3 lessons in creativity — how astronomers developed newer and better models for the evolution of the Solar System — and some reflections

I learnt about the amazing progress astronomers have made elucidating how our Solar System was formed. Given that humans have seen into the furthest reaches of the universe, detected gravitational waves and imaged a black hole, you may think this is an old problem in astronomy, one that has already been solved, and the solution immortalized in many textbooks and popular science books.

Yet, it turns out the story is far more interesting. Recently, astronomers have developed newer models that not only better explain how the Solar System came to be, but also revealed just how creative they were.

Here are three instances of creativity, and the principles behind them, which I believe astronomers applied (intentionally or not), in elucidating the evolution of the Solar System.

Background: The Classical Model

It was generally assumed that the planets, and the order and positions we find them today — the four terrestrial (rocky) planets of Mercury, Venus, Earth and Mars, followed by the gas giants Jupiter and Saturn, then the ice giants Uranus and Neptune — formed as they are, and they continued to orbit the Sun in this order ever since.

In this classical model, the Solar System was formed when a giant cloud of gas collapsed, with the remaining material forming a surrounding flat disk. This protoplanetary disk would eventually form all other planets, moons, asteroids and comets through the process of accretion, where smaller objects collide with each other and build up to form larger bodies. (Interestingly, this model was suggested, in part, by Immanuel Kant, the famous philosopher.)

Unfortunately, this model seems almost too simple to be true, and fails to explain some interesting observations in our Solar System.

For starters, why is Mercury the last planet inward towards the Sun? There’s 88 million miles between Mercury and the surface of the Sun. That’s a lot of space, and it’s weird that it’s devoid of any major astronomical objects (especially if we compare with other planetary systems).

And if you consider the inner planets, which grow in size from Mercury to Venus to the Earth, you would expect Mars to be ten times larger than it is now, but it’s half the size of the Earth, and even less massive.

Adding to the mystery, the size gap between the largest rocky planet, Earth, and the smallest giant planet, Uranus, is strangely large. Uranus is about a dozen times the size of Earth. Why are there no planets with a size in between?

Recognising these peculiarities, astronomers have developed new models for the formation of the Solar System, and these new models are not only excellent cases of brilliant deduction, but also reveals their amazing creativity.

1. If it can’t be here, then make it come: Think in reverse.

For most of modern astronomy, many thought that our Solar System was the only one there is, or that finding an extrasolar planet would be near impossible. Then in 1992, news came of the discovery of a planet orbiting a star other than the Sun. These planets are now known as exoplanets. (This discovery, by the astronomers Mayor and Queloz, was recognized by the 2019 Nobel Prize in Physics.)

It is the type of planets discovered by Mayor and Queloz that questioned what we thought we knew about gas giants and the outer Solar System. Mayor and Queloz discovered a gas giant planet around the star 51 Pegasi (or, in the constellation Pegasus). As a gas giant, it was similar in characteristics to Jupiter in our own solar system, but for one astonishing fact.

It was orbiting its star in an exceptionally close orbit, and I mean, really close. Its orbital period was 4 days. This means the planet is literally baked by the heat emitted from its star. (In comparison, Mercury, our innermost planet, orbits our Sun in 88 days.) More such gas giants were found orbitting exceptionally close to their stars. These planets were thus given the name ‘hot Jupiters’.

But this discovery was puzzling. Based on the accepted model of gas giant formation, gas giants couldn’t possibly form so close to their parent star. At such close distances, the high temperatures would never allow gases to stabilize and accrete. Such formation was only thought to occur beyond the ‘frost line’ of the star, where it is cool enough for gaseous compounds to condense. Astronomers thus have to explain why hot Jupiters are where they are.

And here is where astronomers applied some creativity: switch perspectives; think in reverse; ask a different, even opposite, question.

Instead of thinking about how hot Jupiters could have formed where they are, they thought about what could have brought them, formed far away, to be so close to their star. They didn’t have to abandon established science, but try to make things work with it.

This led astronomers to posit the idea that hot Jupiters were not so different from our own gas giants after all. Like our Jupiter, they also formed beyond the frost line of their star, and then, somehow, migrated inwards.

As a result, astronomers developed alternative models which showed how it is possible for planets to migrate (The basic idea is that after the hot Jupiter had cleared out a path in the gaseous disk surrounding the star, the resulting torque from the remaining gas exterior to the planet’s orbit causes the planet to migrate inward.)

