Tiny star, giant planet raise questions on definitions

Star is brown dwarf; planet may be more star than planet

Published: Friday, Aug. 4 2006 1:22 p.m. MDT

A tiny star with a giant planet is further muddling astronomers' notion of what a planet is. The planet is one of perhaps only two or three planets around other stars to be photographed directly, but it may be more like a star than a planet.

The tiny star, known as Oph1622, is so small that it never lit up, a failed star known as a brown dwarf. Even among brown dwarfs, it is small, with a mass equal to 14 Jupiters, or about 1/75th that of the Sun.

In a paper published Thursday on the Web site of the journal Science, astronomers at the University of Toronto and the European Southern Observatory report that a photograph of Oph1622 also shows a planet about half as large as the star itself, with a mass equal to seven Jupiters.

The two are separated by 22 billion miles, or about six times the distance between the Sun and Pluto. Both are young, about a million years old. Astronomers refer to them both by a recently coined word, planemo (pronounced PLAN-uh-mo), short for planetary mass object — planet-size bodies that may or may not be planets.

"It really stands out as something quite unusual and intriguing," said Ray Jayawardhana, a professor of astronomy at the University of Toronto and an author of the Science paper. "The Oph1622 pair adds to the rich diversity of worlds that have been discovered recently, a diversity that we couldn't really have imagined barely a decade ago."

Within the solar system, astronomers have been debating where to put the dividing line between planets and smaller clumps of rock and ice like comets and asteroids. The discovery of an object larger than Pluto in the outer solar system last year has rekindled debate on whether Pluto, by far the smallest among the current roster of nine planets, should be demoted.

Outside the solar system, the dividing line between planets and stars has also become blurry.

Oph1622, about 400 light-years from Earth, should add to the confusion. Jayawardhana said its companion had the mass of a planet but was born in the manner of stars.

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