Hitchhiker’s Guide to the Solar System

Whenever you learn or experience something new, it is common to compare it with something you are already familiar with. So, when scientists study exoplanets, it makes sense for them to compare these planets to the ones closest to home. The Solar system is a great source of information when it comes to trying to understand the composition and dynamics of planets that reside outside of our system. 

Human understanding of the Solar system itself has changed throughout history, but especially since the development of telescope and photographic technology. Prior to this technological revolution which shifted paradigms, the Solar system was a place that sky gazers have filled with mythology and divine intervention since we first looked up at the sky in wonder. 

To the ancients, and for millennia, the five bright wanderers were tied to omnipotent deities, dancing with enough regularity that their orbital values could be calculated - and even predicted. One such example of these intricate observations were the measurements made by some of the first Mesopotamian astronomers, when studying the orbit of Venus or cycles of the Moon. Another, the brightness, motion and connection to zodiacal light by Indigenous Australian astronomers. 

For thousands of years, the five bright moving “stars” - Mercury, Venus, Mars, Jupiter and Saturn - ruled all knowledge of the Solar system only as bright points of light that differed from the fixed, eternal stars.

In the 17th Century, the human perspective of the Solar system changed. Optics and telescopes opened our eyes to the Jovian moons, the rings of Saturn, dark features on Mars, and craters on the lunar surface - all raising as much excitement as they did questions. At the height of this new era our exploration of the Solar system (in particular from 1684) a total of 16 bodies were considered part of the our planetary system - taking into account the Sun, Moon, and the known Jovian and Saturnian moons.  

Through studying these newly discovered subjects, a range of astronomers - of the likes of Kepler, Huygens, Halley and Newton - started formulating mathematics around their observations, finding relations between the orbital dance of objects, and distance from the Sun. Following the discovery of Uranus by Sir William Herschel, in 1781, these relationships were used to great effect in the discovery of Neptune -- found in 1846 as a result of its gravitational pull nudging Uranus away from the path it would otherwise take across the sky.

The 1800s also saw the number of ‘planets’ in the Solar system rise dramatically, with the discovery of the first objects orbiting between Mars and Jupiter - objects we now know as the asteroids. Indeed, the largest of those asteroids, Ceres, was considered a planet in text books until at least the mid-1930s - around the time that Clyde Tombaugh stumbled across a new, odd, small planet moving on a highly elliptical orbit, even going so far as crossing that of the giant Neptune. With the asteroids demoted, and the new find (Pluto) considered a planet, the system capped out at 9 planets, from the 1930s until very recently.

For 75 years, following the discovery of Pluto, instruments advanced, telescopes got bigger, cameras got better, and rockets sent interplanetary spacecraft to make in-situ observations; we learned so much about our own, unique planetary system. We expanded our knowledge so much that in 2006 we changed our definition of a planet and reclassified the largest of the Solar system’s smaller astronomical bodies (such as Pluto) as dwarf planets.

This brief summary of the evolution of how we consider the Solar system highlights the ever changing banks of knowledge that planetary astronomers contribute to over time.

Now, a new review paper about the Solar system, resulting from collaborations between a dozen scientists and led by an Australian team, creates a new bridge in planetary astronomy, extending to a new class of planets that reside beyond the Sun’s gravitational influence. The paper bridges the gap between our most up to date Solar system science and extends it into the science of Exoplanets, in order to benefit both fields. 

To do this, first author Jonti Horner from the University of Southern Queensland (USQ) and his collaborators took on the behemoth task of describing the Solar system and its planets, their evolution (as it is currently understood), and then examining how knowledge of the Solar system informs our understanding of exoplanetary systems. In much the same way as the revolutions of the last four centuries (the telescope, photography and Newtonian mathematics) revolutionised our understanding of our Solar system - the last 400 years of learning are now helping us to expand our knowledge to systems beyond our own. 

Below, we take a tour of some interesting findings in the paper.

The planetary system that the Earth resides in (the Solar system) is made up mostly of three types of major (planetary) body, all of which orbit our local star, the Sun (also known as ‘Sol’). These bodies have been forged over billions of years, and shifted to their current positions through a cosmic dance of migration - especially with some of the bigger players moving around. 

