Over the past few weeks, the internet has been awash with articles on a planetary parade on the 25th January whereby Venus, Mars, Jupiter, Saturn, Uranus and Neptune will all be visible in the sky after dark. However, many of you may have noticed that we have been able to view this phenomenon for the past ten days or so, and will continue to enjoy this cosmic configuration for another couple of weeks.
Planetary alignments are not rare. In fact, the planets are always in a line across the sky. It may seem strange to some that these other worlds would form in such an ordered way, but once the mechanisms of the formation of our, and other, solar systems are understood, it makes perfect sense.
Planets are children of their parent star. We, and our planetary neighbours, all share the same DNA as the Sun. When a star is young, its spins rapidly. Centrifugal forces cause excess material to be cast from its equator, sculpting a great ring around it – imagine Saturn, but on a stellar scale. From this disc, multiple small bodies accrete, rocks bumping into rocks and merging to create larger objects. Eventually, when they have acquired sufficient mass, gravity works its magic, crushing them from all sides to mould a sphere.
Over millennia, these infant planets engage in a ‘demolition derby’. These gravitational interactions and collisions have ultimately left us with the eight worlds that now make up our celestial neighbourhood. Since they all formed from the same thin disc, their plane is remarkably similar, meaning that they all follow a very similar path across the sky from our perspective. Just like the Sun (and Moon) they can be found along the ecliptic, the imaginary line that passes through the zodiacal constellations. Currently, from west to east, Saturn resides in Aquarius, Venus and Neptune in Pisces, Uranus and Jupiter are in Aries, and Mars is ensconced within Gemini.
The order of the planets in our solar system is rather neat. The four rocky planets are closest to the Sun, separated from the gas and ice giants beyond, by the asteroid belt. Further still, we find the Kuiper Belt and Oort Cloud: discs of millions of small icy bodies, including Pluto and other dwarf planets.
For a long time, it was assumed that this arrangement was normal, with the planets being formed in situ. But recent technological advances have showed us that we are, in fact, not the norm. As Hubble, James Webb, TESS and other telescopes scour the galaxy, we have discovered countless variations, far from the ordered system we know so well. The detection of multiple massive gas giants orbiting closer to their parent star than Mercury is our Sun, has proven particularly confusing. They cannot have been formed there – it is too hot for gas to condense onto a rocky core, and there would not have been a sufficient amount of material to produce such a large body in such a small orbit. Why then is our solar system such an enigma? Some other forces must be at play.
Computer models now suggest that gas giants are indeed formed in the outer regions of the solar nebula, where they have access to more gaseous material, but typically migrate towards the centre due to interactions with other objects. Jupiter’s march towards the Sun however was stunted by its neighbour Saturn. At some point in history, the orbital dynamics of these two gaseous behemoths counteracted their inward migration, and they ultimately returned to the position we know so well.
Jupiter’s slow trek brought with it rocky material from the outer solar system and cleared out the gas close to the Sun, allowing the rocky planets to form from the remaining heavier material. Its presence there for a time, also allowed it to steal much of the rocky material that would have otherwise been available to Mars, which explains its small size in comparison to Earth and Venus. As all the early rocky material interacted with Jupiter, it was flung on chaotic paths across the solar system. This caused the ‘late heavy bombardment’ which pelted the rocky planets with the comets and asteroids that brought water to Earth. Much of the other material was thrown to the outer reaches of the solar system, forming the Kuiper Belt and Oort Cloud (where comets now originate).
It is amazing to consider how events that happened billions of years ago have shaped our history. Not only did the gas giants allow the formation of our home, it aided in the processes that delivered our water, and ultimately allowed life to exist. Even today, we owe our continued existence to Jupiter. It acts like a sentry, standing watch over the Earth, its immense gravity acting like a cosmic shield affecting the orbit of countless asteroids and comets that may otherwise crash into Earth. The relationship is so specific to our survival, that if Jupiter was a mere 10% closer to the Sun, Venus would benefit from this protective action, and if it was 10% further away, Mars would be the fortunate recipient.
Ecosystems are built on symbiotic relationships, and our home is no different, albeit on a vast scale. We are but one planet, but we exist, like everything in nature, in perfect harmony with the members of our cosmic ecosystem.
So, by all means, enjoy the spectacle of the planetary parade over the coming days, but I also encourage you to see the bigger picture, and appreciate the intricate processes that have shaped our environment over our 4.5 billion year history.
See Celestial Events SA advert for contact details
