NAME THAT CELESTIAL OBJECT

Scientific nomenclature can barely keep up with the range of odd objects that astronomers are discovering in the solar system these days. Future discoveries will no doubt suggest new categories and further blur the boundaries of old ones. In the meantime, here’s a field guide to the known residents.

PLANETS lack a standard definition. If a body orbits a sun and was made spherical by its own gravity, astronomers tend to call it a planet. Yet that definition would include at least four asteroids and dozens of Kuiper belt objects.

ASTEROIDS (a.k.a. minor planets) are rocky, metallic, or carbonaceous bodies whose solar orbit takes them beyond Mars, into the so-called asteroid belt. They typically lack sufficient gravity to retain an atmosphere. The latest count of asteroids in the inner solar system, inside Jupiter, is about 700,000.


COMETS are icy bodies that follow elliptical orbits. Those that have orbital periods shorter than 200 years originate in the Kuiper belt. Longer-period comets originate much farther out in the Oort cloud. The known comets number in the thousands.

CLASSICAL KUIPER BELT OBJECTS orbit the sun at 3.9 billion to 4.5 billion miles out. They are sometimes called cubewanos, after QB1, the first Kuiper belt object discovered (1992).

RESONANT KUIPER BELT OBJECTS orbit in synchrony with Neptune. Pluto is the prototype: It orbits twice around the sun for every three solar orbits made by Neptune. About 20 percent of known RKBOs are thought to orbit with a similar 2:3 resonance and thus are called plutinos. Six other resonant orbits have been detected.

SCATTERED KUIPER BELT OBJECTS have very eccentric and tilted orbits that carry them from around 3.3 billion miles from the sun to almost 100 billion miles out. They may have been kicked into their far-flung orbits by Neptune’s gravity.

A New Miniplanet Rises

Detected last November by Brown and his colleagues, using the 48-inch Samuel Oschin Telescope, Sedna is the largest, farthest, coldest, and arguably weirdest world yet discovered in the distant solar system. It may be as large as 1,100 miles in diameter, and it has a mysterious deep red surface—neither particularly dark, like a typical rocky object, nor bright and icy like Pluto and other Kuiper belt objects. Its orbit is even more peculiar. It passes no closer to the sun than 7 billion miles, nearly twice as far away as Pluto ever gets, and it sails beyond the outer edge of the Kuiper belt, possibly into the Oort cloud. Is it a stray Kuiper belt object, a comet from the inner Oort cloud, or something different?
     SEDNA

Discovered: November 14, 2003

Distance from the sun: 8 billion miles; 

ranges from 7 billion to 90 billion miles

Diameter: 800–1,100 miles

Orbital period: 10,500 years

      Features: It’s the coldest body

in the solar system, with

temperatures that approach

400 degrees  Fahrenheit.

Images courtesy of NASA/Caltech (top) and NASA (bottom)