The first type are the familiar inner planets - usually known collectively as the terrestrial planets (after their very broad similarities to the Earth). The group consists of Mercury, Venus, the Earth-Moon system, and Mars. These planets have rocky surfaces and are composed of metals, rocks, silicates and minerals that formed closer to the Sun, during the early stages of the Solar system evolution. These planets also have atmospheres (even Mercury has a very thin atmosphere), though they are all very different from each other. 

Mercury is a shattered husk of a world, thought to be a dense core topped with a relatively thin layer of rubble. In its youth, Mercury was probably twice its current size, before being shattered and stripped of most of its mantle and crust in a cataclysmic collision, in the Solar system’s youth.

Venus can be described as a literal furnace - where a runaway greenhouse has increased the temperature on the surface to be hot enough to melt lead, with an atmosphere so thick that it exerts a pressure, at ground level, as heavy as 90 Earth atmospheres. 

Mars has a thin atmosphere, with ice caps made of carbon dioxide and dust devils that climb for tens of kilometers. Surface temperatures on Mars are usually well below the freezing point of water, though during a warm Martian summer day, the temperatures can reach as high as 20-degrees Celsius. There’s also strong evidence to suggest that water both once flowed freely across its surface, and occasionally bubbles up from below the surface - creating new features observed by orbiting spacecraft.

Moving outwards into the Solar system - the second type of major body are the biggest objects in the system, aside from the Sun - the gas giants Jupiter and Saturn, and the ice giants Uranus and Neptune. 

These enormous planets don’t have any solid surface to stand on, and instead are giant orbs of gravitationally bound materials from the original Solar nebula. 

Jupiter and Saturn both are both primarily made of hydrogen and helium , whereas Uranus and Neptune contain a large portion of the cold ices and volatiles that existed in the far reaches of the early Solar system - like ammonia, carbon dioxide, water, and methane ices. 

These giant planets are many times greater in both mass and size, relative to the inner rocky planets - and are themselves like planetary systems in miniature, with each giant planet being orbited by dozens of moons. Each of the outer planets also hosts a ring system - with Saturn’s being the biggest, brightest and most brilliant.

And lastly, we have the newly minted ‘dwarf planets’ - which include objects like Ceres in the Asteroid Belt, and Pluto, Eris, and Sedna in the icy depths beyond the orbit of Neptune (the largest members of the ‘trans-Neptunian objects’). It is likely that we will discover hundreds more dwarf planets as we develop still more powerful tools to survey the sky - like the Vera Rubin observatory, scheduled to see first light in the near future.

In addition to the eight planets, their satellites, and the hundreds of dwarf planets, the Solar system also contains uncountable smaller objects - most of which reside in three ‘reservoirs’ scattered across our system - two torus-shaped belts that bracket the realm of the giant planets (the Asteroid belt and the trans-Neptunian population), and a vast cloud surrounding the Solar system, stretching halfway to the nearest star (the Öpik-Oort cloud).

From the large range of Exoplanets discovered so far, the majority of systems that exist beyond our own seem to fit a different model - with massive planets orbiting far closer to their host stars than Jupiter orbits the Sun. Exoplanetary systems contain a variety of planets totally unlike those we see at home - from gas giants that skim the surfaces of their parent star (known as ‘Hot Jupiters’) to the ‘Super Earths’ and ‘Mini-Neptunes’ which have no direct analogue in our own backyard. 

A super-Earth is a terrestrial (rocky) planet that is larger than Earth, whereas a mini-Neptune is a gas planet (such as Neptune or Jupiter) which is smaller than Neptune. These planets were thought to be rare because their formation did not agree with earlier theories of planetary evolution, and because none exist in the Solar system.

A key feature of our Solar system are the two-main torus shaped rings of small bodies that provide imaginary boundaries dividing the system into discrete parts - an inner region, an outer region and a beyond the edge region. 

The first is the Asteroid Belt, which resides between the orbit of Mars and Jupiter. This ring of irregular and spherical rocky bodies is the result of Jupiter’s massive gravitational influence, never allowing a planet to form in this region. In fact, most of what we see today - whilst it is plenty - is only a small fraction that remains of this debris field, with Jupiter ejecting vast numbers of objects that once resided in the belt from our system (or inward, to collide with the other planets) as it migrated. 

As the nearest of our small body reservoirs, we know a great deal about the Asteroid Belt, with data coming from numerous sources - including of robotic explorers who have visited these bodies, from our remote sensing, and even from meteorites that have crashed to Earth - we can analyse their composition to determine what they are made from and where they have come from.

