Solar system names changing

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Tariq Malik
Senior Editor
SPACE.com
Mon Jul 28, 7:01 AM ET



The solar system seems to be getting more crowded by the day as its
once nine-world population gives way to a realm of planets, dwarfs and
the dim and distant plutoids.

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But in reality, the solar system is still the same. It’s just the
names for the new stuff astronomers find that are changing.


The cosmic can of worms opened in 2006 when after much debate the
International Astronomical Union (IAU) - an international society of
astronomers - demoted Pluto to dwarf planet status, 76 years after its
discovery. The decision is still widely contended among astronomers,
and the IAU added more fuel to the debate last month when it filed
Pluto and similar objects under a new category: “plutoid.”


Objects such as Eris and the newly-named Makemake, which like Pluto
circle the sun out beyond the orbit of Neptune, also fall among the
plutoid and dwarf planet ranks.


While astronomers continue to debate the definitions and differences
of terms like “planet,” “dwarf planet” and “plutoid,” the IAU has
offered its take on the newest rungs in the solar system’s ladder:


What is a planet?


By the IAU’s 2006 definition, a planet is a celestial body that orbits
the sun, has enough mass that its gravity gives it a nearly round
shape and has cleared its surrounding area of debris.


Pluto, while round and orbiting the sun, is one of a swarm of so-
called trans-Neptunian objects, small icy bodies in the comet
reservoir of the Kuiper Belt that extends out from Neptune’s orbit,
leading to its IAU demotion. But critics have said that asteroids can
be found accompanying established planets like Earth, Mars and
Jupiter, throwing a wrench in that requirement.


What is a dwarf planet?


A dwarf planet, meanwhile, is defined as an object that orbits the
sun, has enough mass and gravity to assume a nearly round shape. It
need not have swept its local region clean, which opened the dwarf
planet gate not only for Pluto, Eris, Makemake and others beyond
Neptune, but also Ceres - the largest asteroid in the solar system.
Ceres circles the sun in the asteroid belt between Mars and Jupiter.


Objects smaller than a dwarf planet are called solar system bodies,
according to the IAU. Other astronomers refer to such things, right
down to relatively small asteroids, as minor planets.


What is a plutoid?


Last month, the IAU also reclassified Pluto, and any other dwarf
planet or round object beyond Neptune, as a so-called plutoid.


Like dwarf planets, plutoids must orbit the sun and be massive enough
to maintain a nearly round shape. But they also must be located beyond
the orbit of Neptune, which circles the sun every 165 years from a
distance of about 30 astronomical units (AU). One AU is the distance
from the Earth to the sun, or about 93 million miles (150 million km).


It should be noted that according to the IAU, the small moons of dwarf
planets or plutoids cannot themselves be considered as dwarf planets
or plutoids.


What’s in a name?


The three designations have taken center stage in recent years as
astronomers spot new objects and refine the orbits and attributes of
other known, but distant, solar system bodies. There are other, even
odder designations – things like plutons, plutinos, centaurs and EKOs
– but when it comes to actually naming something, the IAU wins out.


Take Makemake, which earned its name this summer three years after its
discovery in 2005 by astronomer Mike Brown of the California Institute
of Technology in Pasadena, Calif., whose team has discovered myriad
similar, distant objects.

When astronomers spot a previously unknown solar system body or dwarf
planet contender, it is given a preliminary designation by the IAU’s
Minor Planet Center (136472 for Makemake), and then observed until its
orbit can be better determined. Once that happens, it gains a
provisional moniker (Makemake was 2005 FY9) until an official name is
chosen.

The discoverer of an object can then suggest a name to the IAU, which
meets in a committee to decide on its suitability. For Makemake, Brown
chose the name of a fertility god from Rapa Nui, or Easter Island.

“We take naming objects in the solar system very carefully,” Brown
wrote in a recent blog post, adding that he seeks names from mythology
that seem to have some sort of resonance with the object to bear the
moniker. “Each of these names came after considerable thought and
debate, and each of them fit some characteristic of the body that made
us feel that it was appropriate.”

With that in mind, here’s a look at what astronomer’s know is way out
there now, circling the sun:

The major planets

Since 2006, there have been just eight major planets, by the IAU’s
definition. Starting closest to the Sun, they are: Mercury, Venus,
Earth, Mars, Jupiter, Saturn, Uranus and Neptune.

