Out of the thousands of star systems discovered thus far, there is nothing, not even remotely, like our Solar System.
The only type of news that makes the headlines when it comes to exoplanets is usually a discovery pertaining to a new “Earth-like” planet, since people are fascinated with the prospect of finding another planet similar to our own, and rightly so. The idea of finding life somewhere else in the galaxy is absolutely tantalizing. But it often overshadows a lot of the other things we’re learning about exoplanets, and as a consequence, what we’re learning about the Solar System.
We’ve been trying to figure out the nature of our own system’s formation for a while, long before the first exoplanet discovery. We’re talking about over a century of research, centuries even if we’re going to go back before Newton & Kepler. And our only means of doing so has always been to look at the one planetary system we’ve ever been able to study, which is our own. And that has been the basis for everything.
The moment we were able to get advanced enough technology to detect exoplanets, particularly multiple planet systems, we started noticing just how weird they could be. It seemed like every multiple planet system forced us to almost completely change what we’ve always thought about planet formation, as we keep discovering new types of systems that we didn’t even imagine could be possible.
The first systems ever found consisted of this truly bizarre type of planet which eventually became known as a Hot Jupiter:
If that looks to you like an entire planet on fire, you aren’t far from the truth. These are planets that are gas giants like Jupiter, but orbit extraordinarily close to their star, making them much higher in temperature. Calling them “hot” is an understatement. These planets are absolutely broiling, reaching temperatures of thousands of degrees (both in Fahrenheit and Celsius, which is saying something). One of the hottest ones, Kelt-9b, is about 7,800°F (4,300°C). For comparison, the surface of the sun is about 9,900°F (5,500°C), which is really the same ballpark.
This close proximity also leads to something called tidal locking, which means that the planet’s rotation rate on its axis is identical to its orbital period, and so only one “side” of the planet ever faces the sun. I put “side” here in quotation marks because it’s a little bit more complicated when it comes to gas giants since they don’t have a solid surface in the terrestrial sense. Hot gas will still circulate to the night side, across huge convection cells going around the entire perimeter of the planet, and likely also deep within its interior. A lot of the gas also gets blown away from the planet altogether. This sometimes forms a comet-like tail trailing behind the planet as it orbits its star.
Again, I should really emphasize just how hot we’re talking here. Sure, “thousands of degrees” sounds like a big number in our head, but that’s kind of hard to actually imagine. To give you an idea, the atmosphere of Kelt-9b consists of a titanium and iron vapor. You know... Metal. But as a gas. Kelt-9b’s temperature is above the boiling point of most metals (and yes, they do actually have boiling points when you start getting above 5,000–6,000°F, or 2,700–3,300°C).
As this metallic vapor circulates to the perpetual-night side, it likely cools, potentially to the point where it condenses into molten metal that rains down to lower altitudes, before being circulated back towards the perpetual-day side of the planet.
If that wasn’t crazy enough, the idea of having a gas giant that close to its parent star is pretty bonkers just from a formation standpoint. We’ve always thought the reason why we have gas giants in the outer Solar System is because there is a higher concentration of solid material out there due to the appearance of ices at those distances. More solid material, build bigger planets. Seems pretty straightforward right?
Well, apparently not, because Hot Jupiters are actually pretty common. It turns out that a planet’s orbital parameters can change dramatically during the early stages of its home system before sinking into whatever orbit it finds stable for the next few billion years. This is called planetary migration. The gas giants of the Solar System did a bit of this too. They just did it outwards. Needless to say, the Solar System today would have been very different had Jupiter migrated inwards instead. Hot Jupiters may have formed more or less in the same place Jupiter did, but then slowly moved inwards, perhaps due to a slightly different magnitude and gradient in density of the protoplanetary disk it was forming in.
As weird exoplanet discoveries go, Hot Jupiter systems were only the tip of the iceberg. With the launch of the Kepler mission as well as development of adaptive optics in many Earth-bound observatories, such as Keck & TRAPPIST, our understanding of exoplanets grew exponentially, and we found even stranger systems.
Most prominently, we’ve discovered yet another category of planet called a super-Earth. The name stems from the fact that these were the first planets discovered that had a sufficiently low mass that they likely had a solid surface like the terrestrial planets of the Solar System. And so the “super” in this case is just a latin prefix meaning “bigger than,” which isn’t really a statement of how Earth-like they are. Quite to the contrary, as we would find out later, they are probably nothing like the Earth in many cases, but we’ll get to that later.
While we don’t have any planets like this in our own system, we have discovered these types of planets are extremely common, and are probably the most common type of planet we have discovered yet. However, this may be a result of observation bias, which skews our findings towards larger planets since smaller ones are harder to detect.
