Must Planetary Systems Have Retrograde Orbit Planets?

[petrushkagoogol]

Is it mandatory for any planetary system like our Solar system to have a retrograde orbit planet in it's midst ? Is there any upper limit for the same )

 

[Drakkith]

Is it mandatory for any planetary system like our Solar system to have a retrograde orbit planet in it's midst ?

Our solar system has no planets in retrograde orbit, so it can't be mandatory.

 

[Alltimegreat1]

But it is mandatory to have exactly two planets with retrograde rotations.

 

[Drakkith]

But it is mandatory to have exactly two planets with retrograde rotations.

That is the way our solar system developed, but I don't think the OP meant a stellar system exactly like our own.

 

[petrushkagoogol]

Our solar system has no planets in retrograde orbit, so it can't be mandatory.

Planets. All eight planets in the Solar System orbit the Sun in the direction that the Sun is rotating, which is counterclockwise when viewed from above the Sun's north pole. Six of the planets also rotate about their axis in this same direction. The exceptions—the planets with retrograde rotation—are Venus and Uranus.

 

[petrushkagoogol]

But it is mandatory to have exactly two planets with retrograde rotations.

Can you please explain why ?

 

[Drakkith]

Planets. All eight planets in the Solar System orbit the Sun in the direction that the Sun is rotating, which is counterclockwise when viewed from above the Sun's north pole. Six of the planets also rotate about their axis in this same direction. The exceptions—the planets with retrograde rotation—are Venus and Uranus.

Were you talking about retrograde orbits, or retrograde rotations?

 

[D H]

Can you please explain why ?

Alltimegreat1 was playing a game, and not a nice game. Per the current definition of what is or is not a planet, there are only eight planets in the entire universe, and two of them exhibit retrograde motion.

The current definition of what constitutes a planet is solar system specific ("a planet is an object that orbit the Sun ..."). All of those objects orbiting other stars discovered by the Kepler mission and other techniques? They're not planets. They are instead exoplanets. There are moves afoot to remove that aspect of heliocentricism from the definition of "planet": Replace the concept of orbiting the Sun with orbiting a star, and remove the concept of being more or less round due to hydrostatic equilibrium and you have a quite generic concept.

Orbiting a star rather than the Sun is the obvious first step to making the concept of a planet more inclusive. Removing the concept of being more or less round due to hydrostatic equilibrium is also essential with regard to classifying exoplanets as planets or something else; our instruments are currently far too crude to detect the roundness (or lack thereof) of those exoplanets. The proposed change is that a planet is a non-stellar object that orbits a star and has sufficient mass to clear the neighborhood of its orbit. (This leaves out rogue planets.) Note well: Pluto will still not be a planet per this proposal.

 

[Ken G]

I'm not sure roundness is any kind of issue-- among all the things we don't know about exoplanets, I'd say we know they are round better than we know almost anything else about them. We certainly cannot say we are going to stick to things we can directly observe, because we cannot directly observe any of their properties, they are all inferred by indirect means that require application of the laws of physics. But I agree it is silly to define planets as objects that orbit the Sun, that is indeed pure heliocentrism. So we should take out that they must orbit the Sun, but can keep that they need to be round, or more reasonably, they need to have enough inferred mass for us to infer that they are round.