Why doesn't our solar system have any gas giants close to the sun?

As we learn more about far distant solar systems, it becomes clear just how weird the universe really is. And, more disconcertingly, how unusual our own solar system is. And that starts with our curious lack of so-called "hot Neptunes."

Hot Neptunes- and their even bigger counterparts, known as hot Jupiters - are gas giant planets that orbit very close to their star, often much closer than even Mercury does in our own solar system. Their names are simply ones of convenience - the biggest are the size of Jupiter or even larger, while the mid-sized ones are roughly the size of Neptune, and so their names reflect that.

That, in and of itself, wouldn't necessarily be that weird. After all, there's no reason why our solar system has to be typical. But the problem is that most models of solar system formation say that the gas giants in other solar systems would have to form far away from their star - just like those in our solar system - and only then migrate inwards - which is very unlike what happened here.

So why didn't Jupiter and Neptune come cruising past our planet billions of years ago en route to the Sun? Researchers at UCLA and the Canadian Institute for Theoretical Astrophysics might have found the answer. They created a new model where the disc of gas and dust around a new star was unusually massive. This super thick disc was enough for gas giants to form in close proximity to their star, which is a big change from previous simulations that suggested the star's intense gravity would rip these embryonic gas giants apart.

Of course, the key phrase here is "unusually massive." The disc certainly is far more massive than our own was - that's why the four gas giants formed so far out - but it's entirely possible that these thick discs are the norm elsewhere, and that's why hot Neptunes and hot Jupiters are so common elsewhere in the galaxy.

arXiv via New Scientist.