What's the Biggest Known Planet?

Editor's Note: This feature article is part of SPACE.com's weekly Mystery Monday series.

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. They remain smaller than the dim red dwarf stars, but can range in size up to 70 Jupiter masses. Many simply look like failed stars which never achieved fusion, and yet some colder, smaller brown dwarfs appear more similar to gas giants such as Jupiter or Saturn.

One such brown dwarf made headlines in April 2008. The free-floating object has a mass between 15 and 30 times that of Jupiter, and represents the coolest object of its type at 660 degrees Fahrenheit (350 Celsius). Astronomers consider it a possible representative of a new class of objects, which forms yet another missing link in the overall planetary puzzle.

Going down the scale, the first-ever direct images of extrasolar planets also came late last year. They included one discovery of a three-planet system where the objects ranged from seven to 10 Jupiter masses. Another discovery involved the planet Fomalhaut b with a mass of just three Jupiter masses.

Such objects fall within the generally accepted upper limit of planets being below 13 Jupiter masses. However, the findings could still help scientists revise their planetary definitions based on how objects form, rather than just mass. Both systems contain dusty disks that reflect their young age, and also seem consistent with how planets likely formed in our solar system and elsewhere.

The existing uncertainty about large-class objects even arose during a Pluto debate held in March 2009 at the American Museum of Natural History in New York City. Seager pointed to the three-planet system as just one example of evolving scientific knowledge regarding big planets.

"No one is writing a law or rule that you have to call them this or that," Seager said, referring to the more than 300 extrasolar planets orbiting alien stars.

Perhaps this just means that scientists can take their pick of biggest known planet, if they get their definitions sorted out. But most likely know that ongoing discoveries beyond our solar system could snatch away the heavyweight title at any time.

• Video - Planet-Hunting Kepler Takes Flight
• Video - NASA's Kepler: Hunting Alien Earths
• Top 10 Most Intriguing Extrasolar Planets