The word "planet" has meant many different things over the millennia and even still its definition is evolving.
The word is typically traced back to the ancient Greeks, who believed the Earth was stationary at the center of the universe while objects in the sky revolved around it. The Greek term asters planetai mean "wandering stars" and described the tiny lights that moved across the sky more dramatically than stars when compared over weeks and months. These wandering stars, back then, amounted to Mercury, Venus, Mars, Jupiter and Saturn.
Some think the Greeks and Romans of ancient times considered the sun and Earth's moon as planets. An Elizabethan-era stage play and comedy published in 1597, called "The Woman in the Moon," depicted the solar system with seven planets, including Saturn, Jupiter, Mars, Mercury, Venus, Sol (the sun) and Luna (the moon).??
Nicolaus Copernicus, in 1543, published his mathematical evidence of a heliocentric universe where the six planets revolved around the sun.
Only six planets, including Earth, were known until the 18th Century. In 1781, Sir William Herschel discovered Uranus in that he determined the point of light was a planet and not another star as it had been considered until then.
As planetary scientists and astronomers probe the solar system and beyond, with loads of new discoveries, this idea of a planet has changed and along with it celestial bodies either get thrown onto or off the planet list.
For instance, when Pluto was discovered by Clyde Tombaugh, the icy world seemed to be the king of its neighborhood with no other similar-sized objects in sight. That all changed in 1992 when the first Kuiper Belt Object was found, with currently more than 1,000 such icy bodies spotted in a disk-shaped region beyond the orbit of Neptune, including some around the same size as Pluto. The discovery brings context to Pluto, leading some astronomers to contend Pluto looked more like a Kuiper Belt Object than a planet.
In 2006, the International Astronomical Union (IAU) issued a formal definition of planet, one that led to Pluto's boot from planethood.
The IAU provided three criteria an object must meet to reach planet status:
A planet is a celestial body that
1. It orbits around the sun.
2. It has? sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and
3. It has cleared the neighborhood around its orbit.
Several problems with this definition immediately pop up for astronomers.
Under the IAU criteria, the more than 300 extrasolar planets identified to date would not be considered planets.
"There is no acceptable planet definition for exoplanets," said Sara Seager, an astrophysicist at MIT. The current IAU planet definition necessitates a planet must orbit the sun. Well, an exoplanet, has its own host star and it's not the sun.
Seager joined other astronomers and planetary scientists last week at the Johns Hopkins University Applied Physics Laboratory (APL) in Laurel, Md., for "The Great Planet Debate: Science as Process" conference. The argued about Pluto's status and also discussed worlds beyond our solar system.
The problems, it turns out, are small and big.
Several objects not currently called exoplanets sit along the upper-limit mass cutoff of 13 Jupiter masses, beyond which objects are typically thought to be a class of failed star called brown dwarf. But these borderline objects could go either way, and Seager said a definition must account for them.
The "cleared the neighborhood around its orbit" criterion is also a sticky issue. That's because the farther away a planetary object is from its star the longer it takes to complete its orbit. So depending on the age of the system, that object may not have completed many orbits and thus If Earth were positioned at a distance of 100 astronomical units (100 times farther than it is now), ?our homebase would not fit the IAU definition of a planet, argue Hal Levison of the Southwest Research Institute in Boulder, Colo., and others.
Planet definition still evolving
The different planet definitions put forth at last week's meeting could leave the solar system with as few as eight planets or as many as 13, with the possibility of many more lurking out there yet to be discovered.
Two flavors of definitions include the so-called dynamical definition and the geophysical one. For the dynamical one, a planet is a planet if it has cleared out its orbit of rocky litter either by eating up that material, and becoming fatter in the process, or kicking the junk into other orbits. But that's just a simplistic view. What about Jupiter, which has a slew of captured asteroids that orbit the sun in lockstep with the giant planet?
The geophysical definition would include as planets objects massive enough for gravity to make them about spherical but not so massive that internal nuclear fusion exists, as is the case with stars.
"You go through and look at how the definition [of planet] has evolved over time and they all have one thing in common. The basic characteristic of a planet is they go around the sun, historically," Levison said. "This is a dynamical definition. So to say you can't use dynamics, that somehow it's wrong to use dynamics, in order to characterize a planet is historically inaccurate. That's the way we've always defined planets."
Mark Sykes, directory of the Planetary Science Institute in Tucson, Ariz., supports a geophysical definition of round objects that orbit a star. The key here is that once an object gets that big, important geophysical processes begin. Such an object is large and round enough that heat can build up in its core to trigger geophysical processes ? akin to volcanic activity and tectonic movement on Earth ? and a process known as differentiation in which the less dense material sinks to the center and the volatiles float toward the surface.
It's also roughly the mass at which atmospheres can form, as gases are gravitationally trapped around the object's surface. Internal or surface oceans also become possible, as the volatiles condense toward the object's surface.
The geophysical definition leaves open the planet window for some satellites, including Jupiter's major moons: Io, Europa, Ganymede and Callisto. While Io is the most volcanically active body in the solar system, Callisto is the solar system's third largest satellite and Europa likely has an iron core, mantle and surface ocean similar to Earth's structure.
"These are massive worlds," said William McKinnon of Washington University in Saint Louis. "They are planets in all but name. They just happen to be going around Jupiter."
But does it matter what an object is called, at the end of the day?
"There's an implicit hierarchy. If you're a planet, you are first-class, A-list, you get inside the rope to the club and, otherwise you're nothing," McKinnon said. "There's got to be some way to communicate that these are worlds in their own right, as worthy of study as Mars."
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