The next torus/ring consists of the trans-Neptunian objects, located beyond the orbit of Neptune. These small, cold worlds - some of which have their own moons - are made of rock and ices, and take hundreds of years to complete a single orbit around the Sun. The most famous (and largest) of these bodies is the Pluto-Charon system - visited by the New Horizons mission in 2015. Rather than being a single population of small bodies, the trans-Neptunian region instead consists of several overlapping groups of objects - the Plutinos, Scattered Disk, Edgeworth-Kuiper belt and Detached bodies, to name just a few. Collectively, though, the trans-Neptunian objects are relics, left behind from the Solar system’s formation.

As Jupiter migrated in the Solar system’s youth, so too did the other large planets.  In particular, Neptune moved outwards as Jupiter migrated inwards. Neptune itself is a fairly dense and massive planet, and as it swept through the outer debris field of the early Solar system, objects in this region were captured into resonant orbits, or were scattered inwards (towards the inner Solar system), whilst others were pushed even farther away from the Sun.

Far beyond the trans-Neptunian objects, astronomers are confident that there exists an even bigger, roughly spherical population of a trillion or more objects, known as the Oort Cloud - the place from which the bulk of the long-period comets are thought to originate - though that cloud is far too distant for us yet to observe directly. These tiny, cold bodies of ice and dust (colloquially known as ‘dirty snowballs’) were originally ejected in all directions by the forming gas and ice giants in the early Solar system. They’re still bound to the Sun’s gravitational field, though they extend well out into deep space - with the outer members almost halfway to the nearest star, at distances greater than 100,000 times that between the Earth and the Sun. When one is flung inward, by the influence of a passing star, or the subtle influence of the ‘Galactic Tide’, it can come close enough for its surface ices to sublime - shrouding it in a diaphanous shroud of gas - the spectacular coma and tail of a comet. 

Whilst the small bodies in the Asteroid Belt, the trans-Neptunian belt, and Oort Cloud only make up a tiny portion of the mass of the Solar system (this mostly resides in the Sun, followed by Jupiter), these bodies play a vital role in our understanding of the evolution and composition of our system. Planetary scientists can study where these objects are located, their orbital parameters, and the materials they are made from (through direct evidence such as studying meteorites that have fallen to Earth, or through studying their light/spectrum) to tell the story of our system and how it came to be the way it is.

These studies help scientists understand what they observe when seeing other exoplanetary systems across a range of different evolutionary stages.

The Solar system is full of weird and wonderful features, unique in their own way and according to the world or system they reside in. Here’s a few facts about the bodies in our system that you might not have known about:

• Mercury is the closest planet to the Sun, but is not the hottest. Mercury also has a day that lasts 59 Earth-days long, and a year that lasts 88 days, as it speeds along its very elliptical orbit - the most eccentric of any planet of our system. 

• Venus is the hottest place in the Solar system, and unlike other planets in the Solar system (excluding Uranus), the Sun rises in the west and sets in the East, as Venus rotates backwards.

• Earth is the only planet where water continually exists as a solid, liquid and gas on the surface, making it perfect for life as we know it.

• The northern hemisphere of Mars is on average two-kilometres lower than the southern hemisphere - like two different half spheres have been glued together. Mars is also a planet currently occupied by robots!

• Jupiter’s mass is greater than all the other planets combined, and the planet also features a massive cyclone that is bigger than Earth, known as the Great Red Spot - which has raged for hundreds of years.

• Saturn is the least dense of all the planets in our system - so little is its density that it would float if you could submerge it in a large enough ocean

• Like Venus, Uranus rotates east to west - however, the entire planet is tilted on its side by almost 90-degrees

• Winds on Neptune travel at 2,000 km/h, which is faster than the speed of sound on Earth and as fast as jet fighters

As we learn more about other planetary systems, we are beginning to decipher their structure. We can see their giant planets, and have begun to detect signs of debris belts, like the Asteroid and trans-Neptunian belts in the Solar system. In the future, researchers hope to search for ‘Earth-like’ planets in those systems - places that might be suitable places for life to exist. That search will be greatly helped by our knowledge of the Solar system - but we have to be careful with the assumptions we make.

For decades, it was thought that giant planets (like Jupiter) are critically needed for planets like the Earth to host life. The idea was that Jupiter has acted as a celestial ‘shield’ to the Earth, deflecting asteroids and comets that would otherwise have smashed into our planet, preventing life from becoming established and thriving. 

