Solar system planet and asteroid formation
There are several different forms of planetary formation hypotheses. These include:
- various forms of non-homogenous (‘clumping’) within the primordial solar nebula collapse;
- rotating primordial nebula spinning off material from the proto-sun; and
- interaction of proto-stars forming in binary or multiple systems.
The asteroid main belt is in-between Mars (the outermost of the rocky / terrestrial planets) and Jupiter (the inner-most of the gas giants). The main belt is also approximately co-incident with the ‘snow line’ of the solar system. The snow line is the distance, approximately 2.7 AU, from the Sun at which water could and can condensate from vapour to liquid form within the very low pressure of the solar primordial nebula; closer to Sun the temperature would be too high for liquid water to condense. Most hypotheses predicate that the asteroids were formed as a few planetesimals which failed to coalesce to form a planet in-between Mars and Jupiter because of disruptions by Jupiter’s gravity.
Whilst the above may appear to be relatively well defined, there are many aspects of the current hypotheses which cannot be explained. These include the ‘classic’ problem of angular momentum and the small (!) matter that the plane of the solar system planets is inclined at 7º to that of the Sun’s rotation. Whilst almost all the mass of the solar system is contained within the Sun (the total planetary mass is only ~0.13% that of Sun), the opposite is the case for angular momentum which resides mostly within the planets/planetary orbits. For example, the angular momentum of Jupiter is around 100 times that of Sun.
What we do know for certain is that we have at least two different ‘types’ of planet in the solar system. The innermost planets, Mercury to Mars, known as the ‘Terrestrial’ planets, are rocky in nature and have mass densities in the range 5.5 (Earth) to 3.9 (Mars) grams/cm3. This is markedly different from the giant planets (Jupiter to Neptune) which have much lower densities ranging from 1.6 (Neptune) to 0.7 (Saturn) grams/cm3. Jupiter and Saturn, and Uranus and Neptune, are often referred to as the Gas Giants and Ice Giants respectively
Formation processes for the giant planets may be different from that of the terrestrial planets and may be more akin to inhomogeneity in the original primordial nebula giving fragmented collapse. The bulk of the mass of the nebula would form the Sun, with the fragments producing smaller H and He dominated planets of insufficient mass to produce small stars.
We can also be relatively sure that the current planetary orbits and positions are unlikely to have been where the planet’s formation initiated. The planets are now in stable orbits, there are no disruptive resonances between the major planet orbits, and this is very likely to be due to a period within the early history of the solar system of settling and planet distance migrations.
One interesting theory is that all major planets formed at high (>~50AU) heliocentric distances. The resistive media of the residual primordial nebula resulted in the proto-planets orbits changing by spiralling inwards. Those objects most effected by the reduction in semi-major axis formed the terrestrials (after losing the lighter elements from their atmospheres during to solar heating), whilst those a little less effected by drag those formed the giant planets.
Estimates for the amount of material originally formed at the distance of the asteroid main belt range from 2 – 10 Earth mass. This has been ‘dissipated’ over the eons (the Sun and planets formed ~4.55 billion years ago) by mechanisms such as:
- gravitational orbital disruptions and thereafter accumulations (via collisions) by Jupiter;
- capture of planetesimals to form planetary moon systems – Jupiter has over 60 moons, Saturn also has over 50, and the moons of Mars, Phobos and Deimos, are asteroid in form and nature;
- formation via capture of the Trojan asteroids;
- disrupted orbits to form other objects (e.g. the Centaurs).
We discussed last month the ‘active’ asteroids, the cometary nature of some previously identified asteroids, and the main belt comets. It now seems clear that a comet, by losing its volatile ices and elements can transition into an asteroid. A specific type of transition object, the Damocloids, were identified in 2005.
Damocloids have, and are defined by, specific Tisserand criteria of Tj <2.0. They are associated with the HFC (Halley family of comets), of which ~623 are currently known, which have the same Tisserand band criteria. There are 145 currently known Damocloids, named after the first in the class recognised, the centaur 5335 Damocles (1991 DA). More than half of these are on retrograde orbits. The Damocloids are considered to be the now predominately inactive nuclei of HFCs. They are believed to have origins within the Oort cloud, with their orbits having been dynamically changed due to interactions with the major planets.
In passing, comets can also be grouped into different families, although unlike the asteroids families we saw in our blog of June, comet families share dynamic and orbital characteristics rather than a common progenitor. The HFCs have orbital periods mostly between 20 to ~200 years. Note however that these orbital periods are not ‘constraints’ for Damocloids, some Damocloids on retrograde orbits have periods in many hundreds of years. Approximately 60% of Damocloids have P > 100 years. In addition to the HFC, the Jupiter family of comets (JFCs) have 2.0 <= Tj <= 3.0. JFCs, of which there are currently 636 known, have orbital periods usually less than 20 years and are dominated by the gravitational perturbations of the major planets, especially Jupiter.
Astrophysics of Planet Formation
P J Armitage. Cambridge University Press. 2010
The Sun – Shining Light upon the Solar System
N Taylor. Observatoire Solaire. 2016
We will look at the effects that an asteroid impact on Earth would, and will, have.
The ‘topic’ list for future monthly blogs is now included in our blog index (alongside previous monthly topics)