• Ashwini

GS6: Journey from planetesimals to planet.

Updated: May 18

So far we have discussed the formation of nebulae, star and elements that are present in the Universe. However, we didn't come across the topic of the formation of the planets and other celestial bodies in our Solar System. Well established theory which explains the formation of the Solar System is the nebular theory which owes an excellent, with few shortcomings, explanation for the origin of planets, moons, asteroids and comets.

According to the nebular theory, ‘these objects were formed from the material in the flattened outer part of the disk, which did not become part of the star’. This outer part is called the protoplanetary disk. The material which forms the protoplanetary disk contains elements from the early nebulae, after the Big Bang, and the last remains of the dead stars. The disk from which our Solar System formed contained all 92 elements, some as isolated atoms and some as bonded to others.

Image Credit: NASA (CRAB NEBULA)

Geologists divide the material formed from the atoms and molecules into two classes- Volatile materials and Refractory material. Volatile materials such as hydrogen, helium, methane, ammonia, water and carbon dioxide whereas refractory material are those that melt only at high temperatures, and they condense (the process of conversion from liquid or vapour to solid) to form a solid soot-sized particle of dust in the coldness of space (Temperature of outer space 2.7K or -270 degrees C).

As the proto-sun began to ignite and the process of fusion started, the inner part of the disk became hotter, causing volatile elements to evaporate and drift to the outer portions of the disk. The inner part of the disk was left with dominantly refractory dust, whereas the outer portions accumulated large quantities of volatile materials and ice. Ice- there is a snow line or frost line between Mars and Jupiter beyond which volatile materials are transformed into a solid-state, called Ice. We do not limit the use of ice to water alone.

Remember the changed behaviour of the solar wind during the T-Tauri stage, they were the main reason for pushing the volatile material in the outer parts of the disk. The material present in the disk was assembled in such a way that 99.9% of it aggregated to form the sun and the rest 0.1% remained in the disk with the responsibility of forming the planets, comets, meteors etc. Out of this 0.1% of the material, Jupiter shared the largest portion with 71% and the rest was left for other bodies. Angular momentum of the whole Solar System is 3.3212 × 1045 kg∙m2∙s−1. Surprisingly, the planets which have a less proportion of the disk materials compared to the sun have a large proportion of the share in the angular momentum of the solar system. This is a Paradox!

As this was happening- the segregation of volatile and refractory material, the protoplanetary disk evolved into a series of concentric rings in response to gravity. The material started to clump and bind together, due to gravity and electrical attraction. They started with the size of dust, coalesced to become sand size, then to basketball size and the process went on until they formed planetesimals, bodies whose diameter exceeded about 1 km. Because of their mass, they exerted more attraction and started pulling nearby objects. They became space vacuum cleaners, sucking in small pieces of dust as well as smaller planetesimals that lay in their orbit, and in the process, they grew progressively larger. Eventually, they took the form of protoplanets, approaching the size of what we see today in our solar system. Once this protoplanet cleared its orbit of all the celestial debris, it became a full-fledged planet.

Now the question is, how slow was this process? Some Computer-models suggest that it may have taken less than a 10,00,000 yrs (a million years) to go from dust and gas to the large planetesimals stage. Planets from planetesimals grew in 10 to 200 million years which depended upon various factors such as the temperature of condensation of the aggregating materials.

In the inner part of the solar system, since most of the elements left were refractory in nature only they could survive or condense in the innermost zone and evolved into small, terrestrial planets composed of rocks and metals. Whereas, during the planetary accretion (building up of a planet by aggregation of material) the more volatile element of the nebula which were vaporized in the inner part of the Solar system, carried by the solar stream condensed in the outer part of the disk which gave rise to the giant, gaseous planets. Those fragments which were not incorporated into the planets are found today as asteroids and comets.

Chemical differentiation of nebula based on condensation temperature due to the temperature gradient and the intensity of solar wind resulted in:

  1. Refractory oxides such as Al2O3, Cao and TiO2, failed to volatilize and condensed in the inner part

  2. Fe-Ni metal alloys, Fe-Mg-Ni silicates, alkali metals and silicates, sulfides, hydrous silicates, H2O were condensed and concentrated progressively outwards.

  3. In the outer parts, a very volatile compound such as water and methane condensed (beyond snow line)

Meteorites, which were leftover planetesimals formed at 4.57 Ga, and thus considered to be the birth of the planets and our solar systems.


1. Stephen Marshak- Essentials of Geology

2. John D. Winter- Principles of Igneous and Metamorphic Petrology


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