Classical Kuiper belt object

A **classical Kuiper belt object**, also called a **cubewano,** is a Kuiper belt object (KBO) that has an orbit that is beyond Neptune's, does not have a high eccentricity, and does not have an orbital resonance with Neptune. Their orbits have semi-major axes between 40 and 50 astronomical units. Unlike Pluto, they do not cross Neptune's orbit. They have orbits with low eccentricity and mostly low inclination, like the major planets.

The name "cubewano" comes from the first trans-Neptunian object (TNO) found after Pluto and Charon: **15760 Albion**. Before January 2018, it was only called (15760) . Objects similar to it that were found later were often called "QB1-os", or "cubewanos". However, scientists mostly use the word "classical" instead of "cubewano".

Cubewanos include:
- 15760 Albion (also called  and source of the word 'Cubewano')
- 136472 Makemake, the largest known cubewano and a dwarf planet
- 50000 Quaoar and 20000 Varuna, both thought to be the largest TNO when they were found.
- 19521 Chaos, 58534 Logos, 53311 Deucalion, 66652 Borasisi, 88611 Teharonhiawako,, , 55565 Aya, 55637 Uni, 486958 Arrokoth

136108 Haumea was first marked as a cubewano by the Minor Planet Center in 2006, but they found out it had an orbital resonance with Neptune.

## 'Hot' and 'cold' cubewanos

There are two types of classical Kuiper Belt objects: those that have calm, almost circular orbits farther out ('cold' cubewanos), and those that are closer to the Sun and have more eccentric orbits ('hot' cubewanos).

Most cubewanos are between the 2:3 orbital resonance with Neptune (the plutinos' orbit) and the 1:2 resonance. For example, 50000 Quaoar has an orbit that is almost a circle and close to the ecliptic. Plutinos, however, have more eccentric orbits, bringing some of them closer to the Sun than Neptune.

Most cubewanos, _the cold population_, have low inclinations (< 5°) and almost circular orbits that are between 42 and 47 AU. A smaller population (_the_ _hot population_) has more inclined and eccentric orbits. The words 'hot' and 'cold' have nothing to do with the actual temperatures, but actually mean orbits of the objects. They use an analogy comparing orbits to gas molecules, which move faster as they heat up.

The Deep Ecliptic Survey found the inclinations of both populations. The cold population's inclinations were centered around 4.6° and the hot population's inclinations were larger than 30° (_Halo_).

### Distribution

Most cubewanos (more than two-thirds) have inclinations less than 5° and eccentricities less than 0.1. They mostly orbit in the middle of the Kuiper Belt. Smaller objects further out are more likely to get caught in an orbital resonance and stop being cubewanos.

Cubewanos make a clear 'belt' outside Neptune's orbit, but the plutinos approach, or even cross Neptune's orbit. When scientists compare orbital inclinations, 'hot' cubewanos can be easily found by their higher inclinations. This high inclination of 'hot' cubewanos has not been explained.

## Cold and hot populations: physical characteristics

The two populations are not just different in orbits, they are different in how they look.

The difference in colour between the red cold population, such as 486958 Arrokoth, and more heterogeneous hot population was observed as early as in 2002. Since 2002, scientists have seen that cold population cubewanos are more red than the bluish-colored hot population. Newer studies confirm this, saying that it is true that cold population bodies are redder than the hot population.

Another difference between the low-inclination (cold) and high-inclination (hot) classical objects is the number of binary systems (two objects orbiting each other and the Sun at the same time). Low-inclination classical objects are more likely to be binary systems, and the brightness of both objects in the binary system are close to being the same. On the other hand, binary systems are rarer in lower-inclination cubewanos, and the brightness difference is higher. This observation, and the differences in colour, show that the cubewanos belong to at least two different populations, with different physical properties and history.

## Families

The first known collisional family—a group of objects that might be leftovers from when a larger object broke apart—in the classical Kuiper belt is the Haumea family. It includes Haumea, its moons,  and seven smaller bodies. The objects have similar orbits and physical characteristics. They have large amounts of water ice (H<sub>2</sub>O) and have colors between red (cold classical objects) and blue (hot classical objects) Some other collisional families could be in the classical Kuiper belt.

## Exploration

Only one classical Kuiper belt object has ever been visited by spacecraft. Both Voyager spacecraft passed through the area before the Kuiper belt was found, but New Horizons was the first mission to visit a classical KBO. After its explored the Pluto system in 2015, the NASA spacecraft traveled to the small cubewano 486958 Arrokoth. It arrived there on January 1, 2019.

## List

, there are about 870 objects that are between 40 and 48 astronomical units. Some of them are in the list below.

- 15760 Albion
- 20000 Varuna
- 307261 Máni
- 120347 Salacia
- 145452 Ritona
- 148780 Altjira
- 174567 Varda
- 19521 Chaos
- 486958 Arrokoth
- 50000 Quaoar
- 53311 Deucalion
- 55565 Aya
- 55637 Uni
- 58534 Logos
- 66652 Borasisi
- 79360 Sila–Nunam
- 88611 Teharonhiawako
- 90568 Goibniu

## Related pages

- Lists of astronomical objects