1. A method performed by one or more computers, the method comprising: maintaining an authoritative resources index that, for each entity of a plurality of entities, maps one or more authoritative resources to the entity, wherein each of the one or more authoritative resources is a resource whose occurrence in search results for a received search query has been determined to be an indicator that the received search query is directed to the entity; receiving a first search query from a user device; receiving search results for the first search query provided by a search engine, wherein each of the search results identifies a respective resource; determining that the first search query relates to a first entity of a first entity type based on determining that a count of search results for the first search query that identify resources that have been mapped to the first entity in the authoritative resources index exceeds a first threshold number; and in response to determining that the first search query relates to the first entity of the first entity type, transmitting information identifying one or more entities of a second entity type that have a predetermined relationship with the first entity to the user device as part of a response to the first search query.