by Mike Wall
October 26, 2011
Credit: David Aguilar/Center
For three-quarters of a century, schoolkids learned that our solar
system has nine planets:
Mercury, Venus, Earth, Mars,
Jupiter, Saturn, Uranus, Neptune and Pluto.
But things changed five years ago.
On Aug. 24, 2006, the International
Astronomical Union (IAU)
struck Pluto from the list, demoting it to the newly created
category of "dwarf planet."
The move was spurred by the discovery of
multiple large bodies orbiting even farther from the sun than
distant Pluto - particularly an object called Eris, which appeared
to be bigger than Pluto.
As a result, the IAU came up with a new definition of "planet":
A body that circles the sun without
being some other object's satellite, is large enough to be
rounded by its own gravity (but not so big that it begins to
undergo nuclear fusion, like a star) and has "cleared its
neighborhood" of most other orbiting bodies.
Since Pluto shares orbital space with
lots of other objects out in the Kuiper Belt - the ring of icy
bodies beyond Neptune - it didn't make the cut. So Pluto was newly
classified as a dwarf planet, which tend to be smaller than
"true" planets and fall short on the "clearing your neighborhood"
Although hundreds, or perhaps thousands, more solar system bodies
may eventually join the list, the IAU officially recognizes just
five dwarf planets at the moment.
Here's a brief tour of all five:
Pluto - The
demoted former planet
Credit: NASA, ESA, and M.
Buie (Southwest Research Institute)
Pluto was discovered by American Clyde Tombaugh in 1930. It's
about 1,455 miles (2,352 kilometers) across - less than 20 percent
as big as our planet. And Pluto is just 0.2 percent as massive as
Pluto has an extremely elliptical orbit that's not in the same plane
as the eight official planets' orbits. On average, the dwarf planet
cruises around the sun at a distance of 3.65 billion miles (5.87
billion km), taking 248 years to complete one circuit.
Because it's so far from the sun, Pluto is one of the coldest places
in the solar system, with surface temperatures hovering around minus
375 degrees Fahrenheit (minus 225 degrees Celsius).
Pluto has four known moons:
While Nix, Hydra and P4 are relatively
small, Charon is about half as big as Pluto.
Because of Charon's size, some
astronomers regard Pluto and Charon as a double dwarf planet,
or binary system.
Eris - The
Caltech astronomer Mike Brown led the team that discovered
Eris in 2005. The find spurred the IAU to strip Pluto of its
planethood and create the "dwarf planet" category a year later.
That decision remains controversial to this day, making Eris' name
Eris is the Greek goddess of discord
and strife, who stirred up jealousy and envy among the
goddesses, leading to the Trojan War.
Eris has one known moon, Dysnomia.
Eris is virtually the same size as Pluto, but it's about 25 percent
more massive, suggesting that Eris contains considerably more rock
(and less ice) than its Kuiper Belt neighbor.
Like Pluto, Eris has a highly elliptical
But Eris is even more far-flung,
orbiting the sun at an average distance of about 6.3 billion miles
(10.1 billion km). It takes Eris 557 years to complete one lap
around the sun.
Haumea - The
Credit: SINC/José Antonio Peñas
Haumea, a Kuiper Belt denizen orbiting slightly beyond Pluto, was
discovered by Brown and his team in late 2004.
It's one of the weirdest objects in the
Haumea measures about 1,200 miles (1,931 km) across, making it
nearly as wide as Pluto. But Haumea is just one-third as massive as
Pluto, partly because it's not spherical.
Rather, Haumea is shaped like a giant
The dwarf planet also completes one full rotation in less than four
hours, making it one of the fastest-spinning bodies in the solar
This super-charged spin is responsible
for Haumea's oblong shape, pushing the dwarf planet outward
substantially at the equator. Haumea makes one complete lap around
the sun every 283 years.
Haumea has two known moons,
Credit: IAU/M. Kornmesser
Brown's team also discovered Makemake, spotting the dwarf planet in
Astronomers aren't sure of Makemake's
exact size, but the dwarf planet is thought to be about
three-quarters as big as Pluto. It's therefore likely the
third-largest dwarf planet, after Eris and Pluto.
Makemake orbits the sun from slightly farther away than Pluto, at an
average distance of 4.26 billion miles (6.85 billion km), and
completes an orbit every 310 years or so.
Makemake is the second-brightest Kuiper Belt object (after Pluto)
and can be seen with a high-end amateur telescope, according to the
Ceres - Queen
of the asteroid belt
Credit: NASA, ESA, J.
Parker - Southwest Research Institute
L. McFadden - University of Maryland
is the only dwarf planet not found in the freezing cold,
Belt. Rather, it orbits in the main asteroid belt between
Mars and Jupiter, completing one lap around the sun every 4.6 years.
Ceres is by far the largest object in the asteroid belt, containing
about one-third of the belt's mass. However, at 590 miles (950 km)
across, it is the smallest known dwarf planet.
Because it's so much closer to Earth than the other dwarf planets,
Ceres was discovered far earlier.
Italian astronomer Giuseppe Piazzi
spotted it first, on Jan. 1, 1801. For the next half-century, many
astronomers regarded Ceres as a true planet. That changed when it
became apparent that Ceres was just one of many bodies hurtling
through space in the asteroid belt.
These days, most astronomers regard Ceres as a protoplanet, saying
it likely would have continued growing into a full-fledged rocky
planet like Earth or Mars if Jupiter hadn't shaken up the
asteroid belt long ago.