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There might be a ninth planet in the solar system after all — and it is notPluto.

Two astronomers reported on Wednesday that they had compelling signs of something bigger and farther away — something that would definitely satisfy the current definition of a planet, where Pluto falls short.

“We are pretty sure there’s one out there,” said Michael E. Brown, a professor of planetary astronomy at the California Institute of Technology.

What Dr. Brown and a fellow Caltech professor, Konstantin Batygin, have not done is actually find that planet, so it would be premature to revisemnemonics of the planets just yet.

Rather, in a paper published Wednesday in The Astronomical Journal, Dr. Brown and Dr. Batygin lay out a detailed circumstantial argument for the planet’s existence in what astronomers have observed — a half-dozen small bodies in distant, highly elliptical orbits.

What is striking, the scientists said, is that the orbits of all six loop outward in the same quadrant of the solar system and are tilted at about the same angle. The odds of that happening by chance are about 1 in 14,000, Dr. Batygin said.

A ninth planet could be gravitationally herding them into these orbits.

For the calculations to work, the planet would be quite large — at least as big as Earth, and likely much bigger — a mini-Neptune with a thick atmosphere around a rocky core, with perhaps 10 times the mass of Earth.

It would dwarf Pluto, at about 4,500 times its mass.

Pluto, at its most distant, is 4.6 billion miles from the sun. The potential ninth planet, at its closest, would be about 20 billion miles away; at its farthest, it could be 100 billion miles away. It would take from 10,000 to 20,000 years to complete one orbit around the sun.

“We have pretty good constraints on its orbit,” Dr. Brown said. “What we don’t know is where it is in its orbit, which is too bad.”

Alessandro Morbidelli of the Côte d’Azur Observatory in France, an expert in dynamics of the solar system, said he was convinced. “I think the chase is now on to find this planet,” he said.

This would be the second time that Dr. Brown has upended the map of the solar system. In January 2005, he discovered a Pluto-size object, now known as Eris, in the ring of icy debris beyond Neptune known as the Kuiper belt.

A year and a half later, the International Astronomical Union placed Pluto in a new category, “dwarf planet,” because it had not “cleared the neighborhood around its orbit.”

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