A proplyd, also known as an ionized protoplanetary disk, is an externally illuminated photoevaporating disk around a young star. Nearly 180 proplyds have been discovered in the Orion Nebula. Images of proplyds in other star-forming regions are rare, while Orion is the only region with a large known sample due to its relative proximity to the Earth.
- Ricci, L.; et al. (2008). "The Hubble Space Telescope/Advanced Camera for Surveys Atlas of Protoplanetary Disks in the Great Orion Nebula". Astronomical Journal 136 (5): 2136–2151. Bibcode:2008AJ....136.2136R. doi:10.1088/0004-6256/136/5/2136.
- Sharkey, Colleen; Ricci, Luca (Dec 14, 2009). "Born in beauty: proplyds in the Orion Nebula" (Press release). Hubble/ESA, Garching, Germany. NASA/ESA. Retrieved Aug 4, 2015.