Rogue star may explain why the outer solar system is so strange

Just twenty years ago, we thought our solar system was “normal”. It was pretty accurate as long as you didn’t look at it too closely. But now we know that our system has some remarkable peculiarities, which are especially evident if you look at the dark spaces beyond Neptune. A prime example is Pluto’s eccentric orbit, which is tilted 17 degrees from the disc-shaped plane known as the “ecliptic” in which all other planets move around the sun.

Pluto’s orbit is also much more egg-shaped than the orbits of Earth and other planets. This dwarf ice ball takes 248 years to circle the sun, but spends 20 of those years closer to the Sun than Neptune ever is. It is as if one of the horses on a merry-go-round weaves its way in and out of the path of an adjacent steed. The dwarf planet Sedna, which is roughly twice as far away as Pluto, has an equally wacky orbit.

Old and new explanations

So what gives? Why does the series of worlds of the sun not have almost circular orbits, all located in the ecliptic? This would be normal.

The usual explanation has been that these distant and usually small objects were thrown billions of years ago by the shifting gravitational forces of tyrannical worlds such as Neptune. But earlier this month, a group of researchers led by Susanne Pfalzner from the Max Planck Institute for Radio Astronomy in Bonn, Germany, published a study in The Astrophysical Journal this offers another explanation for the peculiarities of our solar system – a much more dramatic explanation.

Their hypothesis is that a long, long time ago another star – as large as the sun – passed near the nascent disk of dust and gas that would become the worlds of our solar system. Its gravity stirred things up, condemning the objects that would eventually form to erratic behavior and short stature.

In this illustration, a planet is forming in the protoplanetary disk space.Stocktrek Images / Getty Images

“Our group has been researching for years what flyovers can do to other planetary systems without ever considering that we could actually live in such a system”, Pfalzner said in a statement. “The beauty of this model lies in its simplicity.

Here’s how to imagine it: Imagine being in our solar system when it was born, four and a half billion years ago. The rising sun was starting to shine, and the Earth was still forming – no more than countless pieces of dusty rock, gently surrounding the sun. The other planets were also noiselessly condensing out of this protoplanetary disk – a dusty pizza-shaped cloud of gas several billion kilometers in diameter. Day to day, you won’t notice a lot of action.

Eventually, a bright star appeared in the night sky, brighter than the rest. Over the course of hundreds or thousands of years, it would have grown brighter and brighter, eventually looking like a point 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 star cluster that spawned our original star. This cluster is now scattered, its members lost in the rich star fields of the Milky Way. But at the time, we had many stellar neighbors, and researchers say this near miss happened at a distance of about 10 billion miles – a very close call in astronomical terms.

A torn record

The Pfalzner team simulated this encounter using computer models – in other words, software. They varied the distance of the passing star, its mass, its trajectory and so on. What they learned was that the nearby passage would have ripped some of the dust and gas from the edges of our protoplanetary disk, disrupting the formation of the outer solar system.

As well as accounting for the strange orbits of objects like Pluto and Sedna, it helps explain the striking fact that we haven’t found any very large objects in our solar system beyond Neptune.

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

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

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Arline J. Mercier

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