findings are essential for a better understanding of the formation of the solar system.
The research was made possible through an international collaboration between the Subaru Telescope and the New Horizons spacecraft traveling through the outer solar system .
New Horizons flew through the Kuiper Belt
NASA's New Horizons spacecraft was launched in 2006 in order to observe the surfaces of outer solar system bodies up close for the first time in our history. The spacecraft successfully completed a flyby of the Pluto system (2015), and later on (2019) it made a flyby of one of the Kuiper Belt objects, (486958) Arrokoth.
Although there were four other spacecraft which have flown to the outer solar system, New Horizons is the only one that has flown through the Kuiper Belt.
Viewing objects in the Kuiper Belt
When examining Kuiper Belt objects from the ground, we can only observe them at small solar phase angles (the angle between the sun, the object, and the observer).
However, when looking at an object in the outer solar system from a spacecraft in the Kuiper Belt, these objects can be observed at different phase angles and their reflection characteristics can be utilized to estimate their surface properties.
Yet, the camera on the New Horizon spacecraft has a narrow field-of-view and cannot find Kuiper Belt objects on its own.
Fortunately, the Subaru Telescope can use its wide-field camera to discover a variety of Kuiper Belt objects and then narrow down the collection of objects which the spacecraft can fly by and examine.
New Horizons and the Subaru telescope
The collaboration between the New Horizons spacecraft and the Subaru Telescope was initiated in 2004.
During the 2004–2005 observations with the Subaru Telescope’s prime focus camera (Suprime-Cam), the orbital alignment between Pluto and the spacecraft caused a portion of the Milky Way ’s center to appear in the background of the search area for Kuiper Belt objects.
While it was highly difficult to search for solar system objects with multiple background stars, the scientists were able to localize 24 Kuiper Belt objects.
Unfortunately, the Kuiper belt objects discovered until now require too much fuel for New Horizons to flyby, but new ones at greater distances might fall within the available fuel reach of the spacecraft.
In 2020, deeper observations with the Hyper Suprime-Cam (HSC) on the Subaru Telescope began, and, by 2023, 239 Kuiper Belt objects were discovered.
“The most exciting part of the HSC observations was the discovery of 11 objects at distances beyond the known Kuiper Belt,” said team member Fumi Yoshida, a scientist at the University of Occupational and Environmental Health Sciences and the Planetary Exploration Research Center at the Chiba Institute of Technology .
Unexpected discovery of mysterious objects
A large number of objects discovered with HSC are located at distances ranging from 30 to 55 astronomical units (au) from the Sun (with one au corresponding to the distance between the Sun and Earth) and are thought to belong to the known Kuiper Belt.
However, the experts were not expecting to discover what looks to be a cluster of objects in the 70–90 au region and a valley between 55 and 70 au, where only a small number of objects are distributed.
Is there a new population of Kuiper Belt objects located at 70–90 au? "If this is confirmed, it would be a major discovery. The primordial solar nebula was much larger than previously thought, and this may have implications for studying the planet formation process in our solar system,” Yoshida explained.
“This is a groundbreaking discovery revealing something unexpected, new, and exciting in the distant reaches of the solar system; this discovery probably would not have been possible without the world-class capabilities of the Subaru observatory,” added New Horizons mission principal investigator Alan Stern.
In order to determine the precise orbits of these objects, the scientists are continuing observations with HSC.
“I think the discovery of distant objects and the determination of their orbital distribution are important as a stepping stone to understanding the formation history of the solar system, comparing it with exoplanetary systems, and understanding universal planet formation ,” Yoshida said.
New Horizons is currently traveling further out, at approximately 60 au from the Sun, hoping to discover many more distant objects.
The findings will be published in two articles in the Planetary Science Journal and are currently available on the arXiv preprint server.
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