A well-defined planet.

In his Perspective “A black widow's best friend?” (23 September, p. [1712][1]), F. A. Rasio discusses the discovery of a millisecond pulsar found to have a planetary-mass companion ([ 1 ][2]). Rasio points out that the object currently has a mass similar to that of Jupiter, but probably began its life as a stellar companion. He explains that the pulsar's intense radiation field likely eroded away most of the object's mass over time. Rasio argues that even though this object probably did not form in the traditional manner for planets—i.e., by growth in a rotating disk—it should nevertheless be classified as a “planet” by virtue of its present-day mass. He notes that in 2006 the International Astronomical Union (IAU) approved a definition of what constitutes a “planet” in our Solar System (primarily with Pluto in mind): It “(a) is in orbit around the Sun, (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and (c) has cleared the neighborhood around its orbit” ([ 2 ][3]). Rasio wonders whether this new object should be accorded the status of being a “planet,” even though it does not orbit a star similar to our Sun. ![Figure][4] CREDIT: SWINBURNE ASTRONOMY PRODUCTIONS, SWINBURNE UNIVERSITY OF TECHNOLOGY, AUSTRALIA An answer to his question was proposed by the IAU's Working Group on Extrasolar Planets (WGESP) in 2001, when the first system of pulsar planets ([ 3 ][5]), discovered in 1992, was approved as a planetary system. That system is believed to have formed in a manner different from the object on which Rasio is pondering, but the WGESP's definition ([ 4 ][6]) does not take into account the formation mechanism of any planet. The definition includes the statement, “Objects with true masses below the limiting mass for 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 formed).” According to this statement, the IAU status of the newly discovered companion ([ 1 ][2]) to a millisecond pulsar is clear: It is a planet. 1. [↵][7] 1. M. Bailes 2. et al ., Science, 333, 1717 (2011). [OpenUrl][8][Abstract/FREE Full Text][9] 2. [↵][10] International Astronomical Union (IAU), IAU 2006 General Assembly: Result of the IAU Resolution Votes (2006); [www.iau.org/public_press/news/detail/iau0603/][11]. 3. [↵][12] 1. A. Wolszczan, 2. D. A. Frail , Nature, 355, 145 (1992). [OpenUrl][13][CrossRef][14][Web of Science][15] 4. [↵][16] Working Group on Extrasolar Planets (WGESP) of the International Astronomical Union, “Position Statement on the Definition of a ‘Planet’” (2011); [www.dtm.ciw.edu/users/boss/definition.html][17]. [1]: /lookup/doi/10.1126/science.1212489 [2]: #ref-1 [3]: #ref-2 [4]: pending:yes [5]: #ref-3 [6]: #ref-4 [7]: #xref-ref-1-1 "View reference 1 in text" [8]: {openurl}?query=rft.jtitle%253DScience%26rft_id%253Dinfo%253Adoi%252F10.1126%252Fscience.1208890%26rft_id%253Dinfo%253Apmid%252F21868629%26rft.genre%253Darticle%26rft_val_fmt%253Dinfo%253Aofi%252Ffmt%253Akev%253Amtx%253Ajournal%26ctx_ver%253DZ39.88-2004%26url_ver%253DZ39.88-2004%26url_ctx_fmt%253Dinfo%253Aofi%252Ffmt%253Akev%253Amtx%253Actx [9]: /lookup/ijlink/YTozOntzOjQ6InBhdGgiO3M6MTQ6Ii9sb29rdXAvaWpsaW5rIjtzOjU6InF1ZXJ5IjthOjQ6e3M6ODoibGlua1R5cGUiO3M6NDoiQUJTVCI7czoxMToiam91cm5hbENvZGUiO3M6Mzoic2NpIjtzOjU6InJlc2lkIjtzOjEzOiIzMzMvNjA1MC8xNzE3IjtzOjQ6ImF0b20iO3M6MjU6Ii9zY2kvMzM0LzYwNTkvMTA1Ny4xLmF0b20iO31zOjg6ImZyYWdtZW50IjtzOjA6IiI7fQ== [10]: #xref-ref-2-1 "View reference 2 in text" [11]: http://www.iau.org/public_press/news/detail/iau0603/ [12]: #xref-ref-3-1 "View reference 3 in text" [13]: {openurl}?query=rft.jtitle%253DNature%26rft.volume%253D355%26rft.spage%253D145%26rft_id%253Dinfo%253Adoi%252F10.1038%252F355145a0%26rft.genre%253Darticle%26rft_val_fmt%253Dinfo%253Aofi%252Ffmt%253Akev%253Amtx%253Ajournal%26ctx_ver%253DZ39.88-2004%26url_ver%253DZ39.88-2004%26url_ctx_fmt%253Dinfo%253Aofi%252Ffmt%253Akev%253Amtx%253Actx [14]: /lookup/external-ref?access_num=10.1038/355145a0&link_type=DOI [15]: /lookup/external-ref?access_num=A1992GY62900051&link_type=ISI [16]: #xref-ref-4-1 "View reference 4 in text" [17]: http://www.dtm.ciw.edu/users/boss/definition.html