Just five years ago, before any brown dwarfs or exoplanets had been confirmed, there was a wide gap between the largest planet we knew of - Jupiter -- and the smallest known stars, which were 75 times the mass of Jupiter.

"That made things nice and simple. Textbook simple," said the Carnegie Institution's Alan Boss, who heads the 13-member IAU group that's trying to define "planet." "You could say, 'This is a star, and this is a planet.'"

So simple we didn't need a textbook definition.

"Now, we realize things are not so clear," Boss said in an interview. With the recent discovery of free-floating planet-like objects, a months-old definition hammered out by the IAU group after much debate is already outdated, he added.

"We're kind of starting all over again," Boss said. A draft recommendation might be ready for review in two weeks, he added. But first, there are some thorny issues to sort out.

The gray areas between planets and brown dwarfs boil down to three things: their genesis, their orbit and their size.

But Tarter, who coined the term "brown dwarf" back in 1975 when the objects were only theoretical, explains that you can't always tell how an object formed. So she would also include the requirement that the object orbits a star.

While this seems like an instinctive definition for a planet, it doesn't account for the recently discovered planet-like objects roaming freely in space. (Perhaps the ancients saw this coming: The Greek root of the word "planet" means "to wander.")

To further complicate matters, brown dwarfs can meet the above two "planet" requirements. They frequently are found orbiting a star in what scientists call a binary arrangement, implying that they formed out of the original disk of gas and dust.

Much of the current debate, therefore, centers on size.

And when a brown dwarf is about 13 times the mass of Jupiter, it generates enough pressure to force the burning of deuterium, a hydrogen-like element. Planets cannot burn deuterium.

"Most people, but not all, would make the dividing line at whether or not deuterium burns," said A'Hearn, the University of Maryland astronomer.

This still leaves areas of confusion, allowing the possibility of some brown dwarfs that are less massive than other objects that would be classified as planets.

Brown dwarfs are the ill-defined middle ground between planets and stars. A star is a star because it shines on its own, generating light through thermonuclear reactions in which hydrogen is converted to helium. Brown dwarfs, though they can burn deuterium in another type of reaction called "core fusion," fall short of full-blown stellar thermonuclear fusion.

But brown dwarfs can, like stars, be born out of an otherwise unorganized cloud of gas and dust, when gravity forces a direct collapse of the cloud.

And even stars cross the blurry lines of definition.

"The fact is that stars form in disks as inevitably as planets," explained astronomer Gibor Basri of the University of California, Berkeley. This happens in binary-star systems, where one star forms first and the other is created from the leftovers.

Basri, who has written about brown dwarfs for Scientific American, just this week completed the draft of a paper titled "What is a Planet?" that he plans to submit to colleagues for review, then possibly to a scientific journal. He shared the draft with SPACE.com.

In the paper, Basri discussed the two possible ways that free-floating planets, among the most vexing objects, might have formed.

"One is that they formed in planetary systems around stars...and were subsequently ejected from the system," Basri wrote. "The other possibility is that these objects formed in isolation, or at least were not originally bound to a star."

Either way, Basri suggested that existing terms and definitions are no longer sufficient. Some new terms and definitions are needed to cut through the scientific and cultural roadblocks that prevent clear and accurate distinctions between planets and other objects.

"A planet is a spherical object never capable of core fusion, which is formed in orbit around an object in which core fusion occurs at some time."

Or, if that's too much of a mouthful, Basri has a shorter version:

"A planet is a spherical non-fusor born in orbit around a fusor."

Gone, it seems, are the days when our idea of a planet was so simple it didn't even need a definition.