How we are understanding more about the stuff surrounding stars

We are getting a better and better at understanding how planetary systems are formed. This article tracks the historic development of the heliocentric model and how that relates to understanding the formation of our own Solar System.

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Scientists have spent a lot of time over the years trying to figure out how the Solar System was formed and our ability to now observe the debris around stars is helping us to understand this process better. The process that is generally is accepted is that our Solar System formed from the debris left over from the formation of the Sun. This big massive disk of dust didn’t manage to get captured by our young Sun’s gravitational collapse, as it was formed, so left a bit over to make the planets. Bits of dust started hitting other bits of dust and then these combined into clumps of dust and a little while later – presto! We have a home planet to live on.

It took us quite a while to figure out how our Solar System might form as it took us a long time to accept that the Sun was at the centre and the planets whizzed around rather than the complex Solar System model that was in vogue for a long time that put the Earth at the centre of the universe. The first recorded person to talk about the Sun being at the centre of it all was Aristarchus of Samos around 250 BC. It appears he was a bit on his own about this as many of his contemporaries believed in the Earth being at the centre of everything. About a century after Aristarchus, another classical period astronomer called Seleucid of Seleucia agreed with the earlier work.

Aristarchus (Credit: Wikipedia)

Their work didn’t attract a lot of attention and was eventually lost over time until the concept of the Solar System being heliocentric came up again in the 16th century. Nicolaus Copernicus was the main proponent of the heliocentric view that led to a revolution in how we viewed where the Earth sat in the wider scheme of the Universe. He published De revolutionibus orbium celestial in 1543 and gave a predicable geometric element to his model that removed the complexity of the epicycles in the geocentric model. This also directly challenged the established views of the Christian church at the time so it took a while for his views to take off.

Copernicus (Credit: Wikipedia)

At about the time of Copernicus another accomplished astronomer, Tycho Brahe, was pushing his view of the model which complicated things a bit further by suggesting that the plants, other than Earth, rotated around the Sun and that system then orbited the Earth. As the evidence was starting to stack against the geocentric model the hybrid nature of Brahe’s helio and geocentric model was popular amongst geocentrists. Galileo’s invention of the telescope was the really the big step needed to gather evidence for the heliocentric model. He was able to observe sunspots tracking across the face of the Sun, showing rotation, and the phases of Venus all of which could not be explained by the geocentric model.

Tycho Brahe’s helio and geocentric model (Credit: Wikipedia)

Johannes Kepler also improved on the Copernican model by describing the elliptical nature of the orbits and Galileo’s observations through his telescope backed up Kepler’s theories. By the end of the 17th century the heliocentric model was becoming widely accepted and thoughts starting turning towards how might the Solar System be formed. The first hypothesis that attempted to explain the formation of the Solar System has been remarkably durable and is widely accepted, the nebula hypothesis. It was first postulated by Immanuel Kant in his Allgemeine Naturgeschichte und Theorie des Himmels (Universal Natural History and Theory of the Heavens) in 1755. Over time we have been able to observe the processes involved in star formation and have been able to directly observe the disks of material around some stars. The hypothesis has been updated over the years to what is now believed to be a process following the cooling of an accretion disk around a new star. As it cools, dust grains and ice can clump together and they eventually squish together enough material to make objects about 1000m across. These are called planetesimals and eventually crash into each other and get bigger and bigger. After about 100,000 – 300,000 years Moon-sized planetary embryos are formed.

So fast forward to modern times and the ability of the Hubble Space Telescope (HST) to directly view the disks of dust and material around other stars, which has dramatically added to the observational data that improves the theories of planet and Solar System formation. One of the latest observations is around the star HR 4796A, the featured image at the start of the article is the HST view of that star. Whereas before we have just seen the disk of material around the star, these recent observations show a much bigger structure that might help explain how the more distant parts of our own Solar System formed. Plus the analysis helps explain just how complex and dynamic the systems around stars are – especially if you throw in a nearby companion red dwarf star as is the case with HR 4796A.

Photo from Hubble of the area around HR 4796A (Credit: Hubblesite.org)

So we’ve come a long way from thinking that the Earth was the centre of the universe to directly observing the formation of solar systems elsewhere in the galaxy. Thre’s still a few processes we don’t understand but the veil on these is slowly getting lifted as more and more observations are made. Exciting times!