On Oct. 5, 1995, astronomers Michel Mayor and Didier Queloz announced the discovery of the first confirmed planet orbiting a star other than our own sun. (This is a day known in the Trivia Geek’s household as “The Day Star Trek Came True.”)

As a result, the terms extrasolar planet and exoplanet came into somewhat more general usage — namely because TV news journalists started saying them. In the past dozen years, more than 200 suspected or confirmed exoplanets have found their way onto the list of known celestial objects.

Most of these planets have one thing in common. They’re massive enough to exert enough gravitational pull that their parent stars wobbled (the actual technical term) in noticeable and predictable patterns, making the inference of planetary bodies around the stars a matter of straightforward if complex mathematics.

How massive is massive? Well, as a sort of astrogeek shorthand, we measure most exoplanet masses as factors of the mass of Jupiter, 1.8986 × 1027 kilograms — or roughly 318 times the mass of Earth. For example, Mayor and Queloz’s discovery was a planet orbiting the star 51 Pegasi and weighed in at more than 0.47 Jupiter masses (MJ). Of the 29 currently confirmed extrasolar planets, the average mass is roughly twice that of Jupiter. The most massive exoplanet confirmed to exist is XO-3 b, which weighs in at a whopping 12 Jupiter masses.

The largest exoplanet ever discovered, however, is not the same as the most massive, which should clue you in to some of the inherent astronomic weirdness surrounding the entire planetary discovery enterprise.

Planet TrES-4 is 1.7 times the diameter of Jupiter — about 120,000 kilometers wide — but weighs in at just one Jupiter mass. That averages out to an absurdly low planetary density — around 0.2 grams per cubic centimeter — or roughly equivalent to balsa wood. TrES-4 is such a lightweight that scientists can’t figure how it stays together, especially since the planet is bleeding a comet-like tail across its orbital path as its atmosphere escapes into space.

Every new planet discovered brings its own unique mystery to the table, and such has been the case since exoplanet discovery first started — three years before Mayor and Queloz found 51 Pegasi b. It began with a planet so weird that some almost refuse to acknowledge it as a traditional planet, which is why the Mayor/Queloz discovery is so much more famous.

WHAT WAS THE FIRST EXTRASOLAR PLANET EVER DISCOVERED?

Get the answer.

What was the first extrasolar planet ever discovered, a celestial object so unusual that some refuse to even qualify it as a traditional planet?

We actually have a pair of answers, as the discovery of the first two extrasolar planets was simultaneous. Astronomers Aleksander Wolszczan and Dale Frail unveiled PSR 1257+12 B and PSR 1257+12 C in 1992.

What makes these two planets so unusual is that they orbit not a star, but a pulsar — PSR 1257+12 — which Wolszczan and Frail catalogued in 1990. The astronomers discovered the two planets using pulsar timing, which essentially amounts to decoding hiccups in the pulsar’s otherwise clockwork radio frequency emissions. Planets B and C are relatively tiny worlds — both about four times the mass of Earth — that orbit the pulsar at 0.36 and 0.46 Astronomical Units, respectively.

Even though their discovery came first, the two fast-orbiting orbs bear the B and C designations. That’s because in 1994 Wolszczan and Frail discovered the pulsar’s innermost planet, PSR 1257+12 A, which is merely twice the mass of Earth’s moon and orbits at a decidedly cozy 0.19 AU (about 50 percent closer to its parent than Mercury ever gets to our sun at its perihelion).

These planets don’t get much publicity since they’re pulsar companions, rather than associates of a traditional star. Mayor and Queloz discovered the first “acceptable” planet, but as we said, it’s a rather un-Earthlike orb. It’s still not a place we could conceivably send a team of astronauts to explore, assuming we could conjure up Warp Drive in the next month or so. If we’re going to be picky about planetary bodies, let’s take it to the sci-fi-friendly extreme.

The first possible Class M planet out there (to extend the Star Trek parlance) is Gliese 581 c. Only discovered in April 2007, it hasn’t yet graduated to the independently confirmed list yet.

A so-called super-earth, Gliese 581 C is a terrestrial (rock, not gaseous) planet about five times the mass of Earth and orbits its red dwarf parent star in the Goldilocks Zone — a distance neither too hot nor too cold to support liquid water and, therefore, potential life. It’s also only about 20 light years away in the constellation Libra.

If there’s a candidate for first planet-side interstellar human colony, Gliese 581 c is the current frontrunner. Confirming or denying its human habitability would prove quite the feat not just for astronomers but humanity itself — and it would make for some galactically horizon-expanding Geek Trivia.

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. Every week, yours truly will choose the best post from our assembled masses and discuss it in a future edition of Geek Trivia.

This week’s quibble comes from the September 12 edition of Geek Trivia, “Holiest of holes.” My old buddy neilb accused me of tripping up the explicit terminology involving celestial objects that hit the Earth.

“’10-kilometer-wide meteor’ — I don’t think so. If it impacts the Earth, it’s a meteorite. Meteors burn up in the Earth’s atmosphere. In space, they’re meteoroids until proven otherwise. If something is going to leave 300-km wide craters, wipe out entire genera, and change the dominant life-form on the planet, then I do think that you have to name it correctly.”

Well, neilb, you’re right, I did screw up — just not simply for the reason you think. I never should have brought meteors, meteorites, or meteoroids into this at all, as we’ve crept into one of those gray areas of definitional astronomy. Anything 10 kilometers across is an asteroid, not a meteoroid.

While the line between meteoroid and asteroid isn’t explicit or consensual — thanks for another vague point, International Astronomical Union — a rock the size of a small town is pretty well into asteroid territory. I should have listed it as such (and avoided your whole quibble in the process).

Thanks for the catch, 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.

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