The purpose of this article is to list known or theoretical celestial bodies that are very similar to a planet, but don't fit within IAU's definition of a planet.
Definition of a planet[]
The IAU officially defined a planet as a celestial body that respects the following rules:
- Orbits a star
- Has a rounded shape
- Has cleared its neighborhood
- Is not big enough to sustain thermonuclear reactions.
This definition made Pluto be no longer considered a planet and was contested both by scientists and civilians.
The IAU definition of a planet is still incomplete. Rocky planets have Geographic features (for example mountains or craters). It is not specified the accepted error from an ellipsoidal shape. Also, it is not said if a planet-like objects orbiting Brown Dwarfs or Black Holes could be considered planets. Also, the definition says nothing about Rogue Planets.
Note: Many objects listed in this article cannot be named with current accepted IAU terminology. However, scientists in their paperwork called them planets. For example, the New Horizons team often used the term planet when talking about Pluto, to save time and space. Scientists, talking about celestial objects orbiting red dwarfs, called them planets directly because they needed a term to defined them. The same happens when free-floating rogue planets are discovered. For this reason, in this article, planet-like objects will be directly called planets.
Not orbiting a star[]
The most known planets not orbiting a star are:
- Rogue Planets
- Planets orbiting Brown Dwarfs, hypothetical Black Dwarfs, Black Holes or Quasars
- A Hyperbolic Planet, passing through a solar system.
A special kind, not discovered yet, is of planets orbiting Hot Planets. The hosting celestial body, even if not sustaining thermonuclear reactions, might be hot enough behave like a small star for millennia.
Also, a planet located in a Globular Cluster can chaotically move between the many stars passing within the cluster without leaving the system, hitting a star or being captured into orbit.
Not having rounded shape[]
No celestial body is spherical. Most planets are ellipsoidal, tilted, as a result of their rotation. Even stars are partially tilted. In case of rocky or icy planets, there are many Geographic features (like mountains or craters). The following examples show that a few celestial objects might be or not considered planets.
- A nearly spherical asteroid with a radius of only 50 km can be produced from molten lava ejected when two larger bodies collide. The asteroid can then be moved to a far orbit, where it can have a 'clear neighborhood orbit'.
- A celestial body spinning very fast, like Haumea, might have a highly tilted shape.
- A celestial body the size of a planet can support very high mountains or very deep canyons if its core and mantle are solid. In the Solar System, the best example is Iapetus. If rocks are hard enough, similar structures can exist on a planet the size of Moon.
Not Clearing Neighborhood[]
This is the most common aspect that bans a planet-like object to be considered planet. As one can see, the Universe can host very unusual planet-like celestial bodies.
Trojan Planet[]
Many planets in the Solar System have Trojans (asteroids located in the Lagrangean points L4 and L5 of their orbits). Trojans share the same rotation period and nearly the same orbit with their shepherd planet.
Giant Planet & Trojan Model: Theory says that Trojans can have up to 1/20 the mass of their shepherd planet and that their orbits are stable for relatively high periods of time. This clearly proves that Jupiter can host a Trojan the size of Earth or even bigger.
Many stars host planets more massive then Jupiter. So, in theory, they can have Trojan planets up to the size of Uranus.
Binary Star & Trojan Model: Many solar systems are binary. In certain situations, binary systems allow for Trojans to exist. In this case, the system must be made of one very big and one very small star and the small star must have a nearly circular orbit.
Binary objects orbit each other around a barycenter. If the barycenter is too far away from the biggest star, Trojans lack stable orbits.
Black Hole & Trojan Model: This requires a massive enough black hole to be in the center of a system, with a star orbiting it. A Trojan planet can be located in the L4 or L5 point. The planet will actually orbit the black hole, following the same orbit with the star. However, light and heat will come from the star.
Co-Orbitals[]
Saturn's moons Janus & Epimetheus periodically switch orbits one with another. When Janus is closer to Saturn, it moves faster, until it catches Epimetheus from behind. Then, the two moons change positions and Epimetheus gets closer to Saturn then Janus. It is highly possible that much larger, planet-like bodies, share similar orbits.
Two rocky planets: We can imagine the Earth and the Moon in a co-orbital relation. This could happen if the Moon is pushed out of its orbit. Each orbit sweep will occur after decades or centuries and could cause earthquakes or volcanic activity. From the Earth, the Moon will appear much smaller, usually like a star. It will be visible like a disk only during orbit change. For a significant amount of time, the Sun will be between Earth and Moon.
