16. A method performed by one or more computers, the method comprising: obtaining data identifying a first entity of a first entity type and a plurality of candidate entities of the first entity type; selecting, as related entities for the first entity, a plurality of the candidate entities of the first entity type based at least in part on how frequently recognized references to the candidate entities co-occur with a recognized reference to the first entity in resources; ranking the related entities, wherein ranking the related entities comprises: determining, for each of the related entities, a respective subsequent query score based at least in part on how frequently search queries that include recognized references to the related entity are submitted to a search engine by users after submitting a search query that includes a recognized reference to the first entity, computing, for each of the related entities, a respective ranking score based at least in part on the subsequent query score for the related entity, and ranking the related entities based on the ranking scores; determining that a recognized reference to the first entity occurs in at least one of a first threshold number of resources or a second threshold number of previously submitted search queries; and in response to determining that the recognized reference to the first entity occurs in at least one of the first threshold number of resources or the second threshold number of previously submitted search queries, generating a mapping between the first entity and the related entities of the first entity type in an index that maps each of a plurality of entities to one or more respective related entities for the entity.
16. A method performed by one or more computers, the method comprising: obtaining data identifying a first entity of a first entity type and a plurality of candidate entities of the first entity type; selecting, as related entities for the first entity, a plurality of the candidate entities of the first entity type based at least in part on how frequently recognized references to the candidate entities co-occur with a recognized reference to the first entity in resources; ranking the related entities, wherein ranking the related entities comprises: determining, for each of the related entities, a respective subsequent query score based at least in part on how frequently search queries that include recognized references to the related entity are submitted to a search engine by users after submitting a search query that includes a recognized reference to the first entity, computing, for each of the related entities, a respective ranking score based at least in part on the subsequent query score for the related entity, and ranking the related entities based on the ranking scores; determining that a recognized reference to the first entity occurs in at least one of a first threshold number of resources or a second threshold number of previously submitted search queries; and in response to determining that the recognized reference to the first entity occurs in at least one of the first threshold number of resources or the second threshold number of previously submitted search queries, generating a mapping between the first entity and the related entities of the first entity type in an index that maps each of a plurality of entities to one or more respective related entities for the entity. 17. The method of claim 16 , wherein ranking the related entities further comprises: computing, for each of the related entities, a respective co-occurrence score based on how frequently a recognized reference to the related entity co-occurs with a recognized reference to the first entity in resources indexed by the search engine, wherein computing, for each of the related entities, the respective ranking score comprises: computing the ranking score for the related entity from the subsequent query score for the related entity and the co-occurrence score for the related entity.