Geek Trivia: Planetary (m)alignment

Besides the sun and planets, what are the eight known objects in the solar system larger than Pluto?

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.


What are the eight known objects in the solar system—besides the sun and the first eight planets—that are 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.

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The Quibble of the Week

If you uncover a questionable fact or debatable aspect of this week's Geek Trivia, just post it in the discussion area of the article. Every week, yours truly will choose the best post from the assembled masses and discuss it in the next edition of Geek Trivia.

This week's quibble comes from the January 25 edition of Geek Trivia, "Rising from the ashes." Long-time quibbler and Geek favorite Bill Ward called me out for my characterization of the space shuttle Challenger disaster.

"[You said,] '(Technically, Challenger itself did not explode—rather, extreme g-forces caused by a severe malfunction in one of the spacecraft's solid rocket boosters tore it apart. As such, the disaster wrought surprisingly little fire damage on the cargo.)' This is very incorrect.

"What destroyed Challenger was technically an explosion of the liquid hydrogen tank of the external fuel tank, which ripped apart the shuttle. The fuel tank exploded because of a gas of hot flame that burned through an o-ring on one of the two external solid rocket boosters [SRBs], but burn-through of the SRB was not in any way, shape, form, or fashion, enough to have destroyed Challenger itself directly."

Luckily, TechRepublic member Mickster269 found someone who could explain my meaning better than myself: NBC News space analyst James Oberg.

"'The shuttle did not explode in the common definition of that word. There was no shock wave, no detonation, no bang—viewers on the ground just heard the roar of the engines stop as the shuttle's fuel tank tore apart, spilling liquid oxygen and hydrogen, which formed a huge fireball at an altitude of 46,000 [feet]. . . Challenger itself was torn apart as it was flung free of the other rocket components and turned broadside into the Mach 2 airstream. Individual propellant tanks were seen exploding—but by then, the spacecraft was already in pieces.'"

Sorry if I was unclear in my distinctions. For once, I squeaked by Bill on a technicality. Good work everyone, and keep those quibbles coming!

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The Trivia Geek, also known as Jay Garmon, is a former advertising copywriter and Web developer who's duped TechRepublic into underwriting his affinity for movies, sci-fi, comic books, technology, and all things geekish or subcultural.