Mankind's Explanation: 12th Planet


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A Cometís Odd Orbit Hints at Hidden Planet  

 Far beyond the solar systemís nine known planets, a body as massive as Mars may once have been part of our planetary system and might still be there.

   Although the proposed planet would lie too far away to be seen from Earth, its gravitational tug could account for the oddball orbit of a large comet spotted in the outer solar system a year ago.

   Known as 2000 CR105, the comet moves about the sun in a much more elongated pathway than originally though, astronomers now find. Observations over the past year by Brett Gladman of the Observatoire de la Cote díAzur in Nice, France, and his colleagues show that the cometís orbit takes it further than 200 astronomical units (AU) from the sun and as close as 44 AU. One AU equals the Earth-sun distance of about 150 million kilometers.  

   Such an oblong orbit is usually a sign that an object has come under the gravitational influence of a massive body. But 2000 CR105, which mat be an escapee from the distant reservoir of comets known as the Kuiper belt, never gets anywhere near any of the solar systemís familiar team of nine planets. Even at its closest approach to the sun, the approximately 400 km wide ball of ice comes no closer than 14 AU to Neptune, the nearest known candidate for a significant gravitational interaction.

   The astronomers concede that feeble and random pushes from Neptune could have slowly nudged 2000 CR105 into its current orbit. However, preliminary analysis suggests this scenario isnít likely, note Gladman, Matthew Holman of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass., and their collaborators.

   In an article the researchers recently posted on the Internet (, they suggest that a massive body lurking among the tiny, frozen residents of the Kuiper belt could have been the culprit.

   That object could have been Neptune itself. According to one theory, Neptune and Uranus first formed between the orbits of Jupiter and Saturn and were then flung out to greater distances from the sun.

   If that kick propelled Neptune into the Kuiper belt before it settled into its current orbit nearly circular orbit, its gravity could have caused the orbits of several objects like 2000 CR105 to stretch into elongated trajectories.

   Alternatively, the cometís orbit could be the handiwork of an as-yet unseen planet whose mass lies somewhere between that of the Earthís moon and Mars, the researchers say. Itís likely that such an object would have coalesced in the outer solar system from the same debris that formed Neptune, Uranus, and the cores of Jupiter and Saturn, Holman notes.

   Thereís only a 1 percent chance that a planet could have survived in the Kuiper belt or its surroundings over the 4.5 billion year age of the solar system, says Holman. If the planet found a secluded nook of the belt, however, it could remain intact today.

   If the proposed planet is as massive as Mars, it would have to lie some 200 AU from the sun about 7 times Neptuneís distance Holman calculates. Were it closer, observers would have stopped it.

   A planet lurking in the Kuiper belt now or in the past might also explain why many members of the belt have orbits away from the plane in which the nine known planets orbit the sun.

   ďUndoubtedly, something [massive] knocked the hell out of the belt,Ē says Harold F. Levison of the Southwest Research Institute in Boulder, Colo. ďThe question is whether it is there now.Ē

   The stability of the orbit of 2000 CR105 suggests that any planet that influenced the cometís path has long since departed. If astronomers find a family of objects similar to 2000 CR105, the nature of their orbits could indicate whether the hidden planet is in fact still there, Levison says.

- R. Cowen

This article is from Science News, Volume 159, page 213 April 7,2001

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