As recently as two decades ago, we thought our solar system was "normal." That was pretty accurate as long as you didn't look at it too closely. But now we know our system has some remarkable idiosyncrasies, which are especially obvious if you look at the dim spaces beyond Neptune. A prime example is Pluto's oddball orbit, which is tilted by 17 degrees to the disk-shaped plane known as the "ecliptic" in which all the other planets travel around the sun.
Pluto's orbit is also much more egg-shaped than the orbits of Earth and the other planets. This dwarf ice ball takes 248 years to revolve around the sun but spends 20 of those years closer to Sol than Neptune ever is. It's as if one of the horses on a carousel were to weave in and out of the path of an adjacent steed. The dwarf planet Sedna, which is roughly twice as far away as Pluto, has a similarly wacko orbit.
Explanations old and new
So what gives? Why doesn't the sun's retinue of worlds have near-circular orbits, all in the ecliptic? That would be normal.
The usual explanation has been that these distant and generally small objects were tossed around billions of years ago by the shifting gravitational forces of bully worlds such as Neptune. But earlier this month a group of researchers headed by Susanne Pfalzner of the Max Planck Institute for Radio Astronomy in Bonn, Germany, published a study in The Astrophysical Journal that offers another explanation for our solar system's idiosyncrasies — one that's far more dramatic.
Their hypothesis is that long, long ago another star — as big as the sun — passed close to the nascent disk of dust and gas that would become the worlds of our solar system. Its gravity stirred things up, dooming the objects that would eventually form to erratic behavior and small size.
"Our group has been looking for years at what fly-bys can do to other planetary systems never considering that we actually might live right in such a system," Pfalzner said in a statement. "The beauty of this model lies in its simplicity."
Here's how to picture this: Imagine being in our solar system at the time it was born, four-and-a-half billion years ago. The infant sun was beginning to shine, and Earth was still forming — no more than countless bits of dusty rock, gently circling the sun. The other planets were also noiselessly condensing out of this protoplanetary disk — a dusty, pizza-shaped cloud of gas, billions of miles across. Day to day, you wouldn't notice much action.