It was once thought that our Solar System was fairly normal. We modeled our planetary accretion disk theory around the idea that gas giants formed on the outer edges of a planetary dust cloud and rocky ones formed closer to the sun. However, exoplanetary systems have completely blown this idea out of the water, and as a result, we’ve discovered that our solar backyard isn’t so normal after all.
Here are ten unusual things about our Solar System.
10. No Hot Jupiters
Hot Jupiters were one of the first types of exoplanets discovered in the early days of exoplanet hunting. They orbit uncomfortably close to their parent stars and burn incredibly hot. The surprising thing for astronomers is that there are far more close orbiting gas giants than close orbiting rocky worlds like Mercury.
So, why don’t we have one? Well, some scientists have proposed that we did at one point, but this hypothetical Hot Jupiter migrated out of the inner Solar System, wreaking havoc on the inner worlds of the Solar System like a wrecking ball through an antique store. The idea is plausible, but there’s little evidence to support it. The fact of the matter is, we don’t have enough data to suggest why our Solar System has so many gas giants in it. At least not yet.
One theory is that our Jupiter started off as an Earth-sized asteroid and slowly collected gasses in the stellar cloud around the sun, following a 700,000,000 year orbit that some scientists dub “the Grand Tack” which is named after the way a boat will maneuver in and around a buoy. This Grand Tack orbit would have sent asteroids hurtling toward the forming super-Earths in the early Solar System, causing them to shatter and reform—much like the theory of the moon’s early formation (which suggests that a large body, about the size of Mars, crashed into a hypothetical planet called Theia causing the formation of the Earth and Moon).
9. Where Are the Super-Earths?
Super-Earths are massive rocky worlds which are often twice as large as our Earth, and they can get even larger than that. We’ve discovered that 30 to 50 percent of exoplanets observed are close orbiting super-Earths. Their prominence has gotten scientists wondering why there aren’t any in our Solar System too.
In 2004 astronomers discovered 55 Cancri e, a hellish Super-Earth twice the size of our home. The fact that it orbited closer than Mercury does in our Solar System confused astronomers and got them asking some important questions. We still don’t know why hot Super-Earths form, but we do know that they’re too common for their absence in our own system to be ignored. Moreover, a fifth of 55 Cancri e’s mass is thought to be composed of lighter elements. It’s thought that much of the surface elements are trapped in a supercritical state (behaving both as a liquid and as a gas, a phenomenon seen in the depths of gas giants in our Solar System).
As our simulations for planetary system formation have improved, we’ve begun to see that worlds too low in mass (like ours) tend to be absorbed by larger ones. So, why didn’t this happen in our system?
Well, one theory is that early Super-Earths did form, but were destroyed (as described in the previous section).
Another theory suggests that the entirety of our Solar System was erased before Jupiter moved to its current location, forcing the rocky worlds we know and love today to be formed late in the game.
And a third theory suggests that both the sun and a gas giant gobbled up most of the early planet-forming materials, eliminating any possibility for the formation of a Super-Earth.
8. Why Doesn’t the Sun Have a Twin?
Astronomers even searched the skies for a hypothetical sister star called “Nemesis” whose gravitational influence was thought to have sent the asteroid that killed the dinosaurs our way—but they never found it.
Though Nemesis was never actually found, astronomers maintain that this does not mean it never existed. Using radio telescopes to pierce the Perseus nebula, a place dozens of young sun-like stars call home, researchers at Harvard’s Smithsonian Astrophysical Observatory compared the distances between binary star systems in the nebula and noticed something peculiar. In the Perseus nebula, there are three different ranges of distances between stellar twins; most of them are born orbiting close to each other, there is a midrange of stars which are born orbiting each other a bit farther out, and a final classification of stars born more than 500 Astronomical Units away from each other. That’s more than the distance from the sun to Neptune. The current theory is that our sun did in fact have a twin, but only for the first couple million years of its lifespan. After that first couple million years, the two separated from each other due to the distance between them and influence from other gravitational influences.
7. The Problem with Life
Most of the rocky worlds we’ve observed have been discovered around red dwarf stars, and most of the Hot Jupiters we’ve discovered orbit stars larger than ours. The potential for life in many of the exoplanetary systems we’ve discovered is quite low. What’s worse for the case of life in these other systems is that most of the rocky worlds are Super-Earths. Super-Earths are thought to be far too large to be ideal for the development of complex life.
So, why does our system have life? What makes it so special?
The TRAPPIST-1 system was a massive discovery in the scientific community. To think that there was a system with seven rocky Earth-like worlds! But as quickly as people were to be captured by fantasies of visiting these worlds, new data showed that the tidal forces exerted on these planets by their red dwarf parent (even those within the habitable zone) would force them to become tidally locked. This didn’t mean that the development and evolution of complex life on these worlds wasn’t possible, but it definitely suggested that we wouldn’t find much more than bacteria or simple lifeforms in TRAPPIST-1.
