Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune are the eight planets in the solar system, all of which orbit the sun because of its gravitational attraction. Is this, however, the most planets that can circle the sun? Is there enough space for another?

The solar system has a disproportionately large number of planets compared to other known planetary systems. According to The Extrasolar Planets Encyclopaedia, there are 812 known planetary systems with three or more verified planets, with just one other general system, Kepler-90, having the same number of planets as the solar system.

Many of these systems may include tiny inner planets that we can’t see; the solar system isn’t the most crowded planetary system in our galaxy. It does, however, emphasize the fact that eight planets may be close to the maximum size that a planetary system may naturally develop to.

To calculate the total maximum capacity of planets circling the sun, we must enter the domain of theory, neglecting certain natural phenomena that may restrict the number of planets that may form. One of the most effective methods is building and developing a whole new solar system.

Engineering a solar system

Sean Raymond, an astronomer at the Bordeaux Astrophysics Laboratory in France who specializes in planetary systems, told Live Science, “When you’re talking about how many planets may be in a planetary system, there are a lot of different elements you need to consider.”

The size of the star, the size of the planets, the type of planets (for example, rocky planets or gas giants), the number of moons orbiting each planet, the location of large asteroids and comets (such as those in the asteroid belt between Jupiter and Mars and in the Kuiper Belt beyond Neptune), the direction of the planets’ orbits, and the amount of material left over, according to Raymond, all contribute to the structure of a planetary system. To settle into a stable form, a system must undergo hundreds of millions of years of strong collisions and gravitational tugs-of-war between planets.

However, suppose we were a super-advanced civilization with technology and resources well beyond our present capabilities. In that case, we could overcome many of these constraints and construct a solar system with the greatest number of planets imaginable, according to Raymond.

We may imagine that the resources available to make planets in this hypothetical designed solar system were limitless and that they could be manufactured and positioned at a whim. Moons, asteroids, comets, and other potential stumbling blocks would also be feasible to eliminate. The only restrictions would be that the gravity exerted by the planets and the sun would be the same as it is usually and that the planets would have to circle the sun in a stable arrangement without colliding.

According to the International Astronomical Union, a planet is defined as a celestial entity that:

  1. Orbits the sun.
  2. Has sufficient mass to reach hydrostatic equilibrium (making it spherical).
  3. Has cleared the region surrounding its orbit of debris.
An artist’s interpretation of exoplanets orbiting a distant star. (Image credit: Shutterstock)

Size matters

The greatest number of planets in a solar engineering system is restricted by the number of stable planetary orbits fitted around the sun.

“When a planetary system gets unstable, planets’ orbits start to cross each other, which means they may collide or merely gravitationally disperse,” according to Raymond, where planets bounce around other planets and are hurled out of the system.

In a stable system, the minimum safe distance between the orbits of distinct planets is determined by the size of each planet, or more precisely, its Hill radius. The Hill radius of a planet is the distance between the planet and the boundary of its sphere of influence, within which smaller objects, like the moon circling Earth, are impacted by its gravity.

An artist’s impression of two planets colliding into one another. (Image credit: Shutterstock)

Planets with more mass have a larger Hill radius because their gravitational attraction is stronger. According to NASA, the distance between the orbits of Earth and Mars is almost seven times less than the distance between the orbits of Mars and Jupiter, which is around 342.19 million miles (550.7 million kilometers).

As a result, Raymond said, the number of orbits that can fit within the solar system is mainly determined by the size of the planets. Jupiter, for example, is 300 times more massive than Earth; thus, its Hill radius is around 10 times greater, according to Raymond. This indicates that ten Earth orbits might be squeezed into the same space as Jupiter’s present one.

As a result, the planets must be as tiny as possible to maximize the number of planets in a system.

Opposite directions

The size of the planets determines the number of orbits that might fit into a constructed system. However, regardless of the planet’s size, there is another ingenious approach we might use to add a few additional orbits: changing the direction in which they circle the sun.

Each planet circles the sun in the same direction as the sun in the present solar system. The planets were created from a vast cloud of dust spinning in the same direction as the sun. However, planets that circle the sun in the opposite way, known as retrograde orbits, might be viable in our manufactured solar system, according to Raymond. This theory, however, is a little far-fetched; owing to the way planets originate, retrograde orbits are unlikely to occur in nature.