Rocky & giant planet: We can imagine an even more complex system, with a co-orbital system made of a rocky and a giant planet (for example, made of Earth and Jupiter). During each orbit sweep, the small planet will be pushed much closer or further from its sun. The changing orbit will cause alternative warm and ice ages.
Planet & star: An even more complex system can be made of a central object (massive star or black hole), a small orbiting star and a planet. Orbit sweeps will occur once in a few centuries or millennia and will have dramatic effects on the climate. Extra heat from the co-orbital star can trigger warm ages or even runaway greenhouse effects. Also, the planet, having little mass, can be pushed much closer or further from the main star.
If both the star and the co-orbital planet are orbiting a black hole, the only source of heat will come from the star. As distance to the star increases, the planet will experience ice ages. The process will reverse when distance becomes smaller and can degenerate into a runaway greenhouse effect during orbit sweep. However, the planet will have time to cool and even freeze, as orbit sweeps will occur very rare.
Orbital Resonances[]
Many celestial bodies are locked in orbital resonances. This includes many of the Main Belt Asteroids and Kuiper Belt Objects. Pluto and Ceres, for example, are locked in such resonances.
An orbital resonance is usually created between a big and a small celestial body (for example between Neptune and Pluto). Many objects can share the same resonance, thus following similar orbits. The smaller body usually has an elliptic and tilted orbit. Very interesting is the fact that sometimes the small body crosses the orbit of the big body (as happens between Neptune and Pluto).
Orbital resonances are known to exist also between bodies of similar size (for example between the moons of Jupiter or the moons of Saturn). However, in these cases, no moon is known to cross the orbit of another moon.
Objects are more stable when they share a closer orbital resonance (for example 1/2, 1/3, 2/3, 2/5). Objects sharing a resonance of 1/1 are known as quasi-satellites.
Jupiter is massive enough to host in orbital 1/2 or 2/5 orbital resonance a planet larger then Earth in the Asteroid Belt. Neptune could host a planet larger then Mars in a 2/3 resonance like it hosts Pluto.
Even more, in a binary system, a planet can be kept on an orbital resonance with the small star. Technically, such planets might orbit in asteroid belts or might sometimes cross the orbit of their resonance star, appearing not to clear their neighborhood and not fitting into the IAU definition of a planet.
Cubewano[]
This term is used to define Kuiper Belt Objects with orbits that seem not influenced by Neptune and not in an orbital resonance.
We know that other stars also have Kuiper and asteroid belts (and sometimes much larger then ours). There is a significant chance for a Cubewano-like planet to exist inside an asteroid belt. Of course, risks of collisions will be far greater then on a planet with clear neighborhood.
A Cubewano-like planet can be produced after a devastating impact, which blew a significant part of its crust into space, forming an asteroid ring around its orbit. Also, a large enough object like a planet will form orbital resonances with smaller bodies.
Binary System Planet[]
There is a theory that a planet located between two stars of a binary system might be safe for a long enough time. Such a planet will not be located at the barycenter, but at the Lagrangean point L1. In an ideal situation of two identical stars orbiting at the same distance, the planet will be placed at half the distance between them.
In reality, such a planet will not remain on its current orbit for long. A planet can arrive at L1 or L2 point on its own and can move around that point for a limited amount of time, but in the end it will be moved away from such position.
Also, a far enough planet can orbit both stars on a safe orbit.
Elliptical Orbit[]
Main article: Elliptical Orbit.
It is possible for a planet to have a highly elliptical orbit (like comet Halley), crossing the orbits of other planets and asteroids. Such a solar system is not stable for astronomical time units, but still can be possible for enough time compared to a human civilization.
Hyperbolic Orbit[]
Main article: Hyperbolic Planet.
A planet can pass through a solar system. By doing so, it will cross many orbits and neighborhoods.
Chaotic Orbit[]
There are places in the Universe where many celestial bodies come close enough to perturb each other's orbits. Examples can include a Globular Cluster, a solar system with many giant planets or a multiple system or a Galactic Center. In such environments, a planet cannot have a clear orbit, but a rather chaotic trajectory. This also applies for possible planets in the Oort Cloud, which are often affected by gravity from nearby stars.
Conclusion[]
The IAU definition of a planet is incomplete. There are many known or theoretical celestial bodies which don't fit properly into the definition of a planet.