^O'dell, C. R.; Handron, Kerry D. Cometary Knots in the Helix Nebula(PDF). Astronomical Journal. April 1996, 111: 1630. Bibcode:1996AJ....111.1630O. doi:10.1086/117902. hdl:1911/17047. Their masses of about 10−5M☉ would be more like those of our solar system planets (M🜨 = 7024596565000000000♠3×10−6M☉, MJ = 7027190900800000000♠9.6×10−4M☉) than of our largest observed comets (7019100000000000000♠10×1018 gms).
^ 4.04.1Huggins, Patrick J.; Forveille, Thierry; Bachiller, Rafael; Cox, Pierre; Ageorges, Nancy; Walsh, Jeremy R. High-Resolution CO and H2 Molecular Line Imaging of a Cometary Globule in the Helix Nebula. The Astrophysical Journal. 2002-07-01, 573 (1): L55–L58. arXiv:astro-ph/0205516. doi:10.1086/342021.
^ 6.06.16.2Meaburn, J.; Clayton, C. A.; Bryce, M. & Walsh, J. R. The global motions of the cometary knots in the Helix planetary nebula (NGC 7293). Monthly Notices of the Royal Astronomical Society. 1996, 281 (3): L57–L61. Bibcode:1996MNRAS.281L..57M. doi:10.1093/mnras/281.3.l57.
^Henney, W. J. R. T. Schilizzi , 编. Title: Classification of ionized photoevaporation flows. Galaxies and Their Constituents at the Highest Angular Resolutions, Proceedings of IAU Symposium #205, Held 15–18 August 2000 at Manchester, United Kingdom. 2001, 205: 272–3. Bibcode:2001IAUS..205..272H..