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 lighted 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 one-seventy-fifth that of the Sun.

In a paper published yesterday 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.”

In the past decade astronomers have found 200 planets around other stars. Almost all of these have been indirectly detected from a slight shift in the frequency of a star’s light caused by the gravitational pull of a planet.

In 2004, astronomers reported the first direct sighting of a distant planet, with about the mass of five Jupiters, seen next to a brown dwarf known as 2M1207 about 230 light-years away in the constellation Hydra. Astronomers have since seen small, faint companions around a couple other stars, but are uncertain whether those companions are planets or brown dwarfs.

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

Star systems form in two steps. First, a cloud of gas collapses under gravity into one, two and sometimes three stars. A disk of leftover gas and dust then coalesces into planets.

Brown dwarfs, by some definitions, have a mass greater than about 13 Jupiters, the minimum amount for fusion reactions involving a heavy form of hydrogen known as deuterium to begin. (The fusing of ordinary hydrogen requires much higher temperatures and much higher mass, about 75 times that of Jupiter. That is the upper mass limit for brown dwarfs.)

But some astronomers say that many brown dwarfs are embryonic stars that were ejected out of nascent star systems. Others say that brown dwarfs formed just like other stars, but from smaller gas clouds. Astronomers have also observed free-floating planet-size objects not in orbit around a star, and they debate whether these objects formed in the same way as stars or were ejected from other star systems.

But Oph1622’s planet could not have formed the usual planet way, Dr. Jayawardhana said. Most known planetary disks have only 1 percent to 2 percent as much mass as the parent star, and thus the disk rotating around Oph1622 would have been too small to produce a planet half as large as itself, he said.

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