University of Utah and Smithsonian Astrophysical Observatory
Computer simulations by Ben Bromley, associate professor of physics at the U., and his colleagues show how the planetoid Sedna may have had its orbit perturbed by a passing star or may even have come from matter orbiting that star.

Is an "alien" world orbiting our sun?

The tantalizing possibility is raised in a study by the University of Utah's Ben Bromley and Scott Kenyon, a scientist at the Smithsonian Astrophysical Observatory, who theorize that a distant planetoid could have migrated to this solar neighborhood when our sun had a relatively close encounter with another star billions of years ago.

The possibly alien world — one that would not have been created with Earth and its sibling planets during the formation of our solar system — is a newly discovered object far beyond Pluto, a planetoid that is being called Sedna.

Even if Sedna is native to the solar system, as Bromley suspects, their computer simulations and analysis show it was apparently knocked into a bizarre orbit by a passing star.

The report, "Stellar encounters as the origin of distant Solar System objects in highly eccentric orbits," is published in today's issue of the scientific journal Nature.

As soon as NASA announced the discovery of Sedna this past March, scientists began scratching their heads about the strange world. At about 1,000 miles diameter, it is too small to be called a true planet but larger than any known asteroid. Its finders named it after Sedna, the native Alaskan goddess of the sea.

The oddest aspect is that Sedna is exceptionally far from the sun, in the most distant reaches of the solar system. Presently located 8 billion miles from our own star, its elliptical orbit takes Sedna out to 84 billion miles. Also, it is believed to be the reddest and shiniest body in the solar system, according to a co-discoverer of Sedna, Mike Brown of the California Institute of Technology.

Bromley, associate professor of physics at the U., and Kenyon, senior scientist at the Smithsonian Astrophysical Observatory, Cambridge, Mass., have been using a supercomputer at the Jet Propulsion Laboratory in Pasadena, Calif., to simulate the way planets formed from dust and other material in the early solar system.

With the discovery of Sedna, they began simulating ways it might have reached its position. Now they have published their results in Nature.

"Its orbit is very strange," Bromley told the Deseret Morning News. "It gets no closer than about 70 astronomical units, and its orbit carries it way out, past 1,000 astronomical units."

One astronomical unit, abbreviated as AU, is the distance from the sun to the Earth, about 93 million miles.

Normally, when an object like an asteroid has a highly elliptical orbit it's because of an encounter between it and a planet. The planet's gravity pulls it into a strange orbit.

In such cases, the object always eventually yo-yos back to the vicinity of the encounter.

The planet Neptune deflected many objects from the Kuiper Belt, a vast ring of ice and rock debris extending from 30 AU from the sun, where Neptune rolls along, out to 50 AU.

Yet Sedna never approaches within 5 billion miles of the orbit of Pluto, the most distant known planet. So it didn't get there because of a planetary encounter.

"Now, there's nothing out in Sedna's vicinity that obviously caused it to have a peculiar orbit," Bromley said in a telephone interview. The likely explanation: The gravity of a passing star changed Sedna's orbit, pulling it out of its normal position.

A likely site for Sedna before the shift is the Kuiper Belt — assuming Sedna wasn't lost by another star, migrating to our sun's system.

"We ran these simulations and we saw that, in fact, a passing star could put things on orbits like Sedna's," Bromley said.

As the report in Nature states, "a passing star probably scattered Sedna from the Kuiper Belt into its observed orbit."

But maybe it was never in the Kuiper Belt in the first place.

Some of the particles on their computer screens, which represented material in orbit around the sun during an encounter with another star, kept disappearing.

"We didn't know really what happened to them," Bromley said. "In determining their fate, we found that some of them were orbiting the passing star, long since the star had passed the sun's vicinity."

They realized that as well as losing material to that star, "the sun could easily capture" material from the other star.

That was what Bromley refers to as "the Oh wow, that's cool" moment.

Afterward came months of study to refine the objects' interactions.

The flyby of another star probably would have been early in the solar system's history. Only then were other stars likely to be in the neighborhood, Bromley believes.

Most stars develop in star-forming regions where other stars also are born. An example of this is a nebula beloved by amateur astronomers, the spectacular Orion Nebula.

"Our simulations are indicating that it's more likely that Sedna is an indigenous planet" — one of "ours" — "but there is still a reasonable chance that it's a captured planet," Bromley said.

Logically, whatever happened with Sedna could have happened with other objects, possibly many of them.

Their research also may explain the abrupt edge of the Kuiper Belt at 50 AU.

Why does the belt have a sharp edge? "It doesn't seem to be a natural thing," Bromley said.

Their simulations offered a reason, he said. "The edge of the Kuiper Belt could be created by a passing star."

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