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Thousands of years ago, ancient astronomers looked up at the sky and realized that some
of the things they thought were stars were moving in ways they didn't expect.
They called these errant objects planets, a word that means wandering stars.
Five of these objects - Mercury, Venus, Mars, Jupiter, and Saturn - were identified and
named in ancient times.
Through the centuries, the tools astronomers used to study the skies became better and
stronger, and three more planets were identified: Uranus, Neptune, and Pluto.
However, as astronomers continued to study the outer reaches of our Solar System, they
came to a startling realization: Pluto wasn't alone in the space beyond Neptune.
As more and more objects were discovered near Pluto that were close to Pluto in size, astronomers
realized that they couldn't ALL be planets.
Something had to be done.
In 2006 the International Astronomical Union created a new category: dwarf planets.
For an object to qualify as a dwarf planet, it only had to meet two criteria: One, it
must orbit around the Sun.
Two, it must be massive enough for its own gravity to pull it into a spherical - or mostly
spherical - shape.
Planets had a third qualification they had to meet; they must have cleared their orbits
of other objects.
Since dwarf planets often share their orbits with asteroids, comets, debris, and even other
dwarf planets, this third point made it easier to tell the difference between a planet and
a dwarf planet.
There are currently five officially recognized dwarf planets in the Solar System: Ceres,
Pluto, Eris, Haumea, and Makemake.
Of those five Ceres was discovered first, in 1801.
That's because Ceres is located in the asteroid belt between Mars and Jupiter, making it easier
to observe using the telescopes of the time and the only dwarf planet in the inner solar
system.
When it was discovered, Ceres was heralded as the fifth planet from the sun, and Jupiter
was pushed to the sixth position.
This new order only lasted a year before another new planet was observed near Ceres - then
another, then another!
Within a few decades, dozens of new objects had been discovered between Mars and Jupiter.
By the 1850s, astronomers decided enough was enough, and reclassified all of the objects
in the asteroid belt - including Ceres - as minor planets.
Following the IAU's decision in 2006, Ceres was reclassified again as a dwarf planet.
It is the largest object in the asteroid belt, and contains 25% of the asteroid belt's mass.
It completes one rotation every 9 hours and one orbit around the sun every 4.6 years.
Ceres has no moons or rings, but it does have a very thin atmosphere which contains traces
of water vapor.
It is the smallest of the recognized dwarf planets with a diameter of 590 miles or 950 kilometers.
Ceres became the first of the dwarf planets to be visited by a spacecraft when the Dawn
space probe entered its orbit on March 6, 2015.
Pluto is the largest and most well-known dwarf planet.
It was regarded as the ninth planet from its discovery in 1930 until its reclassification
in 2006.
Pluto is small and made of rocky ice, and it rotates slowly with a day that is 6.4 Earth
days long.
Because it is usually farther from the sun than Neptune, it takes 248 years for it to
complete a single orbit around the sun.
Pluto has a thin atmosphere made of nitrogen, methane, and carbon monoxide.
Like Ceres, Pluto has no rings.
Unlike Ceres, Pluto has five moons including Charon, which is so large that Pluto and Charon
are sometimes called binary or double dwarf planets.
Pluto was visited by the New Horizons spacecraft in July of 2015, capturing stunning images
of the former planet and raising new questions about the composition and structure of this
cold and distant world.
Eris was named for the Greek goddess of discord and strife, which is fitting since the discovery
of Eris is what prompted arguments about Pluto's status as a planet.
When it was discovered in 2005, NASA initially called Eris the tenth planet, but discoveries
of multiple other large objects in the area caused them to reconsider.
Eris is slightly smaller than Pluto, but it is estimated to be more massive and it is believed
to be covered with methane ice.
It has one known moon, Dysnomia.
Eris and its moon are two of the most distant known objects in the solar system, traveling
much farther from the sun than Pluto does.
It is estimated that Eris will take 558 years to complete one orbit around the sun.
The discovery of another dwarf planet, Haumea, was announced in 2005.
Unlike the rest of the dwarf planets, Haumea is oblong instead of shaped like a ball!
Haumea appears to have water ice on its surface, and it is one of the fastest-spinning large
objects in the solar system, completing a turn once every four hours.
Its orbit is only a little longer than Pluto's, taking 284 years to travel once around the sun.
Haumea, named for the Hawaiian goddess of fertility and childbirth, has two known moons.
The moons, Hi'aka and Namaka, are named for two of the goddess' daughters.
The fifth and final dwarf planet currently recognized is Makemake.
Makemake was also discovered in 2005 and was named for the creator god in the mythology
of the Rapa Nui people of Easter Island.
Like the other dwarf planets of the Kuiper Belt, Makemake is covered in ices - made of
methane, ethane, and possibly nitrogen ice.
Although its diameter is only about two-thirds of Pluto's, Makemake's icy surface makes it
the second-brightest object in the Kuiper Belt, after Pluto.
In 2016 it was discovered that Makemake has at least one moon, which is as-yet unnamed.
Astronomers believe that there are hundreds of objects in the solar system that are likely
to be dwarf planets.
As scientists continue to study them, it is likely that more and more will be added to
our list.
Until then, I hope you enjoyed learning about Ceres, Pluto, Eris, Haumea, and Makemake - the
dwarf planets of the solar system.
Goodbye till next time!