The solar system is familiar. It's home. You've probably learned about all the planets in school, memorising their names and their order of distance from the sun.
The four planets closest to the sun are rocky, with solid surfaces you can walk or land a spacecraft on. Then you have the four outer planets (excluding Pluto), huge spheres of gas surrounded by rings. In between lies the asteroid belt like a cosmic moat.
Planets are weirder than our own solar system would lead us to expect
It's a tidy configuration, and for about a century and a half, it was all we knew about planets. Then, in 1995, everything changed.
That's when astronomers discovered the first planet orbiting another star, a Jupiter-like gas giant named 51 Pegasi B. Over the next two decades, astronomers would discover thousands more worlds. According to estimates, as many as hundreds of billions of planets populate the Milky Way galaxy. The solar system, we now know, is far from alone.
The multitude of planetary systems seems to be yet another fact of our cosmic inconsequence, in which our corner of the universe is just like any other. But while planetary systems abound, astronomers are finding that in some respects, the solar system stands out.
It's still too soon to know for sure how odd the solar system is (odd like your quirky uncle, or odd like a leprechaun riding a unicorn?), but scientists are already trying to explain why it might be so. If it turns out to be a cosmological anomaly, then so might be Earth - and life. Maybe, we really are special.
A space oddity
Once you get over the fact that planets are as common as stars, you're faced with their startling diversity. "We kind of always vaguely hoped and expected planets to be common," Laughlin says. "And that's absolutely right - they are common. But they are weirder than our own solar system would lead us to expect."
With the help of the Kepler space telescope, which has discovered thousands of planets around other stars, astronomers are finding planets and planetary systems of all shapes and sizes. They've found miniature systems - cute little guys comparable in scale to Jupiter and its four biggest moons. In other systems, planets orbit at large angles compared to the rotational axis of its star. A few systems even orbit two stars at once, getting Star Wars fans excited about a real-life Tatooine planet with two suns.
In the solar system, planets are either small and rocky or big and gassy. But now, astronomers have found that most other planets don't fit in either category. Instead, they're in between: smaller than Neptune but bigger than Earth. The smallest of these, sometimes called super-Earths (a somewhat misleading term, since a super-Earth isn't necessarily like Earth at all – it’s just a planet that’s a bit larger) might be rocky. But bigger ones, dubbed sub-Neptunes, are puffier and mainly made of gas.
There aren't even any asteroids down there
What's stranger is that a lot of these planets orbit extremely close to their stars - closer than Mercury is to the sun. When astronomers first discovered these tightly orbiting planets in 2009, most were sceptical. "It seemed so crazy that people really didn't believe it," Laughlin says. But then Kepler, which launched in 2009, confirmed that they do indeed exist - and they're everywhere. In fact, half of the stars in the galaxy might host super-Earth-type planets on close orbits.
That, Laughlin says, is one of the biggest differences about the solar system. "We really have nothing interior to Mercury's orbit," he says. "There's zilch. There aren't even any asteroids down there."
The other oddity about the solar system is Jupiter itself. Big planets aren't that common, and most of them are on orbits similar to Earth's or Venus's. Only about a couple percent of stars have a Jupiter-sized planet at a distance comparable to Jupiter's orbit.
"Having nothing interior to Mercury's orbit and having Jupiter itself - a massive planet on a Jupiter-like orbit - combine to make us unusual," Laughlin says.
No one knows why the solar system might be unusual, but Laughlin has an idea. The explanation involves an elaborate scenario in which a nomadic Jupiter swooped in and destroyed infant planets, altering the fate of the solar system and clearing the way for the Earth we know and love.
Planets are born on the heels of their stars, which form when a cloud of gas collapses into a dense ball. The leftover gas and dust that surrounds the newborn star spins and flattens into a disc like a pizza. It's in this disc that matter clumps together and creates new worlds.
Astronomers used to think that the planets in the solar system formed right where they are today. Near the young sun, it was too hot for volatile compounds such as gases and ices to exist. The only construction materials were rocks and metals, so only small, rocky planets could form there. Only farther out in the cold could gases and ices accumulate and grow into the gas giants we see today.
We used to look at the giant planets, and think those are big, so they never moved
But astronomers discovered gas giants orbiting extremely close to their stars, where it would've been too hot to form. These so-called Hot Jupiters, astronomers realised, must have migrated from farther out, away from the heat. Planetary migration could even be common, and the gas giants in our own solar system might have had a well-traveled past themselves.
"We used to look at the giant planets, and think those are big, so they never moved - that was our anchor point," says Kevin Walsh, a planetary scientist at the Southwest Research Institute in Boulder, Colorado in the US. In theories of planetary formation, scientists had assumed the gas giants remained fixed. But now, he says, "that anchor point? It's gone."
Walsh says it was Jupiter that made a move early in the solar system's history. The scenario is named the Grand Tack model, after a particular zigzag manouvre in sailing. Like a sailboat, Jupiter zigged and zagged across the solar system.
According to the theory, Jupiter formed a little closer to the sun than it is today, at a distance of about three astronomical units (an astronomical unit, AU, is the average distance between Earth and the sun). At this time, the solar system was only a few million years old - just a baby - and still filled with gas.
As Jupiter circled the sun, the gas outside of Jupiter's orbit pushed on the planet, nudging it into the inner solar system. When Saturn formed, outside of Jupiter's orbit, it disrupted the gas that was forcing Jupiter inward. That disruption halted Jupiter's migration as it got to about 1.5 AU of the sun.
Let's suspend our disbelief. Let's take this seriously and ask what the consequences are
Meanwhile, the interactions between Jupiter and the gas inside of its orbit were pushing the planet back outward. Now that nothing was forcing Jupiter inward, the inner gas could push it back out, bumping it to its current orbit of 5.2 au.
