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A rogue star may explain why the outer solar system is so odd

Scientists have a new explanation for Pluto's tilted orbit and other idiosyncrasies.
by Seth Shostak /
An artist's animation of a protoplanetary disk, which will eventually turn into a solar system.
An illustration of a protoplanetary disk of the sort that scientists believe gave way to our solar system.Universal History Archive / Getty Images
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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.

A protoplanetary disk surrounds a young star.
In this illustration, a planet is forming within the gap of the protoplanetary disk.Stocktrek Images / Getty Images

“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.

Eventually, a bright star appeared in the night sky, brighter than others. Over the course of hundreds or thousands of years, it would have grown steadily more luminous, eventually looking like a dot 40 times brighter than the full moon is today. You could easily have read a newspaper by its glow, if there had been newspapers.

This passing star was an unnamed cousin of the sun, born in the cluster of stars that spawned our home star. That cluster is now dispersed, its members lost in the rich star fields of the Milky Way. But back then we had many stellar neighbors, and the researchers say this near miss took place at a distance of about 10 billion miles — a very close call in astronomical terms.

A torn disk

Pfalzner’s team simulated this encounter using computer models — in other words, software. They varied the distance of the passing star, its mass, its path and so forth. What they learned was that the nearby passage would have torn some of the dust and gas away from the edges of our protoplanetary disk — which disrupted the formation of the outer solar system.

In addition to accounting for the strange orbits of objects like Pluto and Sedna, it helps explain the striking fact that we’ve not found any really big objects in our solar system beyond Neptune.

A century ago, this hypothesis would have been modestly interesting dinner conversation. There was no way then to know that it was more than an intriguing idea. But today astronomers use computers as frequently as they use telescopes. And unlike the latter, computers can let us see things billions of years in our own past.

Dr. Seth Shostak is senior astronomer at the SETI Institute in Mountain View, California, and host of the “Big Picture Science” podcast.

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