1. A method performed by data processing apparatus, the method comprising: identifying an entity that is referenced by a received search query; identifying a plurality of content items that have been identified as relevant to the entity; for each of the identified content items, identifying: a content category to which the content item has been classified; and a rank score indicative of the relevance of the content item to the entity; selecting a set of knowledge modules to be presented in a knowledge panel for the entity, the knowledge panel (i) being a portion of a search results page in which content related to the entity is presented separate from a listing of search results for the received search query and (ii) including presentation space in which knowledge modules are presented in multiple different arrangements, the set of knowledge modules including at least two different knowledge module types that each presents a different category of content items in a given data format, the selection being based, at least in part, on the rank scores and identified content categories for the content items, wherein the set of knowledge modules include: a first knowledge module, of a first type, that presents a first category of content items in a first data format; and a second knowledge module, of a second type different from the first type, that presents a second category of content items different from the first category of content items, the second knowledge module including a first content item obtained from a first network resource and a second content item obtained from a second network resource different from the first network resource, the second category of content items having a second data format different from the first data format; determining, based on a combination of the rank scores for two or more content items assigned to each knowledge module, an order in which the set of knowledge modules are to be presented in the knowledge panel; positioning the knowledge modules within the knowledge panel in the determined order for the knowledge modules; determining, for a given knowledge module of the set, an order in which the two or more content items assigned to the given knowledge module are to be presented in the given knowledge module based on the rank score for the two or more content items assigned to the given knowledge module; and providing, to a user device, the knowledge panel including the set of knowledge modules that are presented in the determined order for the set of knowledge modules, the knowledge panel being presented with the search results page for the received search query, and the two or more content items assigned to the given knowledge module being presented in the determined order for the two or more content items assigned to the given knowledge module.