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.
New
horizons
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.
Pandora's
box?
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."
advertisement
