Planets, Stars: What's the Difference? Good Question

"What's the Biggest Known Planet? "
Space.com (August 17, 2009)

"Pluto huggers and haters may hog the spotlight, but there's another debate on the bigger end of the planetary scale. Astronomers have in recent years uncovered super-massive objects that blur the boundary between planet and full-blown star.

"The complications go beyond simply defining stars as undergoing thermonuclear fusion. Planet hunters peering at distant stars have found huge orbiting objects which dwarf Jupiter, the largest gas giant planet in our solar system. Such finds may represent the missing links on the sliding scale between planets and stars.

" 'Taken together, these discoveries are going to change what we call a planet,' said Sara Seager, an astrophysicist at MIT. 'Until now people have been arguing about how big can an object be and still be a planet.'

"Brown dwarfs represent the largest objects which inhabit that hazy space between planets and stars...."

This article gives a pretty good overview of what we know - and what we don't know yet - about objects that are more massive than Jupiter, but don't have the fusion reactions going on inside that make an object a "star."

Although some of the debate may sound silly - a matter of what word gets used to describe a collection of objects, and what objects go in the collection - that sort of definition of terms is important. For scientists, at any rate, who need to be able to communicate about what they're doing.

It also helps - a lot - to be able to organize information, before trying to analyze it.

'What's the biggest planet?'

The biggest planet could be CFBDS J005910.83-011401.3 - unless it's a star, or something else. I'll call it "Cifbidsu" - pronounced "k[ih]fb[ih]ds[oo]," with emphasis on the middle syllable, "bid" .
Anyway, Cifbidsu is about 40 light-years away, not near or associated with any known star, weighs in at about 15 to 30 times the mass of Jupiter, and has a surface temperature of about 660 degrees Fahrenheit (350 Celsius). That makes it the coolest brown dwarf known. Assuming that it's a brown dwarf.

There's a pretty good writeup on CFBDS J005910.83-011401.3 in the Space.com archives: "Missing Link Between Planets and Stars Found" Space.com (April 10, 2008).

More, at "Major Breakthrough: First Photos of Planets Around Other Stars" Space.com (November 13, 2008).

Related post:

• "Planets Circling Other Stars Photographed"
  (November 13, 2008)

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Related posts, at

• "A Serious Search for Other Worlds, Life, and - Maybe - Extra-Terrestrial Intelligence"