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A different breed of planet?

L. Cook / Gemini Obs. / NASA / ESA / STScI
An artist's conception shows the binary system 2M J044144, with a primary brown
dwarf surrounded by a disk as well as a planet-sized companion.


If a gas giant forms like a star, but ends up small enough to be a planet, what do you call it? Astronomers are scratching their heads over a planet classification puzzle that is way bigger than Pluto.

Three and a half years ago, the International Astronomical Union tried to find a standard for defining the lower limit for things that are labeled "planets." The definition that came out of the IAU's meeting was the result of some spur-of the-moment cutting, pasting and politicking, as documented in my book, "The Case for Pluto."

The latest discovery, focusing on an oddball star system about 450 light-years away in the constellation Taurus, adds a twist to the controversy over the upper limit for planethood. The system includes an object that is well within the usual mass range for a planet - five to 10 times as massive as Jupiter. But here's the problem: The location and age of the object suggest that it had to congeal from its own cloud of gas and dust, like a star rather than a planet.

The curious case is described in a research paper to be published in the May 1 issue of the Astrophysical Journal Letters.

"Whether this object should be called a 'planet' is up for discussion, as it doesn't fit neatly into our current theories of planet formation," Wellesley College astronomer Kim McLeod said in a news release about the find. "Most people have a good sense of what a planet is: It orbits a star, is big enough to have become spherical, and - this last bit thanks to the Pluto debate - is enough of a gravitational bully to have cleared out other objects from its orbital path."

We'll leave that "last bit" aside for now. The point is that the IAU's definition doesn't really address the top end of the planet category. Take the case of the smaller star in a binary-star system: It goes around a star (its bigger companion). It has a spherical shape. And it may be all by its lonesome, meaning that it has "cleared out the neighborhood of its orbit."

Is that star a planet? If not, why not?

Defining stars ... and failed stars
One answer is that stars are objects so massive that their crushing gravity lights up an internal fusion furnace, like the blaze that powers our own sun. By that measure, astronomers figure that anything more than 80 times as massive as Jupiter would light up its hydrogen, becoming a full-fledged star. Objects between 13 and 80 times Jupiter's mass fall into a slightly different category: They could light up their deuterium, but not their hydrogen, and thus they'd be considered "failed stars," or brown dwarfs.

Based purely on mass, the mystery object would be judged a planet. But lots of astronomers tend to judge a planet by how it formed. "In that definition, you define a planet as an object that forms in a disk around a larger body," Kevin Luhman, an astronomer at the Center for Exoplanets and Habitable Worlds at Penn State University, told me today.

In their paper, Luhman, McLeod and Penn State's Kamen Todorov contend that the mystery object did not form that way. They found the object while making a survey of brown dwarfs in the Taurus star-forming region, using the Hubble Space Telescope and the Gemini Observatory. It's apparently orbiting a brown-dwarf companion that's about 20 times as massive as Jupiter, at a distance of 2.25 billion miles (3.6 billion kilometers).

The astronomers' observations suggest that the object formed over the course of just a few million years. That's not enough time for building up planets from the bits of material in a disk surrounding a star or brown dwarf. Astronomers call this scenario "core accretion."

There's another scenario that could put planet-building on a faster track, known as disk instability. In that scenario, a big clump of gas and dust in the disk quickly collapses to form a gas giant. But Todorov and his colleagues say there wasn't enough gas and dust surrounding the larger brown dwarf to account for the smaller object's mass.

They conclude that the object was formed like a star is formed, through the collapse of its very own cloud of gas and dust. That cloud would be separate from the cloud that gave rise to its bigger companion. In effect, the brown dwarf and the smaller object would be born as fraternal twins.

"The most interesting implication of this result is that it shows that the process that makes binary stars extends all the way down to planetary masses," Luhman said in today's news releases. "So it appears that nature is able to make planetary-mass companions through two very different mechanisms."

If it quacks like a duck...
Is the mystery object a planet? The astronomers behind the observations say no.

"It wouldn't quite do to call it a planet, contrary to the case for the hackneyed duck," McLeod said in the Wellesley news release. "It walks like a duck (orbits something bigger) and quacks like a duck (has the same mass) but didn't come from a duck's egg (the big object's disk)."

The object might not even look like a duck: It might be made completely out of gas, instead of a sheath of gas wrapped around a rocky core like Jupiter's. It might look like a cool little sun, and not like the marbled world shown in the artist's conception at the top of this Web page. Luhman said the observations suggest that the mystery object and its brown-dwarf companion may be part of a quadruple-mini-star system, along with a red star and brown-dwarf companion found in the same celestial neighborhood.

Todorov and Luhman (Penn State) / Gemini Obs. / AURA
An adaptive-optics image from the Gemini North Telescope shows four objects,
apparently in a quadruple system. A brown dwarf and planetary-mass companion
are at lower right. A red star and brown-dwarf companion are at upper left.


Based on the IAU's unofficial working definition for extrasolar planets, the mystery object might still fit into the planetary pigeonhole. That definition classifies planets strictly on the basis of mass and orbit, "no matter how they formed." But one of the astronomers who hammered out that definition, Alan Boss of the Carnegie Institution for Science, said he'd avoid using the word "planet" in this particular case.

"While people like to use the 'p-word' to describe objects with masses below 13 Jupiter masses, given the attention given to exoplanets these days, they should more properly be called 'sub-brown dwarfs,'" Boss, author of "The Crowded Universe," told me in an e-mail. He pointed out that he predicted the discovery of such objects, theoretically including SBDs less massive than Jupiter itself, in a paper published nine years ago.

Alan Stern, a planetary scientist at Colorado-based Southwest Research Institute and principal investigator for NASA's New Horizons mission to Pluto, sees it differently:

"I think it's interesting," he told me in an e-mail. "It shows planets form lots of ways, so choosing origin method [as a criterion] is a poor way to go. Sort of like saying a human has to be made by both parents, except a human can be made in a test tube, or by cell division (identical twins). A planet should be defined by its characteristics, not its circumstances of location or origin or anything else. When you pull up to one, you know it's a planet or it is not. Why is this hard for some? Why do some want to make something hard out of something so easy? (And yes, that's a rhetorical question.)"

Luhman acknowledged that the mystery object falls into a gray zone that could get a lot grayer. "This is probably going to come up more and more in future years as we find more and more objects in this gray zone," he told me.

Like the debate over Pluto, this kind of discussion can easily get caught up in the details of astronomical classification. Or it can go to show that our universe reflects an incredible range of diversity. Maybe it's time to stop dwelling so much on the definitions and celebrate the diversity instead. What do you think?

More about planetary diversity:


To learn more about the planet quest, check out my book, "The Case for Pluto." The next event on the book-tour schedule is my talk at the National Academy of Sciences' Marian Koshland Science Museum in Washington at 6:30 p.m. on April 15. In the meantime, you can join the Cosmic Log corps by signing up as my Facebook friend or hooking up on Twitter.