A Diamond in the Rough: The Rare Structure of our Solar System

 by Max Harvey

Over the many years of human existence, people have looked to the stars and questioned whether or not we are actually alone in our universe. Whilst many hypotheses have been nurtured over the years, one thing is undoubtedly true and it perfectly encapsulated by Arthur C Clarke:

“Two possibilities exist: either we are alone in the Universe or we are not. Both are equally terrifying.”

Whilst it seems like an obvious statement at first, these powerful words do give rise to the question as to whether or not there will come a point in time where we can say confidently that we are the only life in this universe.

A great amount of our reasoning on this matter stems from our analysis of other star systems in other galaxies, looking to see whether or not there are extraterrestrial planets that could potentially be capable of supporting. The actual mechanism of the evolution of life is left to the biologists and I have previously posted a blog article about the chemical requirements for life to form, however, when it comes to the formation of solar systems and the nurturing of these potentially life-bearing systems, there is a lot to consider.

In another previous article, I detailed the Goldilocks zone and how it was dependent on star luminosity combined with the distance between the star and the object in question. Whilst there are arguments that exceptions do exist to this rule we can assume for the majority of cases that in order for life to have even the slightest chance of survival, the object upon which this life is present must be situated somewhere within this habitable zone which is similar to the distance Earth is from the sun if you assumed that the other planet's star was similar to ours.

Relating back to the words of Arthur C Clarke at the beginning, the real question that one should be asking when we are trying to determine whether or not we are alone or not in this universe is concerning how likely it is for a rocky planet to form in this so-called habitable zone. Determining this could be the first step in obtaining a final answer as to whether or not life as we know it is a regular occurrence or a ‘diamond in the rough’.

To understand planetary formation within the bigger picture of star system formation, modern scientists must use a multidisciplinary approach spanning from astrological observations, to chemical isotope analysis, to geological rock comparisons in order to fully understand how it was that our solar system and so many others formed in the way that they did.

Arguably the best way to observe the formation of star systems is to look at other star systems that are in an earlier stage of life to ours. This can in turn give us the answers that we need when determining our solar systems hidden history. This process is exactly what astrophysicists have been doing for the last few decades in an attempt to gather the information that could explain how our solar system gained its characteristic features that we know all too well.

However, there comes the problem that when we look at other star systems throughout space, very few of them look comparable to ours. We are all used to the composition of our own solar system, with its 4 terrestrial planets close to the sun and the more massive gas and ice giants further away. However, many other solar systems have a different makeup. Instead of small rocky bodies orbiting close to the sun, astrophysicists have found that there are in fact much larger rocky bodies orbiting in these star systems. These objects have been named affectionately as 'Super-Earths' due to their large masses (2 - 8 times that of Earth) and orbit at much closer distances to their sun than even mercury. An example of one such star system is the Gilese 876 system where the super-earth 876d has a mass 7 times of Earth but also orbits its star at a frightening speed, completing a full circuit is just 2 days.

Also within this star system are three gas giants that are known as 'Hot Jupiters' due to their close proximity to the sun. In fact, each of these four planets orbits closer to the star at the centre of the system than mercury does to our star. This pattern of a 'condensed' solar system is seen across many other systems that we can observe using telescopes on Earth and in space. It appears that the solar system that we live in is in fact rarer than we might have first believed.

The reasoning behind why a high proportion of star systems form in the same manner as Gilese 876 is actually due to the effect of the gas giants that orbit the stars. In the early days of a solar system, these large gas giants frequently collide with debris in the early asteroid belt, reducing their angular velocity and hence causing them to spiral inwards towards the sun. These gas giants subsequently clear even more debris from the inner regions of the solar system, removing the building blocks for the possible formation of a terrestrial planet in the habitable zone, and consequently the existence of life. It appears that this 'type 2 migration' of these gas giants is very common and results in the compacted star system as seen in the case of Gilese 876.

Having now seen that our solar system is in fact rarer than we thought, the even more important question is how our solar system didn't end up with the same fate. The answer lies in the lord of the rings, Saturn. It appears that in our solar system, the gravitational effect of Saturn upon Jupiter actually halted its inwards descent, enabling the 4 terrestrial planets to form and eventually bear life in the case of Earth which was lucky enough to form in that magic habitable zone. It seems to be the case that it is only as a result of the fine balance between these two competing gas giants in our solar system that our Earth was able to form in the necessary region of space.

The discovery of these condensed star systems and the precise requirements required for a solar system like ours to form, both point to the conclusion that the presence of habitable planets that fall within the goldilocks zone is rarer than previously thought and may even lend itself to the suggestion that maybe we really are alone in the whole of the universe despite the vast number of star systems that exist. Maybe it is true that Earth is, in fact, a diamond in the rough and not just a common occurrence.