Hypothetically, how many planets can one star support?

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This is a question that has come across my mind thinking about how our Sun is capable of not only supporting 8 planets, 5 recognized dwarf planets, and an uncounted amount of asteroids and comets with the most outward orbits extending to roughly 1 light year.

This makes me wonder, how many planets can a stars gravitational pull support without any unstable orbits for prolonged amounts of time? For one thing, it varies on what type of star we're dealing with (red dwarf or blue giant, newly born or near the end of its life) and how massive it is. Obviously, the more massive an object the more gravity it will have. In the case of a blue giant, stable orbit velocities will have to be very high if the planets in close proximity want to remain in orbit.
Red dwarves will have slower stable orbit velocities, but their gravity doesn't extend out as much because of their significantly lowered mass and density.

Nonetheless, let's kick off a discussion about this, and see what we ideas we can come up with.

You have it backwards.

The tighter the orbit, the higher the orbital velocity.

KEPLER’S FIRST LAW: The orbit of a planet around the Sun is an ellipse, with the Sun in one of the focal points of the ellipse.

The orbit of a planet around the Sun is not a perfect circle. It is an ellipse - a "flattened" circle. The Sun occupies one focus of the ellipse. A focus is one of the two internal points that help determine the shape of an ellipse. The distance from one focus to any point on the ellipse and then back to the second focus is always the same.

KEPLER’S SECOND LAW: As the planet moves around its orbit during a fixed amount of time, the line from the Sun to planet sweeps a constant area of the orbital plane, regardless of which part of its orbit the planet traces during that period of time.

A planet’s orbital speed changes, depending on how far it is from the Sun. The closer a planet is to the Sun, the stronger the Sun’s gravitational pull on it, and the faster the planet moves. The farther it is from the Sun, the weaker the Sun’s gravitational pull, and the slower it moves in its orbit.

KEPLER’S THIRD LAW: For a given orbit, the ratio of the cube of its semi-major axis to the square of its period is constant.

A planet farther from the Sun not only has a longer path than a closer planet, but it also travels slower, since the Sun’s gravitational pull on it is weaker. Therefore, the larger a planet’s orbit, the longer the planet takes to complete it.

Now, back to your original question. A star can support any number of planets, depending on the mass of the star, the masses of the planets, and the separation of their orbits. Generally speaking, the smaller the planets, the more numerous they are and the closer their orbits can be; but the larger the planets, the less numerous they are, and the farther apart their orbits must be to remain stable. Note that there is more distance between the outer "gas giants" than between the inner "terrestrial" planets.

The formula is complex, and depends on the star's mass, the planetary masses, and how far the planets orbit from their central star. It gets even more complex for multiple star systems.

I hope that this at least partially answers your question.

Yeah. The OP said you need a faster speed to orbit a high gravity star. He didnt get it backwards. He go it right.


I dont know if there is a 'limit'.

The number and variety of bodies that have been known, and continue to be discovered, in our own solar system is astounding. And now we are also finding exoplanets around other stars (already 1500).

But the odd thing is this:99 percent of the mass of the entire solar system is just ONE body. That one body being the Sun.

Of that remaining one percent of the matter that composes the solar system that is NOT the Sun-ninety nine percent of THAT is - just one body. That being the one planet Jupiter.

Ninety nine percent of the solar system is the star at the hub, one percent is Jupiter, and the microscopic residue left over is (that one percent of the one percent)- is what makes up everything else: the other gas giants, the rocky planets (including earth), the icey dwarf planets like Pluto, the moons of all the planets, the asteroids, the comets,the loose dust, the loose gas, whatever there is. All from just that one/10,000 of the matter of the solar system that was left over after the Sun, and Jupiter formed!

They have made computer models of hypothetical versions of our solar system. When they add extra planets those extra planets tend to get bounced out of the solar system by the gravitation of the real planets.

So my guess is that a star is limited in some way to how much matter ends up orbiting around it. Though the way the matter is packaged (many small planets, or a few large ones) can vary. Some stars may have "brown dwarves" (gas bodies larger than Jupiter that generate heat, but are too small to have nuclear fusion at their cores, and so cant become full blown stars)orbiting around them, and we have long known of many double and triple star systems (suns with other suns orbiting around them).

So its a complicated subject.

There is no theoretical limit, especially seeing how the classification "planet" is disputable, astronomy uses "natural sattelite" instead, which includes the comets and every item in the astroid belts (if any).

there are indeed practical limits, since the mass of an orbital object determines the area it "sweeps" empty by either absorbing or flinging out anything that gets too close. (for this point, treat an astroid belt as 1 object, a planet with a mass equal to the combined mass of the astroids in the belt, and moons are added to their 'planets' as a single object as well, when seen from the star), and a star has a maximum distance where you can be, becouse at some point, orbital speeds exeed escape velocity (for that distance to the parent star).

If there is a limit I think it will be related to range, not amount.

I think the farthest point a planet can be is the point where the star+planet gravity weakened by range equals zero, the planet doesn't move around the star nor rotates around itself and there are no other gravity sources around the planet that could break the balance. That's probably the theoretical maximum range of a planet.

But I might be wrong. I am not a scientist.