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.
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