Mercury: Birth of the Inner Planets

Artist view of a protostellar accretion disc. (©David Darling) From U. Hawaii, here.

About 4.7 billion years ago, a massive cloud of dust and gas began to collapse. As it collapsed, it began to spin faster and faster; random collision between molecules and dust particles flattened the cloud into a rotating disk. Near the center of the disk, the density of dust and gas became so great that the material couldn't escape; neither could heat. Pressure- and thus temperature- skyrocketed. Finally the temperature and pressure became so high that atoms of hydrogen began to fuse together to form helium, releasing enormous amounts of energy.

And so our sun was born.

This famous image, often referred to as "The Pillars of Creation" shows a number of protoplanetary disks (aka. "proplyds") emerging as the Eagle Nebula is ablated by stellar winds and light pressure from nearby young, hot stars. The protoplyds are the darker, denser knots of dust and gas. Each of these are several or more times farther across than the diameter of Pluto's orbit.

The so-called nebular hypothesis is a fairly old idea, as scientific ideas go. The original idea- that a cloud of dust and gas collapsed to form the solar system- dates back to the 1730's. Other hypotheses have been developed, but the nebular hypothesis is the one that has best withstood the steadily increasing torrent of observations.

A Hubble Space Telescope image of the AB Aurigae disk. Note in the lower left corner of the right-side picture the small ellipse showing the size of Pluto's orbit on the scale of that image.
From here.

The disk that remained after fusion ignited the central ball of gas into a true star (the presence of fusion is the criterion that distinguishes a "star" from an enormous ball of gas) had developed areas of higher density that would eventually become planets. Gravitation- more important for larger particles- and electrostatic forces- more important for smaller particles- continued to pull material together to form larger and larger objects: dust, grains, rocks, and planetesimals. After the sun ignited, the sudden flux of heat would drive off the outermost layers of gas in a powerful solar wind. Additionally, radiant energy would tend to drive volatile materials- hydrogen, helium and water- outward, away from the inner solar system. I want to focus primarily on the inner four planets here (Mercury, Venus, Earth and Mars), but this is why the gas giants (Jupiter, Saturn, Uranus, Neptune) are larger and composed mostly of gasses.

Protoplanetary disk HH-30, showing the polar jets, thought to dampen angular momentum.
Credit NASA/ESA.

Even after the sun ignited, the disk continued to slowly contract. To get the system to contract, you have to remove angular momentum. This has been a puzzle, but recent images from the Hubble telescope and other observatories have shown that polar jets (material being ejected along the star's axis of rotation) and the strong magnetic field of the newly-born star may provide a mechanism to remove angular momentum from the stellar system. The University of Hawaii site has a terrific computer simulation/animation (partway down the page) showing how twisting magnetic fields in a young system could transfer angular momentum into these polar jets.

Areas of higher density pulled more material into themselves. Once this process of planetary collapse got underway, the rapidly growing protoplanets became so gravitationally dominant that they rapidly cleared their orbits of smaller objects. (This ability of a planet to clear its orbit is the reason that Pluto is no longer considered a planet by many astronomers: Pluto has not accomplished this feat.) The time from the beginning of planetary collapse to completely sweeping up the remaining debris is only a couple of hundred thousand years.

Hubble image of Fomalhaut showing the enormous separation between the ring center and the actual star; this is interpreted to indicate that there is a large planet present in the system in addition to the visible objects. The planet is not directly observable with current technology, but the rings would not be stable in this configuration without another source of gravity. According to the press release accompanying this image, the suspected planet is probably a little inside the ring's inner edge, which would correspond to 1 1/2 to 2 times the distance of Pluto from the sun.

Within 100 to 200 million years after the great cloud of gas and dust began to collapse, the overall structure of the solar system was much as we see it today. Many of the particulars had not evolved into their current conditions (e.g. atmospheres, surface conditions), but the bulk compositions and general orbital positions were much as they are now. The current estimate for Earth's age is 4.55 billion years. Plenty has happened since then, and our home almost certainly did not have oceans until a couple of hundred million years later. So had you taken a look at it then, it might not have been recognizable. But for practical purposes, everything that makes Earth (and the other planets) was in place, just organized a little differently.

We happen to be edge-on to see the protoplanetary disk around Beta Pictoris. This disk appears to have an interesting secondary disk. Presumably, collisions between particles in the two disks will ultimately cause them to merge into one.

This is as good a time as any to point out that what drew me to science- and what draws many scientists- is the aesthetic: the sense of awe and of beauty- as much as or more than anything else.

Next: Meteorites and the composition of the rocky planets.