Possible Alien World Around Mini-Star Tests Definition of a Planet
A brown dwarf, estimated to be approximately 20 times the mass of Jupiter (at left), and its companion, about 7 times the mass of Jupiter (at right) are shown in this illustration.
Credit: NASA, ESA, and K. Todorov and K. Luman (Penn State University)

A planet-like object has been found to be orbiting a cold, miniature star called a brown dwarf, calling into question just what it means to be a planet.

Brown dwarfs are not quite massive enough to be full-fledged stars; they are not dense or hot enough in their cores to ignite nuclear fusion, the process that powers stars.

"Brown dwarfs are nature's 'almost stars' ? gassy bodies that aren't quite hot enough in their cores to fuse hydrogen," said researcher Kim McLeod, an astronomer at Wellesley College in Wellesley, Mass.

And the new discovery of an apparent planet orbiting one of these mini stars seems to blur the line even more between a planet and a brown dwarf.

In this case, the "star" weighs about 20 times the mass of Jupiter, but its planet weighs not too much less ? about 5 to 10 times Jupiter's mass. With the objects in such similar weight categories, some astronomers are questioning whether those labels even apply.

"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," McLeod said. "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."

Another confusing aspect about this system is the apparent age of the smaller companion.

The weird object appears to have formed in less than 1 million years, which is the approximate age of the brown dwarf. But that's much faster than the predicted time it takes to build planets according to some theories.

"This is the youngest planetary-mass companion that has been found so far, and its extreme youth provides constraints on how it could have formed," said co-researcher Kevin Luhman of the Center for Exoplanets and Habitable Worlds at Pennsylvania State University. "The formation mechanism of this companion in turn can tell us whether it is truly a planet."

Scientists envision a few possible ways planets can form. In one scenario, a disk of dust and gas surrounds a star, and that dust clumps together to form a rocky planet which then accumulates a shroud of gas around it. Another option is that a lump of gas in the disk collapses into a gaseous ball to create a gas giant planet. Thirdly, instead of starting from a disk, a planet might collapse directly from a cloud of gas and dust, the same way a star or brown dwarf would form.

In the case of the planet-like companion in the recently observed system, researchers are favoring the third possibility because it could explain how the companion formed so quickly.

The planet-like object was spotted in observations taken by the Hubble Space Telescope and the Gemini Observatory telescopes in Hawaii and Chile. The researchers detail their findings in the May 1 issue of the Astrophysical Journal Letters.

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