For fans of Pluto to ponder

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Planetary (m)alignment

 

In honor of NASA's Pluto-bound New Horizons probe that launched last month,

we're celebrating two astronomy geek anniversaries this week. On Feb. 7,

1979, the planet Pluto's highly eccentric, inclined orbit brought it closer

to our sun than the next "innermost" planet, Neptune.

 

Twenty years later, on Feb. 11, 1999, Pluto slipped back outside Neptune's

orbit, becoming the ninth planet in our solar system once again. That

presumes, of course, that you're still willing to consider Pluto a planet.

 

In some respects, Pluto received its planet status partially because an

institution commissioned specifically to search for a ninth

planet--Arizona's Lowell Observatory, founded by astronomer Percival

Lowell--was responsible for its discovery in 1930. In fact, Lowell predicted

the eventual detection of a ninth planet as early as 1908.

 

(But don't proclaim Lowell a visionary just yet. He also believed fervently

in the now laughably dismissed Martian canals.)

 

When Lowell Observatory astronomer Clyde Tombaugh pulled together

indisputable photographic evidence of Pluto's existence in 1930, it matched

Lowell's predictions about a so-called "Planet X" closely enough to justify

declaring Pluto a planet. However, there was little publicized consideration

of whether it qualified for such a designation or what the standard of

planethood might be.

 

However, Pluto may not be able to live up to any such standard. We've

already mentioned its unusual orbit, but Pluto is also a great deal smaller

than most observers originally estimated.

 

Lowell believed in the existence of a ninth planet due to some perceived

irregularities in the orbits of Neptune and Uranus. Lowell and his

contemporaries believed that only a planetary mass could explain the orbital

strangeness, and when Pluto appeared in photographs in 1930, many assumed it

would fit the bill.

 

Instead, it turns out that Neptune has much less mass than early

20th-century estimates contended. And as observations of the seventh and

eighth planets became more precise, the "necessary" and calculated size of

Pluto--originally thought to be larger than Mercury--diminished.

 

The 1978 discovery that Pluto had a moon (Charon) meant that the total mass

of the Pluto system was actually quite small and that Pluto itself--less

Charon's mass--was smaller still. Indeed, not only is Pluto smaller than all

of the other local planets, but it has less mass that eight other

non-planets in the solar system.

 

BESIDES THE SUN AND PLANETS, WHAT ARE THE EIGHT KNOWN OBJECTS IN THE SOLAR

SYSTEM LARGER THAN PLUTO?

 

Seven planetary moons are larger than Pluto, including Earth's own moon,

which is roughly 50 percent larger than the so-called planet. Beyond that,

Jupiter's moons Ganymede, Callisto, Io, and Europa, the Saturnian moon

Titan, and Neptune's largest moon Triton are all more massive than Pluto.

 

Contestant number eight in our little rundown is a trans-Neptunian object

discovered barely more than a year ago: 2003 UB313.

 

A trans-Neptunian object is, not surprisingly, any object that regularly

orbits our sun at a distance greater than the orbit of the planet Neptune.

The Oort Cloud and the Kuiper Belt are thus collections of trans-Neptunian

objects.

 

On Jan. 5, 2005, astronomers at California's Mount Palomar Observatory

discovered object 2003 UB313, nicknaming it Xena . Current estimates place

its diameter between 2,500 to 3,500 kilometers, larger than Pluto's own

2,400-kilometer diameter.

 

Moreover, Xena has a moon (nicknamed Gabrielle), a fact that has helped

foster a rather passionate astronomical debate: Is 2003 UB313 a planet?

 

Complicating matters is the fact that the International Astronomical Union,

which oversees the formal classification and naming of all astronomical

bodies and phenomena, has no explicit definition of a planet. In fact, if

Pluto had somehow managed to avoid detection until today, it's very likely

that, given its small size and strange orbit, science would have classified

it as a trans-Neptunian object rather than a planet.

 

Moreover, if Pluto is a planet, what would stop Xena from earning the same

designation? Now, before we start waxing dorktacular about how imminently

cool it would be to have finally found the long-sought 10th planet, consider

this: Science first caught Xena on film in 2003, but it still took more than

a year of analysis to realize that the object was even there, much realize

that it was so much larger than any heretofore-observed trans-Neptunian

object.

 

Therefore, it's imminently plausible that there are untold numbers of

similar sized objects out there just waiting for detection-all of which

would qualify as additional planets if the loose, Pluto-inclusive standard

of planet designation holds. In the meantime, the Pluto vs. Xena debate

shows little sign of coming to a resolution in the near term, leaving us

with some unanswered questions-and a great deal of Geek Trivia.