Recently, though, we found the first Earth-like planets orbiting around remarkably similar stars to our own. In fact, KOI-456.04 is thought to hold a lot of promise, according to scientists. After re-examining data collected by the now-retired Kepler Space Telescope, astronomers decided to survey KOI-456.04’s parent star and discovered that the exoplanet was less than twice the size of Earth. That’s not all, though, more Earth-like worlds have been observed around sun-like stars.
Though we’ll have to wait for more data to emerge on the composition of these worlds, the prevailing theory is that complex life is likely to only show up in worlds around stars like our own, making our Solar System more than a little special.
6. Where are the Red Dwarfs?
Back in the ’80s, when the idea of Nemesis was first theorized, it was thought that the sun’s companion would be a red dwarf. Red dwarfs make up the majority of stars in our galaxy and most of the exoplanets we’ve observed so far orbit these types of stars. In fact, we recently discovered a system of multiple Super-Earths orbiting red dwarf GJ 887.
Stars like our own only make up about 10% of those observed in our galaxy. The absence of a red dwarf twin or a red dwarf, in general, makes our Solar System just a little bit stranger.
5. The Division of Rocky and Gas Giant Planets Is Too Even
One of the most common elements of the orbits of the 4,000 exoplanets discovered so far is that they aren’t uniform. Far from it. Many of them have intermingling large and small-bodied worlds—imagine a complicated dance between Hot Jupiters, distant Jovian class worlds, and Super-Earths and you get a basic idea of the chaos the majority of the systems in our galaxy experience.
Well, researchers went so far as to compare the average size of exoplanets to their neighbors, hoping to find some kind of pattern that would make sense and explain why the planetary neighbors in our system seem to be so similar to one another.
The consensus? There was no pattern, and thus, no answers.
4. Our Star is Unusually Calm
Our sun can get a bit rowdy every now and again, but for the most part, it’s fairly calm—especially when compared to other stars in our galaxy, the majority of which are extremely active.
In fact, an analysis of 369 stars which are similar to our own showed that of these stars, a large majority of them were far more active than ours. There are many factors to consider when thinking of the relative calm nature of our star. We have data going back at least 8,000 years thanks to tree rings and other, similar ways of examining the effects of solar activity on our planet, but that period of time is nothing compared to the age of the sun. It’s unknown if we’re simply the lucky ones, who won the stellar lottery, or if our star is going through a quiet phase.
But at the moment, it does appear as though our star is far calmer than others like it.
3. Many Massive Planets in Exoplanetary Systems Have Eccentric Orbits
We already know that Hot Jupiters make up a large number of exoplanets in the Milky Way and we know that our Solar System is lacking them. But what about their orbits?
While the orbital shape in exoplanetary systems tends to be largely circular, this is not true when it comes to Hot Jupiters. In fact, according to some research, gas giants in general seem to have more eccentric orbits in other systems. And in fact, even if there are a decent number of gas giant exoplanets with circular orbits, most of them tend to orbit close to their parent star. Is the only thing keeping Jupiter in place really Saturn’s gravitational pull?
Why is that?
Unfortunately, without more data relating to the early Solar System’s history, we may never truly know.
2. Most Planets Don’t Have Stars
It’s estimated that on average stars in the Milky Way have about 10 planets orbiting around them. That’s fairly impressive. However, with the discovery of an abundance of exoplanetary systems came with it another discovery, the existence of rogue planets. Scientists think that the abundance of these rogue planets is due to early ejection from their birth systems. So, it’s possible that at one point, not only did our parent star have more planets in it, but that they were jettisoned from their orbits into the cold depths of space as well.
Rogue planets are thought to dwarf the number of those orbiting stars in the Milky Way (and possibly even the universe), and many of those planets have partners such as moons, brown dwarf companions, and gas giants like Jupiter. So, if we can call these “rogue planetary systems,” and the majority of planets in our galaxy don’t have a parent star, then doesn’t the fact that we have a star of our own (a rare one at that) make our Solar System just a little bit more unique?
1. The Absence of Water Worlds
Within the loose classification of worlds dubbed super-Earths, a majority of them are thought to be “water worlds.” While our Solar System does have the ice giants (Uranus and Neptune), and Neptune does have a relatively decent amount of water, water worlds are generally not as massive and have a crust. It’s possible that at one point in the evolution of our Solar System there were water worlds orbiting around our star, but unlike the idea of Jupiter originating close to our sun, there’s not much evidence to support this idea.
But water worlds may be far more impressive than you might be thinking (especially if it conjures images from that horrible Kevin Costner movie). They may have oceans hundreds of kilometers deep, and because of their immense mass and gravity, have a better chance of holding onto their internal heat.
They’re not just smaller versions of Neptune or Uranus; at least, we don’t think they are. This may mean that these so-called water worlds could be far more likely to produce life than we originally thought, setting them apart from other, rockier Super-Earths, and making our Solar System just a bit weirder for not having one of its own.