With the exception of Earth, the planets were named after major
players in Greek or Roman mythology, a tradition that began in
antiquity and was continued with the somewhat recent discoveries of
Uranus (in 1781) and Neptune (in 1846).

The same approach was applied to Pluto when astronomer Clyde Tombaugh
spotted it in 1930, though the demoted world’s name was actually
suggested by then 11-year-old Venetia Burney Phair of England. She
named the world after the Roman god of the underworld.

Dwarf planets and plutoids

In addition to Pluto, the dwarf planet population of the solar system
currently includes Ceres, Eris, Makemake and another world currently
dubbed 2003 EL61.

Named after a Roman goddess of grain, Ceres was discovered in 1801 and
was initially considered a planet until astronomers began to spot
other asteroids circling the sun in nearby orbits. While a dwarf
planet, the round, potentially water-ice bearing space rock does not
qualify as a plutoid because it circles the sun well inside the orbit
of Neptune.
Unlike Ceres, Eris (ee'-ris) does qualify as both dwarf planet and
plutoid. It is about 70 miles (112 km) wider than Pluto, orbits the
sun from about 9 billion miles (14 billion km) away and is one of the
brightest objects in the Kuiper Belt. Discovered by Brown and his team
in 2005, Eris was initially nicknamed “Xena” and its solitary moon
“Gabrielle” after the lead characters of a television show. It takes
its official name, Eris, from the Greek goddess of chaos and strife as
a fitting tribute to the debate it and other object sparked over the
definition of a planet. The object’s moon, Dysnomia, is named after
the daughter of Eris, who served as the spirit of lawlessness.
Makemake (pronounced MAH-keh MAH-keh) is the newest dwarf planet and
plutoid to gain a name. The tiny red-hued world is though to be
covered with a layer of frozen methane and is smaller and dimmer than
Pluto. Earlier this month, the IAU officially announced that the
object was named Makemake after the Polynesian god of fertility and
creator of humanity among the people of Rapa Nui, also known as Easter
Island, in the Pacific Ocean. Brown learned of the name while
researching various mythologies for potential monikers and found it
particularly striking. “Eris, Makemake, and 2003 EL61 were all
discovered as my wife was 3-6 months pregnant with our daughter,” he
said in a statement.
Waiting in the wings

The list doesn’t stop there. There are still more objects waiting for
wings for either their own classification or official name:

While it doesn’t have an official name yet, 2003 EL61 is an object
also discovered by Brown’s team and an independent group led by Jose-
Luis Ortiz of the Sierra Nevada Observatory in Spain. It has its own
moon, is about 32 percent as massive as Pluto and about 70 percent
that of Pluto’s 1,413-mile (2,274-km) diameter. But it’s also shaped
like an ice-covered football, making it one weird, distant space
cookie.
Then there’s Sedna (sed’nah), an object about three-fourths the size
of Pluto that is so far out from the sun it takes about 10,500 years
to make a single orbit. Sedna is about 1,100 miles (1,770 km) wide and
circles the sun on an extremely eccentric orbit that ranges between 8
billion miles (12.9 billion km) and 84 billion miles (135 billion km).
Brown’s team led the discovery of that object in 2004 and named it
after the Inuit goddess of the sea. Sedna does not qualify as a
plutoid because of what some astronomers see as a quirky threshold for
how much sunlight it reflects: Sedna is too dim.
Quaoar (KWAH-o-ar), another find by Brown and co-discoverer Chad
Trujillo, is 780 miles (1,250 km) wide, half the size of Pluto and
takes 288 years to orbit the sun from about 4 billion miles (6.5
billion km) away. It was named after the creation force of the Tongva
tribe of the Los Angeles basin.
Brown’s team also found Orcus (awr-kuhs), or 2004 DW, an object about
994 miles (1,600 km). It is nearly 47 AU from the Sun, was discovered
in 2004, and is so Pluto-like in its attributes that astronomers named
it after the Etruscan counterpart of the Roman underworld deity.
And there’s still Varuna, or KBO 20000 Varuna, an icy object 40
percent as large as Pluto and 560 miles (900 km) wide that was first
spotted in 2000 by astronomers using the Spacewatch telescope in
Arizona.
But don’t get too comfortable with this list. There’s surely more to
follow.

By the IAU’s own admission, at least a few dozen more dwarf planets,
if not hundreds, remain to be found as they silently orbit the sun at
the edge of the solar system.