But if hot Jupiters could migrate, why didn’t our Jupiter migrate from its position beyond the frost line? Perhaps it did.

And so astronomers developed a newer, more dynamic model of the evolution of the Solar System, one in which planets migrated. This new model, called the grand tack, suggests our Jupiter had also migrated inwards towards the Sun.

2. What’s unique about us?: Ask what is different.

Let’s assume this is true, that our Jupiter did migrate inwards into the Sun. If so, then there is another mystery: why didn’t it stay there? How did it end up so far out in our Solar System? It must have moved somehow.

Astronomers thus considered what’s different about our Solar System in particular. Interestingly, we don’t have one gas giant. We have two. You may be familiar with it; it is the most famous planet other than Earth, that orange sphere with its eye-catching rings, Saturn.

And so, if Jupiter migrated, Saturn would have too. Its inward migration would be slower and occur later since it was less massive than Jupiter and had been formed further outward. But importantly, Jupiter and Saturn’s resultant gravitational interaction would result in Jupiter being pulled outwards again, with Saturn following.

And so we can explain why Jupiter and Saturn are where they are, without discounting new discoveries about how planetary systems formed.

Sometimes, to creatively solve a problem, we need to consider uniqueness, and apply to that unique context.

3. Just add something in and see: Challenge assumptions, or try something new.

Sometimes, all the available information we have is insufficient, or we cannot deduce a solution from what we have alone. We need to add that something new, something different, that additional golden nugget.

While some understandably argue that introducing (completely) new elements, without any antecedent reason or evidence, is dangerous (since we can come up with anything to explain or justify something), sometimes we do need to think outside of the confines of the problem, to add in something beyond what we are given. The components of the end result are not necessarily the same as what was there in the beginning.

So, just as absence of evidence is not evidence of absence, perhaps we should at least consider: what if? Having that additional golden nugget might just explain how things came to be.

(Of course, after venturing out on a limb, we then have to work backwards to ensure our hypothesis fits in with our current observations and understanding. Steve Jobs once said, we can only connect the dots looking backwards. It’s just that sometimes, we have to consider the dots that aren’t there, and explain how come they’ve disappeared.)

But first, some background. The classical model has had one major development before the grand tack model was posited. In the Nice model (named for the French city, not the adjective, though I guess there’s no reason it shouldn’t be nice!), it is suggested that the gas giants were formed much closer together. Slowly, these planets started to migrate outwards to their current positions.

This would explain why Uranus and Neptune have hydrogen as well as frozen gases and ice such as ammonia and water. They were first further inwards than where they are now, allowing them to sweep up hydrogen and helium available closer to the star, before being swept out beyond the ‘frost line’, where frozen gases and ice can exist.

It also explains why Uranus and Neptune are relatively large. Planets which initially formed in more distant orbits would have taken a longer time to travel around the Sun, and would have accreted lesser material.

However, for Uranus and Neptune, they would first have been able to accrete significant material when they were closer to the Sun, in smaller orbits, before being ‘pushed’ outwards, after interacting with, you guessed it, Jupiter. (Interestingly, models suggest that Uranus and Neptune may not even have been in their current order; their orbits were swapped when they migrated outwards.)

But models also discovered that the outwards migration of Jupiter and Saturn, as described in the Nice model, would have caused the orbits of the inner planets, including the Earth, to become so eccentric, to the point that we could, or should, have collided with each other, perhaps Venus.

But we are still here. So what saved us?

Astronomers thus decided to think outside the box. They put in their models a third ice giant planet. In what is called the jumping Jupiter scenario, this ice-giant would have been ejected by Jupiter. Meanwhile, Jupiter would have lost momentum and ‘jumped’ to an outer orbit, unable to wreak havoc on the inner Solar System.

Why is an additional ice giant necessary? Models discovered that if we only have Uranus and Neptune, one of them would have to be ejected. But they are still here. So the only way the models have been able to arrive at the current state of the solar system, is if we had a third ice giant, which was sacrificed to ‘jump’ Jupiter’s orbit into a higher level.

If so, our sacrificial ice giant could be anywhere in the Milky Way galaxy; we could have lost it at any point the Solar System orbited the galactic centre.