Not only that, but we have discovered entire systems comprised entirely of Earth-sized and super-Earth planets. They are especially common around low-mass stars called red dwarfs, also known as M dwarfs, often in very close-knit systems. The most well known system of this kind is one you have probably heard of before. It’s called TRAPPIST-1.
This system consists seven planets, all within 0.7–1.2 Earth radii in size, all orbiting their parent star within about 6% of the Earth-sun distance. Their parent star is quite small, only slightly larger than Jupiter in size, but much, much higher in mass, being a star.
Systems like this might form from bands of high density within the protoplanetary disk, leading to the gravitational runaway trapping of material within that region (called a trapping zone) as dust migrates inwards due to aerodynamic drag and dynamical instabilities. (This has no relation to the system’s name “TRAPPIST,” by the way, which is simply the name of the observatory that discovered it, a backronym referencing Trappist beer.)
Once the planets form, they all migrate inwards together, eventually ending up in near-resonant configurations, where their orbital periods are related by simple integer fractions like 3/2, 4/3, 5/3, and 8/5. They are all tidally locked with their parent star, which again means that one side is in perpetual day, and one in perpetual night.
Kepler has found plenty of other systems very similar in nature to TRAPPIST, including some systems made entirely out of larger-radius super-Earths. These larger radius planets occupy a weird grey area in our understanding, since they are right at the boundary where they could potentially be terrestrial or a gas planet. Without an analog within our own system, it’s hard to judge what these planets are like until we have a mass empirically measured for them, perhaps from radial velocity measurements of their respective parent stars.
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However, from what we’ve seen so far, these planets might actually be something in between. These might in fact be ocean planets, something that doesn’t really exist in the Solar System, though Earth might be close to qualifying as one. One example is Kepler-22b, which is about halfway between the radius of Earth and Neptune:
Kepler-22b is likely covered in an extremely thick atmosphere, though not quite comparable to the enormous gas envelopes of the gas giant planets, which form from runaway gas capture. Beneath that atmosphere is a dense, hot, ocean of liquid water covering the entire globe. The pressures at the base of this ocean are likely so great that there may be a fairly gradual transition to the planet’s solid crust, or even transitioning straight into the mantle, instead of having a distinct “sea floor.”
Despite the abundance of liquid water, such an environment may not be as hospitable for life as it would seem, and the lack of continents and associated geological processes (such as erosion depositing photo-chemically enriched material into the ocean) may mean no ingredients for organic molecules and the basic building blocks of life, at least as we know it. This type of planet, however, may comprise of one third of all planets in the galaxy.
All these discoveries have been moving towards a pretty startling trend. We keep finding all these types of planets that we think are so weird and bizarre, only to find that they are quite common. Typical even.
In fact, the strangest system out of all of the planetary systems we know of, as luck would have it, is actually the Solar System.
Out of all of these exoplanet systems that are so strange and fascinating, what a lot of people don’t realize is just how absolutely peculiar and outlandish our home system is.
We already knew that having life on Earth makes us at least a little bit of an outlier, but we really had no idea just how much of an outlier we are, in just about every characteristic of our home system.
It is really, really, really weird to have a system with so many different types of planets in the same system, all spread out over such widely varying orbital distances, with such widely varying characteristics, mediated by an asteroid belt of unformed protoplanetary material, And on top of all of that we have the Earth, a planet so perfect in mass and temperature that it can sustain an atmosphere with sufficient pressure for liquid water, but not so much as to cover the entire planet in ocean, leading to extremely complex geological processes through the interaction of the ocean and the continents, along with plate tectonics, a volcanically active interior, and a convective outer core supporting a powerful magnetic field.
And then of course, there’s the fact that life exists here, perhaps as a consequence of all that.
When it comes to all the exoplanet systems we discovered, on the other hand, as diverse as they are, a recent study showed they all share one thing in common: most planetary systems all have planets of roughly the same size as one another, roughly the same mass, and are spaced out rather evenly or according to orbital resonances.
And so you typically get systems with handful of gas giants ranging from Neptune to Jupiter sized (like 55 Cancri), or you get systems with half a dozen super-Earths (like Gliese 581), or you get systems with just as many terrestrial Earth-sized and sub-Earth planets (like TRAPPIST-1). Rarely do we ever see systems with many planets of all different kinds and sizes.
