Artist's concept of Bellerophon orbiting near 51 Pegasi.
Today marks the anniversary of an important 1995 announcement: Swiss astronomers Michel Mayor and Didier Queloz became the first Earthlings to discover a Sun-like star with its very own orbiting planet! Curious as to why this solar system looked so dang familiar, scientists the world over swung their telescopes toward the Pegasus constellation and took a closer look.
51 Pegasi is a yellow dwarf star found about 50 light years from Earth. Its apparent magnitude of 5.49 makes it visible even with our little naked eyes - providing we've got a pleasantly dark sky and the ability to distinguish which dot is which (I don't). Comparatively, 51 Pegasi is estimated to be 3 billion years older than our Sun, 4-6% more massive, and contain more metals and less hydrogen.
Always eager to force a system to conform to known ideals, the planeteers of 1995 proclaimed 51 Pegasi's exoplanet a terrestrial one (gasp! Another Earth!? Aliens?), and gave it the incredibly creative name of "51 Pegasi b." (Wah-wah... This is actually an organized naming system for extrasolar planets. The first planet found is given its star's name followed by the letter b, the second is followed by c, and so on. It is based purely on discovery, and therefore not indicative of station or orbit order around the star. Our planet's informal title, Bellerophon, is much cooler and thus will be known as such from here on out.) Scientists of the day found out rather quickly that Bellerophon orbits uncomfortably close to the star, and current theories of planet formation could not account for this. Giant planets are most certainly unable to form at this distance! As you can guess, discussions of planetary migration ensued and scientists began extensive observation of the exoplanet's characteristics.
As of 2009, Bellerophon is believed to be a gas giant comparable to Jupiter, which circles its star every four Earth days. The guess is that it has to have a wide radius and thick atmosphere to withstand the gusts of the star's solar wind - not to mention its heat intake, harboring "air" at a mere 1200°C.
Psh, this is old news, yes! But its discovery broke the conventional view of planets with short orbits, and forced astronomers to redefine the accepted characteristics of gas planets. Since then, space exploration has included more radial velocity planet searches, and consequently many more systems similar to the Milky Way have been found. To be precise, exactly 263 stars with 307 planets circling them have been discovered as of 2008 (...aaand none of which are Earthlike).
As for our subject matter, about three years ago some American scientists threw 51 Pegasi back into the limelight by mentioning Bellerophon as one of the planets "most likely to support extraterrestrial life." Personally, I think they're crazy. But that's another story for another time...
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