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Formation of Solar System

Where did our solar system come from?

Solar System GIF

Solar Systems form from the gravitational collapse of giant molecular clouds.

Observations combined with computer models: suggest that solar systems like ours formed from giant molecular clouds.

Ours may have been 60 light years wide – although they can be much larger.

HST (Hubbble Space Telescope) Image: Gaseous Pillars In M16-Eagle Nebula Pillars Of Creation In A Star- Forming Region

HST (Hubbble Space Telescope) Image: Gaseous Pillars In M16-Eagle Nebula Pillars Of Creation In A Star- Forming Region

These broke into smaller fragments, 3 to 4 light years across – which are still immensely huge nebulas.

“This dense cloud of gas and dust is being deleted. Likely, within a few million years, the intense light from bright stars will have boiled it away completely. Stars not yet formed in the molecular cloud’s interior will then stop growing. The cloud has broken off of part of the greater Carina Nebula, a star forming region about 8000 light years away. Newly formed stars are visible nearby, their images reddened by blue light being preferentially scattered by the pervasive dust. This unusually-colored image spans about two light years and was taken by the orbiting Hubble Space Telescope in 1999.” – http://apod.nasa.gov/apod/ap030630.html

Molecular cloud broken off from Carina Nebula

Disappearing Clouds in Carina Credit: Hubble Heritage Team (STScI/AURA), N. Walborn (STScI) & R. Barbß (La Plata Obs.), NASA

These nebulas would then collapse into dense cores, 2,000 to 20,000 AU (astronomical units) in size.

Within these dense cores, protostars form – although hot and bright, they are not true stars yet. They only become stars after they ignite into nuclear fusion.

Hubble early Protostar IRAS 20324+4057


98% of cloud: H, He, trace Li – all produced by Big Bang nucleosynthesis

2% of cloud: heavier elements created by nucleosynthesis in earlier generations of stars.

(When stars age, they eventually go supernova. This creates new, heavier elements from H and He, which become part of other nebulas that then make newer stars & solar systems.

Why do planets orbit around stars?

We see planets orbiting around stars – why don’t planets just fall right into the sun?

We see planets rotate on their axis? Why would this be happening?

Money Makes the world Go Round The Office Dwight

Ever notice how flying acrobats, gymnasts, ice-skaters and half-pipe snowboarders, tuck in their arms and scrunch up their bodies while spinning in the air?

By keeping their arms and legs tucked close to their centers of mass, they are able to rotate faster.

This is because, just like linear momentum, the momentum of rotation, called angular momentum, is also conserved.

Angular momentum depends on the speed of rotation and the distribution of weight from the center of mass.

Our page on angular momentum

Skater angular momentum Natalia Kanounnikov

Circus Physics: PBS

Angular momentum ice skater

How does this relate to our solar system?

Here’s an analogy that I learned from Mike Dunlavey,  at physics.stackexchange.com. I have rewritten it in my own words.

When these nebulas – giant gas clouds – first start off, they are never perfectly still. They start with all sorts of moving gas and dust particles.  Everything in space has motion, right?

Let’s consider two cars passing, in opposite directions, on a road. When the cars pass, there is a certain distance between them. They don’t seem to be spinning, or have angular momentum, right? But what if one of the cars threw out a magnet on a rope and captured the other car.  Now thee two joined cars would start spinning, like a bola!

Center of mass rotation angular momentum

GIF by Larry Miller, using a gravity simulation package written by Eugene Butikov of St. Petersburg State University.

That’s what always happens when things moving past each other are pulled together.

When nebulas collapse, each gas and dust particle is like a car; and the gravity that attracts them to each other is like the rope & magnet.

So now we have particles spinning like a bola – and that’s angular momentum, just like an ice skater speeds up and she pulls her arms inward.

Now this angular momentum is conserved as gravity pulls the cloud inward.

Let’s see how a solar system forms as angular momentum is conserved:

cleaning out the neighborhood orbits Pluto

Formation of Solar System

Animation of solar system formation

A nebula slowly condensing into a star, and proto-planets.

The early sun begin off as large, hot and gaseous, but initially has no nuclear fusion.

Once nuclear fusion begins, the resulting massive explosion blows away much of the remaining planetary dust and gas.

solar system forming GIF 2

adapted by G. Rieke, from JPL, M. Roessler, http://cougar.jpl.nasa.gov/HR4796/anim.html

The Angular Momentum Problem

A possible weak link in the condensation theory is sometimes known as the angular momentum problem. Although our Sun contains about 1000 times more mass than all the planets combined, it possesses a mere 0.3 percent of the total angular momentum of the solar system.

