Assertion: nebula's shell | /r/HasProperty | rare phenomenon
Facets:
Source sentences:
text: Until recently, it was thought that shells around planetary nebulae were a rare phenomenon.
text: Until recently, it was thought that shells around planetary nebulae were a rare phenomenon.
text: Until recently, it was thought that such shells around planetary nebulas were a rare phenomenon.
text: Until recently, it was thought that such shells around planetary nebulae were a rare phenomenon.
text: Until recently, it was thought that such shells around planetary nebulae were a rare phenomenon.