But another hypothesis suggests that this ice giant is still here. Indeed, the path of some long-distance comets originating from the Kuiper Belt and Oort Cloud has led some models to suggest that there is a mysterious gravitational body in the far reaches of the Solar System, disturbing comets and flinging them into the inner Solar System in a discernible pattern. If proven to be true, we could have a ninth planet again (sorry Pluto). We could be thus be sitting on the most amazing discovery of the Solar System in decades!

All this pieces-fitting-into-the-puzzle would have been impossible if we stuck to only what we had. We need to challenge assumptions, and not necessarily only accept what is given. Sometimes, when we’re stuck, we need something more, an additional golden nugget that glues together what we have. Sometimes, we just need to add something new.

Implications of a more dynamic model of Solar System formation

What were the consequence of these migrations? Could it explain the strange peculiarities of our Solar System?

Firstly, Jupiter’s migration through what is now the orbit of Mars would have starved the Red Planet of additional material to feed upon. Jupiter would have forced the material in the inner Solar System into a narrow band of dense material. This could then explain the smaller size of Mars, which would be orbiting further outward of this band of material, and not able to accrete on it.

In addition, Jupiter’s inward migration could explain the lack of Super-Earths (planets ranging in size between Earth and Uranus) in the Solar System, something which is otherwise common in other planetary systems. Models suggest that Jupiter’s inward migration would cause any such planetary object in the inner Solar System to collide with another once every 20 to 200 orbits (extremely fast on astronomical scales). The resultant material continuously collides with one another, losing momentum and eventually spiralling inwards and crashing into the Sun. This material would have brought any Super-Earths, which had formed in the inner Solar System, along with it. This clearing would also explain why there is no significant body inwards after Mercury.

What about us? Earth and the other rocky planets would likely formed from what remains at the outer Inner Solar System after the Super-Earths crashed into the Sun. And this could have important implications for life on our planet. The hydrogen and helium in the inner Solar System would have been brought into the Sun, and Earth’s atmosphere would be formed from the remaining gases of nitrogen and oxygen, which are crucial to life on Earth. Perhaps life can only form on these ‘second-generation’ planets.

Furthermore, Neptune’s outward migration would have caused icy bodies to move towards the Inner Solar System. Some of these bodies would have impacted the Earth, delivering to it the water and organic molecules which are important precursors of our carbon-based life. (Organic molecules would have been unable to form at the high temperatures in the inner Solar System at that time, while they would have remained intact on these icy bodies).

Reflections

Of course, I’m no astronomer, so I can’t tell you the implications for planetary science, nor explain in detail the fascinating physics of planetary migration. Also, the above descriptions are likely too simplistic. There are a whole host of factors and features to consider. How would this have affected asteroids, the satellites (moons) of the giant planets?

More importantly, none of these models are definitive. They cannot be ‘proven’, as they can only be probabilistic models. By manipulating inputs, such as mass and position, other sequences of actions could also result in (and thus ‘explain’) the current state of our Solar System. Indeed, alternative processes with lower probabilities could have been the actual evolution of the Solar System. Unless we have a time machine, we would never know.

If so, why then do we try? What is the point of trying to deduce the formation of the Solar System if we can never know?

Other than the pursuit of truth and knowledge for its own sake (which is important!), I would argue that such exercises, even hypothetical models, are useful, whether or not we can ultimately discover the truth.

Such an approach may unearth new revelations, which will force us to also consider why not; rejecting an existing model or hypothesis also needs adequate reason. The exercise of creativity may also a new way of doing things (or how things came to be, in this context). All this adds to our wealth of knowledge and understanding, and could help us find better solutions.

But finding these solutions would require the exercise of creativity; to ask different questions, to consider what’s unique, and to just try something new.

Additional insights

Beyond those lessons in creativity, and their practical value, by learning how the Solar System came to be, I came to appreciate just how unique and interesting it is. Comparing ourselves to the thousands of other star systems in the Universe, it seems that the Earth and our solar system is not the ‘normal’, average system; we are indeed ‘special’ to some extent.

And while the Solar System is on a scale that is just on the edge of our comprehension, we should not forget that how truly lucky we are to live on a planet and in a planetary system which have allowed us to survive and thrive. We should thus better take care of what we’re exceedingly lucky to have.

In sum, while I learnt that sometimes we should not just accept what we’re given (which is why we should experiment with newer models), we must also not fail to treasure what we have.