The lengthening lists of new planet finds have allowed astronomers to start building on the science and knowledge of planet and solar system formation to draw some fascinating conclusions.
According to Astrophysicist Lars Buchhave (Harvard-Smithsonian Centre for Astrophysics) there are three sorts of solar system. The key to the classification system is the elemental composition or ‘heaviness’ of the progenitor nebula – specifically how metallic they are. Rather than the observation of a smooth transition between these three types, the collected data on exoplanets shows distinct types of solar system with little in between.
Planets around the most metallic stars tend to be big – gas giants in the Jupiter class and above. The reason being that the presence of these heavier elements allows more planetary development before the protostar ignites, allowing the growing planet to get heavy enough to attract the lighter elements of hydrogen and helium.
Planets around the least metallic stars tend to be mainly rocky planets, but larger than the rocky planets around our own sun
Those suns in the middle range of metallicity are associated with a third (unfamiliar) type of planet called gas dwarfs. These planets have rocky cores, but are large enough to hold an atmosphere of hydrogen and helium.
So where does our own solar system fit in? Apparently nowhere. Our solar system with our four small rocky planets and four gas giants is an unusual one. In terms of the metallicity spectrum, Earth’s Sun is an example of a metal-rich star, common in the spiral arms of the Milky Way galaxy, so I guess at least our gas giants make sense based on this latest theory.
This got me thinking. Maybe our solar system was formed in the collision of two proto-systems early on? Would this explain the weird fact that Venus rotates in the opposite direction to all the other planets that spun off the ecliptic? Perhaps a metal-poor (Population II) star that went supernova leaving its drifting rocky planets to be snapped up by our Sun?
This oddness in our planetary composition is just the latest in a series of weirdness that relates to our solar system. I’ve noticed this before. The more we look – with the benefit of science – the more atypical we are. Like how both the Moon and the Sun are exactly the same angular size in the sky.
Some of these strange coincidences allowed the development of life as we know it. Jupiter has had a very positive role in protecting life on Earth, acting as our planetary ‘guardian’, preventing many of the asteroid impacts that would have sent life back to the drawing board again and again.