Second Kuiper Belt? Planets Beyond Neptune?

Astronomer used to think that the Kuiper Belt was a few hundred times larger than the Asteroid belt. They thought it was a donut shaped structure that started at Neptune and extended out 2 billion miles. Recent observation suggests it extends out at least 5 billion miles. This is based upon finding about 2000 objects, which include Pluto and its moons.

In other observed solar systems, similar belts stretch outward across hundreds of AU.

In 2023, about a dozen large objects that lie beyond 60 AU—nearly as far from Pluto as Pluto is from the Sun were discovered.

New Horizons is at 57 AU continues to streak beyond the edge of the known Kuiper belt. A dust counter on New Horizons has run continuously during the mission. Dust is a telltale sign of colliding planetary bodies, and so the New Horizons team expected the amount of dust to fall off steeply after the probe left the Kuiper belt. The number of impacts is not declining. The simplest explanation for that is that there is more stuff out there that we haven’t detected.

In 2024, data from the New Horizons space probe and the Japanese Subaru Telescope in Hawaii, have discovered a number KBOs (kuiper belt objects) far beyond the expected outer edge of the Kuiper Belt. This outer edge (where the density of objects starts to decline) was thought to be at about 50 AU, but new evidence suggests the belt may extend to 80 AU, or farther.

Logically, if we are seeing similar belts at other solar system stretch outwards hundreds of AU, then astronomers will likely find the evidence that there are more Kuiper belts or that the one we have stretches continuously out hundreds of AU. The Kuiper belt continues or there are multiple belts with more spareness and more gaps.

Unpublished results from a recent survey of the outer Solar System using the 4-meter Víctor M. Blanco Telescope in Chile. It surveyed a different slice of sky and turned up only one object beyond 50 AU. Other surveys have also come up dry. “Why are we not seeing these things?” Bernardinelli asks. “Did everyone get unlucky? It’s possible, but it’s hard.”

838 of the Kuiper Belt objects were found from 2013-2017 by the Outer Solar System Origins Survey.

There were estimates that the amount of material in the Kuiper belt is 2% of the mass of the Earth but that it started it at 10 times the mass of the Earth. However, there was a computational analysis of the movement of visible objects that suggests that there could be a larger than Earth sized object 250-500 AU away from the sun. Japanese researchers found several objects with unusual orbits and calculated that the unusual orbits had to be caused by an object 1.5 to three times bigger than the earth out ten to twenty times farther from the Sun than Neptune.

The results of the KBP scenario support the existence of a yet-undiscovered planet in the far outer solar system. Furthermore, this scenario also predicts the existence of new TNO populations located beyond 150 au generated by the KBP’s perturbations that can serve as observationally testable signatures of the existence of this planet.

Kuiper belt objects are faint and slow moving. They are a pain in the butt to find. Targets for New Horizons are even harder to see because the spacecraft, as seen from Earth, appears to be flying straight toward the bright center of the Milky Way.

Artificial intelligence makes the screening much less painful. “We went from a week of agony for an army of people to 6 hours of vetting.”

The search results from 2020 and 2021 observations have detected an increase in density of objects at distances d~80 au, compared to the nearly empty region beyond the outer edge of the Kuiper Belt at d~50.

We have only detected about 0.01% of the large objects in the Kuiper belt. Our estimates and guesses could be very wrong.