How did Pluto form (is it an asteroid or a planet?)

Written by Edo Loenen and Marco Westerkamp.
Is it an asteroid or a planet?

Pluto was discovered in 1930 by Clyde W. Tombaugh, who used a 33-cm photographic telescope at Lowell Obersvatory in Flagstaff, Arizona. The search for Pluto at Flagstaff had been initiated by the founder of the obervatory, Percival Lowell, who had estimated the ninth planet's position on the basis off its presumed gravitational pull on Uranus and Neptune. Scientist now know, however, that the mass of Pluto is insufficient to affect the orbits of Uranus and Neptune. The fact that Pluto was found relatively close to the position that had been predicted by Lowell was due to luck. After the discovery of Pluto, it was quickly determined that Pluto was too small to account for the discrepancies in the orbits of the other planets. The search for Planet X continued but nothing is found yet.
To answer the question about the formation of Pluto , it is necessary to know what Pluto is. Because the formation of a planet is different from the formation of, for example, an astroid.
Therefore the following subquestions must be answered:

What is a planet?
Officially Pluto is a planet. The IAU (International Astronomical Union) declared in a pressrelease that: "No proposal to change the status of Pluto as the ninth planet in the solar system has been made".
We don't know the exact definition of a planet and the reason why the IAU made it's decision aren't known by us.
In what way does Pluto differ from the other eight planets?
• Inclination of it's orbit compared to the ecliptic is 17.148°
• Large orbital eccentricity: 0.248
    Earth's eccentricity: 0.0167
• Composition:
• Pluto is composed of:
    • core of hydrated rock (70% of mass)
    • mantle of water ice
    • atmosphere containing methane ice (and possibly: N2, CO, CO2)
This is very different from the other outerplanets, because they are mainly composed of gas.
Therefore Pluto's density is larger than the other outerplanets.
• Pluto has a high albedo: ±0.5
    Extraordinary is that it is irregular, Pluto has the largest global-scale contrast in the solar system. Which indicates that the planet is active.
• Charon (Pluto's satellite) is extraordinary large compared to Pluto:
    radius Pluto : radius Charon
        1 : 0.5
    in comparison with:
        radius Earth : radius Moon
            1 : 0.3
        radius Mars : radius Phobos
            1 : 0.003

Because of all the differences between Pluto and the other eight planets, it isn't hard to believe that Pluto belongs to an other group of objects. Pluto therefore might be formed in a different way. Because the size and composition of Pluto resemble the sizes and composition of Kuiper Belt Objects, science nowadays believes that Pluto might be formed as a Kuiper Belt Object.
Full article

What are Kuiper Belt Objects?
The Kuiper Belt is a large population (over 70,000) of small bodies orbiting the sun beyond Pluto.
These Kuiper Belt Objects (or trans-Neptunians) are mostly confined within a few degrees of the ecliptic. This is why The Kuiper Belt is called Belt.

There are three types of KBO's: Plutino's, Classical and Scattered KBO's.
• Plutino's
Plutino's are KBO's who revolve arround the sun in an orbit which has a 3:2 resonance with Neptune. This is the same resonance Pluto has. This is why the are called Plutino's (little Plutos).
• Classical KBO's
CKBO's do not orbit in the 3:2 resonance with Neptune.
They are called "classical" because their orbits have small eccentricities, as is expected from bodies formed by quiet agglomoration in the early solar system.
• Scattered KBO's
Scattered KBO's are KBO's which possess large, eccentric, inclined orbits that have perihelion distances near 35 AU.
SKBO's are hard to detect due to theire large distance.
The SKBO's form a fat doughnut around the Classical KBO's and the Plutino's, extending to large distances.

It is likely that the Kuiper Belt Objects are extremely primitive debris from the formation of the solar system. The inner, dense parts of the proto-planetary disc condensed into the major planets, probably within a few millions to tens of millions of years. The outer parts were less dense, and accretion progressed slowly. Evidently, a lots of small objects were formed.
As far as we can tell little is known about the exact conditions of the formation of KBO's.We weren't able to find information about this subject.
Full article

How did Pluto end up in it's current orbit?
There are several models created to explain the orbit of Pluto:
    • Dormand & Woolfson - 1977
        Collision between major planets
        Pluto (satellite of one of those planets) ejected at collision
        Interacted with Triton (One of Neptune's satellite), reversing it's orbit.
    • Harrington & Van Flandern - 1979
        Triton and Pluto natural satellites of Neptune
        Object of mass 5 Mearth passed through system
        Orbit Triton reversed, Pluto ejected
    • Farinella et. al. - 1979
        Pluto natural satellite of Neptune
        Triton captured from heliocentric orbit
        Interaction between Pluto and Triton
        Orbit Triton reversed, Pluto ejected
        Charon formed due to tidal forces
    • Dormand & Woolfson - 1980
        similar to Farinella et. al. but more precise
None of these models proved to be true.

The current model is one were Pluto was a natural satellite of Neptune and Triton was in a heliocentric orbit.
Their orbits were changed by a collision. This collision also created Charon.

The mechanism for producing Pluto in its present orbit accompanied by Charon and also Triton as a retrograde satellite cannot be uniquely defined by modelling. Nevertheless, the collision-model with Pluto as a natural satellite of Neptune and Triton as a body originally in a heliocentric orbit is strongly indicated.
Full article

In order to answer our subquestion, we have to answer two questions :
What is Pluto?:
Despite of all the differences between Pluto and the other planets, Pluto officially is a planet. However Pluto probably wasn't always a planet. Current models indicate that Pluto was formed as a KBO, became a satellite of Neptune, collided with Triton and then ended up in it's current orbit and became one of the nine planets.
How did Pluto form?:
Pluto didn't form exactly like the other planets. Because of the similarities between Pluto and KBO's it is most likely that Pluto was formed the way KBO's were formed. As far as we know little is known about the formation of KBO's.


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