And yet here in the Solar System, we have planets ranging from the minuscule size of Mercury (which is smaller than one of Jupiter’s moons!) to the behemoth that is Jupiter itself, a planet so large there is still debate as to whether it actually formed like a planet typically does, from a solid core. Here is a little infographic with a size comparison of the planets of the Solar System compared with TRAPPIST-1 (sizes are shown to scale, but not distances):
In fact, another weird part in all of this is that while the Solar System is such a huge outlier as planetary systems go, the Jovian system (Jupiter & its moons) by itself actually has a lot more in common with most exoplanet systems.
Just compare it to TRAPPIST-1 (planet sizes are to scale, and distances are to a different scale, for visibility):
Jupiter’s four largest moons, Io, Europa, Ganymede, and Callisto are all of comparable size to each other, on orbits that are spaced out in resonances, just like TRAPPIST and other close-knit terrestrial planet systems. Really, the main difference is the fact that Jupiter itself isn’t a star, or even a brown dwarf by the current definition.
And that actually may be the root of all the weird aspects of our Solar System.
Even before we started discovering exoplanet systems, we always knew that Jupiter’s influence in how the system formed was extraordinarily significant. And that’s because of its absolutely huge mass for a planet. In fact, discounting the Sun, Jupiter has about 68.2% of the Solar System’s entire mass. Think about that. 68.2% of the mass of all the things orbiting the sun is concentrated in one, singular place. Jupiter.
We have the asteroid belt because of Jupiter, since its mass causes dynamical instabilities in the orbits of objects that cross orbital resonances interior to it. Jupiter migrated outward in the early Solar System, which in turn forced the other gas giants to also outward-migrate, leading to a cataclysmic era known as the Late Heavy Bombardment. As the giant planets moved outward, they plowed through regions of unformed, icy, protoplanetary material, gravitationally slingshotting them every which way, which is why the surfaces of all terrestrial worlds in the Solar System are so heavily cratered, excluding the surfaces that are subject to processes like erosion, such as the Earth.
And most significant to us, Jupiter is what led a protoplanet the size of Mars to come crashing into the proto-Earth billions of years ago in the Early Heavy Bombardment when much of the Solar System was still forming, erupting a sizable chunk of the proto-Earth’s crust into outer space:
A decent amount of this material went into orbit around the proto-Earth, forming a short-lived ring that eventually coalesced into the Moon, which we now have to thank for our ocean’s tides, which most biologists agree is a significant contributing factor to the origin of life on Earth.
In that regard, in many ways, we owe much of our entire existence to Jupiter and the unique way our Solar System developed.
Jupiter likely formed in a trapping zone just outside the Solar System’s ice line, where the temperature drops below freezing, allowing “volatiles” like water, methane, ammonia, and other hydrocarbons to solidify into ices. This alone isn’t too unusual. We already mentioned that trapping zones are likely common in many forming planetary systems, and are perhaps even a critical part of that process. And it makes sense to have one where Jupiter is, since the ice line provides an abrupt jump in the density of material within the disk, the perfect conditions for runaway gravitational trapping.
What may set the Solar System apart however, is that it may have had multiple high density regions within the disk. In addition to our high density jump at the ice line, there may have also been a high density jump somewhere on the inner edge of the protoplanetary disk, similar to how TRAPPIST-1 formed. Meaning, our Solar System has the characteristic of being two planetary systems in one. The terrestrial planets (the inner Solar System) and the gas giants (the outer Solar System).
Jupiter, of course, dynamically disrupted a lot of what was going on in the inner Solar System, and it’s possible there could have been other planets that may have gotten tossed or knocked into the Sun, or out of the Solar System altogether. Ceres could have been another major planet, if it weren’t for Jupiter gravitationally disrupting that region, which will forever be the asteroid belt, and Ceres forever a dwarf planet.
Depending on what kind of data Juno returns over the remainder of its mission, we might need to reconsider what class of object Jupiter actually is. Clearly it’s very different than a star, but it is also vastly different than the other gas giants, both in its mass, its role in the Solar System, and its potential lack of a distinct, solid core. Perhaps Jupiter should be considered a brown dwarf, or perhaps be considered a new class of object that lies somewhere in between planet and brown dwarf. Perhaps, “super-planet” or something along those lines.
So maybe our search for an Earth-like planet shouldn’t be restricted to just looking at systems with planets of similar radius and temperature to the Earth, but also be looking for systems with a Jupiter-like planet a middle of the way out from its parent star.
Or perhaps our Solar System isn’t that unusual after all, and the fact that we haven’t seen anything like it yet is a matter of observation bias, since planets with orbital periods as long as Jupiter and the other gas giant planets are much more difficult to detect, since it takes so many years to make a single orbit, and usually a confirmed detection for a planet requires more than one pass of its star.
Time will tell!