October 06 – 51 Pegasi b

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”).

Count them if you like.

Count them if you like.

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).

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).


Smallish asteroid 299 Thora is a fairly typical main belter, discovered on October 6th 1890 by Johann Palisa. It is about 17 km wide, and zooms around the Sun every 1,387 days at 19 km/second.

Thor wades through a river while the æsir take the bridge, by Lorenz Frølich

Thor wades through a river while the æsir take the bridge, by Lorenz Frølich

The name Thora was chosen by a Professor Schlieber of Berlin, after the Germanic and Norse god of thunder and lightning, Thor. According to Norse legend, Thor was the son of Odin, and husband of Sif (although, as with the Greek gods, this didn’t stop him fathering children by other women). Thor, as you probably aware, carried a hammer capable of flattening mountains, and had Thursday named in his honour.

So, if you know anyone named Thora, tell them they can blame it on their pagan Viking parents.


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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