October 06 – Discovery of Exopanet 51 Pegasi b (1995)

October 6th 1995 was a significant day in the hunt for extrasolar planets, with the discovery, in the constellation of Pegasus (the winged horse), of the first one found to be orbiting a main sequence, Sun-like star. That star was 51 Pegasi, and the planet is known as 51 Pegasi b, shortened to 51 Peg b if you’re in a hurry, and lengthened (unofficially) to Bellerophon if you’re not. Bellerophon was the Greek character who tamed Pegasus, so you can see what they did there. The “b”, by the way, indicates that this was the first planet discovered around 51 Peg. There is no “a”, as that letter would be used, in uppercase, to denote the star itself, and would only be needed if the star had a companion (“B”).

The discovery of 51 Peg b was made using the radial velocity method, and announced by Michael Mayor and Didier Queloz. It was later confirmed by other observers (always important). Using “radial velocity” to find planets just means watching the spectral lines of stars over times to see if they display a regular red- and blue-shift. This would indicate that they are being made to “wobble” by an object in orbit.

Radial velocity used to be a very popular method for planet hunting, but in recent years it has been surpassed by “transit photometry” (watching for teeny tiny dips in the light coming from a star as a planet passes in front of it).

Location of 51 Peg (image credit: Stellarium)

Peg 51 is located between two of the stars forming the asterism known as the “square of Pegasus”, Markab (α Peg) and the red giant Scheat (β Peg).

Despite orbiting a Sun-like star, 51 Peg b was still nothing like the type of place planet hunters were looking for (they all wanted to find an Earth-like planet at about the same orbit as ours). 51 Peg b is about 150 times the mass of the Earth, wider than Jupiter, is closer to 51 Peg than Mercury is to the sun, giving it a mean temperature of around 1,000°C, and has a year lasting about 4 Earth days. All of which has made me decide not to move there (my border perennials wouldn’t like it one bit).


1890 – Asteroid 299 Thora discovered by Johann Palisa.

1964 – The Soviet Union launches Kosmos 47, an unmanned Voskhod test flight.

1990 – Launch of the NASA / ESA Ulysses probe to study the Sun.

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