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