Jupiter, for example, has a lot more angular momentum than does our Sun—in fact, about 60 percent of the solar system’s angular momentum. All told, the four jovian planets account for well over 99 percent of the total angular momentum of the solar system. By comparison, the lighter (and closer) terrestrial planets have negligible angular momentum.

The problem here is that all mathematical models predict that the Sun should have been spinning very rapidly during the earliest epochs of the solar system and should command most of the solar system’s angular momentum, basically because it contains most of the mass.

However, as we have just seen, the reverse is true. Indeed, if all the planets’ orbital angular momentum were transferred to the Sun, it would spin on its axis about 100 times as fast as it does at present.

Possible solutions:

Many researchers speculate that the solar wind, moving away from the Sun into interplanetary space, carried away much of the Sun’s initial angular momentum. The early Sun probably produced more of a dense solar gale than the relatively gentle “breezes” now measured by our spacecraft. High-velocity particles leaving the Sun followed the solar magnetic field lines. As the rotating magnetic field of the Sun tried to drag those particles around with it they acted as a brake on the Sun’s spin. Although each particle boiled off the Sun carries only a tiny amount of the Sun’s angular momentum with it, over the course of nearly 5 billion years the vast numbers of escaping particles could have robbed the Sun of most of its initial spin. Even today, our Sun’s rotation rate continues to slow.

Other researchers prefer to solve the Sun’s momentum problem by assuming that the primitive solar system was much more massive than the present-day system. They argue that the accretion process was not entirely successful during the system’s formative stages. Matter not captured by the Sun or the planets might well have transported much angular momentum back into interstellar space as it escaped. This proposal is difficult to test, because the escaped matter would be well beyond the range of our current space probes. Perhaps the remote Oort cloud of innumerable comets is the “escaped” matter.

Despite some minor controversy as to how this angular momentum quandary can best be resolved, nearly all astronomers agree that some version of the condensation theory is correct. The details have yet to be fully worked out, but the broad outlines of the processes involved are quite firmly established.

The angular momentum problem


First photographs of solar systems forming

Astronomers using NASA’s Hubble Space Telescope have applied a new image processing technique to obtain near-infrared scattered light photos of five disks observed around young stars in the Mikulski Archive for Space Telescopes database. These disks are telltale evidence for newly formed planets…. The dust in the disks is hypothesized to be produced by collisions between small planetary bodies such as asteroids. The debris disks are composed of dust particles formed from these grinding collisions.

The tiniest particles are constantly blown outward by radiation pressure from the star. This means they must be replenished continuously though more collisions. This game of bumper cars was common in the solar system 4.5 billion years ago. Earth’s moon and the satellite system around Pluto are considered to be collisional byproducts. “One star that is particularly interesting is HD 141943,” said Christine Chen, debris disk expert and team member. “It is an exact twin of our sun during the epoch of terrestrial planet formation in our own solar system.”

Astronomical Forensics Uncover Planetary Disks in NASA’s Hubble Archive

In northern Chile we find the Atacama Large Millimeter/submillimeter Array (ALMA), an array of radio telescopes. This array was able to image HL Tauri, a young star in the constellation Taurus.

Protoplanetary disk Atacama Large Millimeter Array HL Tauri


Here we see a photograph of a protoplanetary disk: this is a disk of matter surrounding a young star, in the process of creating a solar system.
Hubble Orion Nebula Wide Field and Planetary Camera five protoplanetary disks

Life cycle of our sun

Life cycle of Sun by Oliverbeatson

Image by oliverbeatson at Wikipedia

External links


Learning Standards

SAT Subject Test in Physics
Circular motion, such as uniform circular motion and centripetal force

2016 Massachusetts Science and Technology/Engineering Curriculum Framework
HS-PS2-1. Analyze data to support the claim that Newton’s second law of motion is a
mathematical model describing change in motion (the acceleration) of objects when
acted on by a net force.

HS-PS2-10(MA). Use free-body force diagrams, algebraic expressions, and Newton’s laws of motion to predict changes to velocity and acceleration for an object moving in one dimension in various situations

Massachusetts Science and Technology/Engineering Curriculum Framework (2006)
1. Motion and Forces. Central Concept: Newton’s laws of motion and gravitation describe and predict the motion of most objects.
1.8 Describe conceptually the forces involved in circular motion.

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