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Des Moines Register
08-29-06
Solar Shake-up
Pluto moves to new class of planets
By KELLI BROWN
REGISTER STAFF WRITER
Eight is enough. After years of debate since Pluto’s far-off discovery in 1930, the
International Astronomic Union last week voted to drop the tiny, icy body from the
traditional lineup and move it into a subcategory of dwarf planets.
So the lineup now is: the four inner rocky planets (Mercury, Venus, Earth and
Mars); then an asteroid belt; then the four outer gas giants (Jupiter, Saturn,
Uranus and Neptune); and finally the icy Kuiper Belt with Pluto.
The decision comes a week after the international organization set the world astir
by proposing three additional planets: Ceres, a large asteroid between Mars and
Jupiter that had planetary status in the 1800s; Charon, Pluto’s moon; and Xena,
the farthest object yet discovered in our solar system.
Xena’s discovery brought Pluto’s status into question in the same way that the
discovery of the asteroid belt claimed Ceres’ planetary status in the 1800s,
explained Iowa State University assistant professor Charles Kerton.
After astronomers discovered Ceres in the early 1800s, some time passed before
other asteroids were found in the area between Mars and Jupiter. The thinking at
that time was that Ceres was a planet, Kerton said. However, years later when
other asteroids were found, astronomers were forced to rethink Ceres’ status and
later reclassified it.
“Before we found the Kuiper Belt (in the 1950s), Pluto was looked on as a very
odd object (for a planet),” Kerton said. “And then, once we started discovering
others (in the Kuiper Belt), we started realizing that Pluto wasn’t that odd after all,
but just one of the larger ones.”
In 2005, American Michael Brown announced the discovery of Xena, a Kuiper
Belt object more distant and larger than Pluto. It was at that point, that the
questions regarding Pluto’s status began to grow even more.
The gist of the IAU’s new definition of a planet says that it must be nearly round,
orbit a star and dominate its portion of space.
Pluto has joined a category called “dwarf planets” with Ceres and Xena. Pluto’s
moon Charon remains a moon. The IAU lists at least a dozen more solar bodies
that could fall into the dwarf planet category in the coming months and years.
Kerton says that although Pluto’s status seems like a demotion, it shouldn’t. “The
language of Pluto being demoted and losing its status is only indicative of its
size,” he said.
“For astronomers, (dwarf planets) are very important to understanding how the
solar system is now and was formed. We can see how they’re moving around the
sun and look back and see how they and other planets have changed through
history.”
With the new definition, very little changes for the academic world, Kerton said.
“Personally for me, it puts in a definition what we’ve been teaching our astronomy
students for the past few years,” he said.
He suggests that school teachers focus on the eight main planets and mention
that the other two areas exist. “It makes the outer solar system a lot easier,”
Kerton said.
“Before we didn’t know why Pluto was out there. Now we know that a planet
didn’t form between Mars and Jupiter and that the leftover material there is the
asteroid belt. Ceres just happens to be the largest. Now it’s the same for Pluto.”
MERCURY
The small, rocky planet moves around the sun at just over 30 miles per second.
But the length of its day (how fast it rotates on its own axis) is the same as
58days on Earth.
- Circumference: 9,525 miles
- Distance from the sun: 36 million miles
VENUS
The planet most similar to Earth is under such extreme pressure that it destroys
exploratory probes within hours of landing. It rotates so slowly that its day is
longer than its year (the time it takes to orbit the sun).
- Circumference: 23,627 miles
- Distance from the sun: 67 million miles
EARTH
Oceans cover 70 percent of the planet thanks to a narrow surface temperature
range that allows it to exist as a liquid.
- Circumference: 24,901 miles
- Distance from the sun: 93 million miles
MARS
The "red planet" is small and rocky with large quantities of ice about 3 feet below
the surface. It also features the largest volcano in the solar system and a canyon
so long it would stretch from New York to Los Angeles.
- Circumference: 13,263 miles
- Distance from the sun: 142 million miles
JUPITER
Jupiter's stripes are formed by strong winds, and the "Great Red Spot" is a giant
storm that's been observed for 300 years.
- Circumference: 279,118 miles
- Distance from the sun: 484 million miles
SATURN
The most distant planet known to the ancients is known for its giant rings of icy
particles and debris. The planet's striped look comes from winds that are four
times the strength of the hurricane-force winds of Earth.
- Circumference: 235,298 miles
- Distance from the sun: 886 million miles
URANUS
The smaller, ringed planet has no solid surface and is made entirely of gas. Its
peculiar rotation could mean the planet was "knocked on its side" after a collision
with a large object early in its history.
- Circumference: 99,787 miles
- Distance from the sun: 1.8 billion miles
NEPTUNE
Now the most distant planet, Neptune has winds three times stronger than
Jupiter's and nine times stronger than Earth's. Its moon Triton is the coldest
measured body in the solar system at nearly -400 degrees.
- Circumference: 96,683 miles
- Distance from the sun: 2.8 billion miles
SEE THE PLANETS
The five closest planets can be seen without a telescope. With good conditions
you should be able to see four of them in late August:
Mercury, Venus and Saturn: Get up early to catch this threesome low in the
eastern sky in the hour before sunrise. The stars will be fading and the planets
should shine bright.
Mars: At this time of year, the red planet is lost in the evening twilight. It will be
visible again at dusk by April or May of next year.
Jupiter: The largest planet is visible low in the southwest sky during the late
evening. Look for a bright, star-like object. With a small telescope you can even
see some of the moons as points of light surrounding it.
Did you know?
You might think that we’re in the center of our galaxy based on all those stars
you see, but we’re not. We’re 30,000 light-years away from the center of the
Milky Way.
Another way to look at it is how far we are from the next star. In the center of the
Milky Way there are as many as 100,000 stars in one cubic light-year.
In our part of the galaxy, there isn’t even another star within four
light-years.
If you were traveling in a spaceship at 10 miles per second (36,000 miles per
hour) it would take you 70,000 years to get to the next closest one.
Could Earth be hit by an asteroid or meteor?
For the most part, our atmosphere protects us by burning up the majority of
space material as it enters. In fact, the atmosphere can protect us from NearEarth-Objects (NEOs) up to about 130 feet in diameter.
Scientists have discovered more than two-thirds of the asteroids in the solar
system that are larger than this threshold and continue to watch for others that
cross our orbit.
However, they say it’s not unlikely that we could be hit by a relatively small NEO
with the force of a nuclear bomb “sometime in the next couple of centuries.” The
last hit Siberia in 1908.
On the other hand, a globally catastrophic impact on average occurs only once
every 1 million or 2 million years.
The 3/4-mile crater, pictured at left, is from a meteorite impact 49,000 years ago
in Arizona.