The model has gotten planetary scientists excited because it describes many unexplained features of the solar system. Jupiter's back-and-forth journey clears out all the gas beyond 1 AU, a condition that other astronomers have suggested is needed to form Mars. Previous models produced a Mars that's too big, but the Grand Tack makes one that's just right.
The Grand Tack also generates an asteroid belt very much like the one we see today, with the proper masses, orbits, and mix of compositions. Although it doesn't explain why Jupiter exists in the first place - no one really knows the answer yet - it does show how it got to its relatively wide orbit.
Still, Laughlin admits that the whole thing sounds a little too complicated, and a bit crazy. "You have to be really sceptical of that, and indeed, I was sceptical and in some sense, I still am sceptical," he says. But given its success so far, Laughlin and another theorist, Konstantin Batygin of the California Institute of Technology in Pasedena, US, decided to take the Grand Tack a step further. "Let's suspend our disbelief," Laughlin says. "Let's take this seriously and ask what the consequences are."
Jupiter on the warpath
Those consequences, it turns out, are violent. Computer simulations showed that when Jupiter infiltrates the inner solar system, it wreaks havoc. The region is filled with gas, dust, and half-formed planets - lumps of up to 1,000 km in diameter called planetesimals. As Jupiter circles inward, it plows through all this material, triggering a cascade of collisions between the planetesimals, which smash each other into tiny pieces. Once they're as small as about a kilometre wide, they're so light that the surrounding gas drags them down into the sun.
Given the preponderance of super-Earths around other stars, there's a good chance a couple of them can also form along with the planetesimals. But as Jupiter approaches, the super-Earths become gravitationally locked with the fledgling planets. When Jupiter sends the planetesimals plunging into the sun, they pull the super-Earths with them.
The kinds of systems that have super-Earths don't tend to have giant planet companions
When Jupiter returns to the outer solar system, it leaves behind just enough debris to form Earth and the other small, rocky planets. Thanks to Jupiter's path of mayhem, the would-be planets close to the sun never had a chance - which explains why the area inside Mercury's orbit is clear and empty today. Otherwise, the inner solar system would've been filled with super-Earths, instead of Earth and the other rocky planets.
That's the idea, anyway. It's a great story - an intricate chain of events that just emphasises the weirdness of the solar system. If it's true, then it should have happened in other planetary systems as well. Because a planet like Jupiter wipes out super-Earths, other systems should only have one or the other.
So far, the data are promising. "The initial indications look pretty good," Laughlin says. "The kinds of systems that have super-Earths don't tend to have giant planet companions at larger distances."
Astronomers won't know for sure until at least the launch of the Transiting Exoplanet Survey Satellite in 2017 (see above for a NASA video about the satellite and its mission). TESS will be hunt for planets around stars in the solar neighbourhood, and are thus bright enough for the subtle measurements that astronomers need.
Still, Laughlin isn't going all in on this hypothesis. "We're just finding out that the solar system is unusual," he says. "This is really one of the first efforts to understand this brand-new realisation. I'm sure there will be lots of other ideas—some of which will probably be very compelling."
The bigger question, though, is just how weird is the solar system. "Every indication right now looks like we might be rare," Walsh says. But at the same time, the planetary census is far from complete. "The jury is still out," he says.
Astronomers simply haven't detected many planets like those in the solar system yet. "It's more difficult to see the systems like our own by any of the existing planet finding methods," says Jim Kasting, a planetary scientist at Penn State University in State College, US. "The fact that we haven't seen many systems like our own doesn't mean they're not common. It just means they're not that easy to see."
In particular, planets smaller than Earth are still just beyond the reach of current telescopes. Not even TESS will be able to detect Earth-sized planets on Earth-like orbits around sun-like stars.
And finding bigger planets like the ones in the outer solar system will require more time. One of the main techniques to detect planets-which Kepler and TESS use, for example - is to look for the slight dimming of starlight when a planet passes in front of it. But because planets with wide orbits take so long to go around their stars (Saturn, for example, takes 29 years), astronomers will have to look for decades before seeing such a transit.
There are planets on which life, if it were able to start, could flourish
But data isn’t lacking when it comes to super-Earths on orbits tighter than Mercury's - or super-Earths in general. "We know those are extremely common," Laughlin says. Astronomers also know that gas giants on Jupiter-like orbits are not common. And the sun, while not rare, is only similar to about 10 percent of the galaxy's stars. So to some degree, at least, the solar system is weird.
Of course, "weird" is subjective. Some estimates say that up to a fifth of sun-like stars in the galaxy have planetary systems like ours, which puts the total fraction at a couple percent. A percent or two sounds small, but remember that the galaxy could have hundreds of billions of planetary systems. One percent still leaves tens of billions of other solar systems.
"I would be very surprised if the solar system were really strange," says Jack Lissauer, a planetary scientist at NASA Ames Research Center in California, US. "There are so many stars out there. Even if it's only one percent, it's still not really rare."
Whether other solar systems give rise to a truly Earth-like planet - a pleasant place for life - is a question with even fewer answers. "There's zero evidence that Earth-like environments are common," Laughlin says. "There's zero evidence that life is common."
But for some, the huge numbers raise hopes of an Earth twin. "I think there are planets like that," Lissauer says. "There are planets on which life, if it were able to start, could flourish."
Kasting is also optimistic. "I think our solar system is not unique," he says. "There are likely other planetary systems that are not much different. But, of course, we don't know that - that's why you need to build telescopes and make the observations."
And instead of weirdness, you might very well see something familiar.