by Kasey Cunningham

In the November 25 issue of Science Magazine, DTM staff scientist Alan Boss gives his answer to a planet classification question raised in an earlier article written by Northwestern University Professor of Physics, F. A. Rasio. In the article, Rasio debates whether or not to classify the September discovery of a planetary mass object orbiting a pulsar as a “planet”. Rasio points out that the mass of the planetary object is comparable to that of Jupiter, and it could therefore be defined as a planet.

However, Rasio also reveals that the pulsar’s companion most likely did not form as most planets do, by growth from a rotating disk of gas and dust, so he is unsure if the International Astronomical Union (IAU) would agree to classifying the planetary mass object as a planet.

As the current President of the IAU Commission on Extrasolar Planets, Boss pointed out that in 1992, the IAU defined a system of pulsar companions as “planets”, based on their masses, even though they probably did not form in the same way as most planets. The pulsar planets fall under the following definition given by the IAU, which Boss quotes in his letter to Science Magazine: “Objects with true masses below the limiting mass for the thermonuclear fusion of deuterium (currently calculated to be 13 Jupiter masses for objects of solar metallicity) that orbit stars or stellar remnants are ‘planets’ (no matter how they are formed).”

Based on this definition, Boss reveals to Rasio, and the readers of Science Magazine, the planetary mass object is, in fact, a “planet”. Because the pulsar planet is likely to be the whittled down remnant of a star, the discovery shows that there is more than one way to form a “planet”.