The gravitational forces between two planets would be somewhat diminished if they orbited the sun in opposite directions, and the minimum safe distance between their orbits might be lowered.

“If two planets in opposite orbits are traveling in the same direction, they will have more time to collide when they pass each other, resulting in a stronger gravitational kick,” Raymond said. “However, if they are traveling in the other way, they zip past each other and interact for a shorter period of time,” meaning they may be closed without colliding or dispersing.

Let’s made every other orbit in our designed system a retrograde orbit, similar to a carousel with neighboring individuals traveling in opposite directions. We might reduce the amount of space required between each circle and, as a result, fit in more planets.

Sharing orbits

We have assumed that each orbit in our manufactured solar system has just one planet up to this point. Multiple planets sharing an orbit, on the other hand, is plausible, according to Raymond. In our present solar system, we can observe an illustration of this.

The Greeks and Trojans are two groupings of asteroids that orbit Jupiter. According to Raymond, as the gas giant circles the sun, these clusters are around 60 degrees in front of and behind it. However, scientists believe that planets with comparable orbits might exist. Trojan planets is a name given to these hypothetical worlds.

“People are looking for instances of these Trojan planets among planetary systems since they’re predicted to emerge naturally,” Raymond said. He noted that no one had yet noticed any.

We’ll need as many of these Trojan planets as possible if we want to maximize the number of planets in our manufactured solar system. However, just as the number of orbits around the sun must be spread out enough to be stable, the number of planets that can fit into one orbit must be separated enough to remain stable.

A diagram showing 42 Earth-size planets sharing a single orbit. (Image credit: Sean Raymond)

A pair of astronomers used Hill radii to determine how many planets may share an orbit in a research published in the journal Celestial Mechanics and Dynamical Astronomy in 2010. They discovered that a single orbit could accommodate up to 42 Earth-sized planets. Furthermore, according to Raymond, as with the number of orbits in a system, the smaller the planets are, the more they can fit into the same orbit.

Of fact, the possibilities of these many planets organically sharing a single orbit are nearly nil since each one would have to have the same size and shape at the same moment to be stable, according to Raymond. This amount of co-orbital structure, on the other hand, would be achievable in a designed solar system, considerably increasing the number of planets we could fit in.

A diagram showing the number of orbits and trojan planets available due to planet size; one-tenth of Earth’s size (left), Earth-size planets (center), planets ten times larger than Earth (right). (Image credit: Sean Raymond)

Theoretical maximum

Now that we’ve figured out the essential elements that go into designing a planet-packed solar system, it’s time to do some math and see how many planets we can fit within.

Fortunately, Raymond has already done this for us by creating computer simulations, which you can see in further detail on his PlanetPlanet blog. However, although these calculations are based on ideas used by astronomers to produce credible simulations, they have not been peer-reviewed and should be treated with a grain of suspicion.

Raymond designed a system that spans 1,000 astronomical units (AU) from the sun to maximize the number of planets. (One AU is the average distance between the sun and Earth’s orbit, about 93 million miles or 150 million kilometers.) According to the European Space Agency, the solar system’s defined boundary, also known as the heliosphere, is now roughly 100 AU from the sun. However, the sun’s gravitational pull may stretch far beyond. Raymond’s concept also utilizes planets of identical size and retrograde orbits that alternate.

A diagram showing the maximum number of Earth-size planets orbiting the sun (57 orbits each containing 42 planets). Blue lines show regular orbits and red lines show retrograde orbits. (Image credit: Sean Raymond)

With all of this in mind, you could fit 57 orbits with 42 planets apiece for a total of 2,394 planets if you utilized Earth-size planets. However, if you used planets one-tenth the size of Earth (about the same mass as Mars), you could accommodate 121 orbits with 89 planets each, for a total of 10,769 planets. And if the planets were around the size of the moon (one-hundredth the mass of Earth), there might be 341 orbits, each with 193 planets, totaling 65,813 planets.

These figures are astronomical, and humanity’s capacity to construct such complex systems is well beyond our grasp. However, this amusing thought experiment demonstrates that the solar system has more room for planets than the current eight. It’s quite improbable that any more would have evolved spontaneously.

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