But there’s a catch - more recent and detailed computer simulations have revealed that Jupiter’s role is more complex. Whilst it can take objects that threaten the Earth and fling them away, never to return, it can also take objects that would otherwise come nowhere near our planet, and fling them our way. Rather than being a shield, Jupiter is actually more of a threat - were it absent, the impact rate on Earth would actually be reduced!

An interesting story of our Solar system is that of the ‘missing planet’, often known as the hypothetical Planet X. Inspired by tales of the discovery of Neptune, a century and a half ago, astronomers have proposed a number of potential ‘undiscovered planets’, lurking in the distant reaches of the Solar system, to explain oddities in the Earth’s extinction record, and, more recently, the orbits of objects beyond the orbit of Neptune.

Whilst ‘Nemesis’ - the planet that was in vogue in the 1980s - has now fallen on the scrapheap of history, in recent years, interest in the existence of a planet beyond Neptune has returned. Some scientists believe that this planet exists out there, and that it is exerting a subtle gravitational influence on several extreme trans-Neptunian objects, following elongated orbits well beyond Neptune’s orbit. In particular, some studies of the orbital parameters of these bodies argue that there must be a large planet (often called ‘Planet Nine’, to distinguish it from previous suggested extra planets), roughly 2-5 times the mass of Earth, in this region to explain these occurrences. 

If Planet Nine does exist, it would be very cold - receiving only a small fraction of the sunlight/heat that Earth would receive - and given its distance, it would be very hard to try and spot it against the background stars and other objects. 

Though, if it did exist, it would reshape our entire thinking about our system. Where did Planet Nine form? How did it get to where it is? Does it have any moons or rings? What is it composed of? The discovery of Planet Nine would change the evolutionary story of Sol’s system, with learnings applied to exoplanetary systems.

Exoplanets are astronomical planetary bodies which orbit other stars. The first detected exoplanets were found in the late 1980s and early 1990s but since then, over 4,200 have been confirmed - almost 1800 of which exist in systems that contain more than one known planet. 

Exoplanets have been discovered through a number of different observational methods, including analysis of the radial velocity of the host star being disturbed by the orbiting planet (much like how Jupiter causes the Sun to orbit around a barycentre), or through the dips in light curves from the host star, as the exoplanet transits across its surface from our line of sight. 

A number of exoplanet hunting telescopes that have been launched into space, such as the famous Kepler and TESS observatories, have really increased the number of confirmed planets orbiting other stars. These observatories, equipped with much more sensitive equipment, have really provided a high quality of data with many scientists now believing that most stars across the galaxy would host at least one planet or likely a planetary system around it.

Whilst our study of the Solar system has great implications for the study of other planetary systems, the search for alien worlds is now yielding results that inform our understanding of the Solar system. Indeed, that work has revealed that the Solar system might be somewhat unusual, when compared to the myriad other planetary systems found to date. For example, astronomers have, in recent years, found an abundance of super-Earths and mini-Neptunes, two types of planet that were initially thought to be rare - and which are totally absent in our own planetary system.

In fact, we are now finding more super-Earths and mini-Neptunes than any other kind of exoplanet - which has prompted some researchers to wonder why we don’t see any such planets in our own backyard. As our methods for finding exoplanets continue to improve, it is likely that we will find many more of these peculiar planets, but it may well be that we eventually discover that even smaller planets are still more common - finding planets of a similar size to the Earth, or even smaller, is still incredibly challenging!

Exploring the Solar system as if it were an exoplanetary system can help us consider the limitations of our current studies of exoplanets. For instance, only exoplanets with orbits close to their host star can be readily analysed, as those with longer orbits further from their star may not be noticed. This method also emphasises that our Solar system is in only one stage of its evolution, while other systems may be in earlier or later stages, and therefore look very different. 

Learning more about the Solar system and exoplanetary systems helps us to better understand these systems in comparison with each other. This enables scientists to then build up their knowledge to further understand our universe, especially when it varies greatly from what we expect. 

What kind of mysteries await discovery about our own Solar system that we can then apply to Exoplanet systems? Or, what do we notice about exoplanetary systems that we find at odds with our system? Is there a two-way learning objective we can establish through observations?

This kind of information will hopefully help us discover new exoplanets, new worlds, and new perspectives of our relative place in the universe. 

Asteroid video credit: Cosmographia.
Jupiter video credit: NASA Goddard.