9. A system comprising one or more computers and one or more storage devices storing instructions that when executed by the one or more computers cause the one or more computers to perform operations comprising: receiving a first search query from a user device, wherein the first search query has been determined to relate to a first entity of a first entity type, and wherein a plurality of second entities of a second entity type have a predetermined relationship with the first entity; determining a respective ranking score for each second entity of the plurality of second entities of the second entity type, the determining including: determining a frequency of occurrence of authoritative resources for the second entity of the second entity type in search results for previously submitted search queries, each authoritative resource for the second entity being a resource whose occurrence in the search results for the previously submitted search queries has been determined to be an indicator that the received search query is directed to the second entity; and determining the respective ranking score for the second entity based at least in part on the frequency; ordering the second entities of the second entity type according to the ranking scores; receiving search results for the first search query provided by a search engine, wherein each of the search results identifies a respective resource; and transmitting the search results and information identifying each of the plurality of second entities of the second entity type to the user device as part of a response to the first search query, wherein, when presented on the user device, the information identifies each of the plurality of second entities in an order that matches the ordering.
12. The system of claim 9 , wherein the ranking score for a particular second entity of the second entity type is further based at least in part on how frequently a recognized reference to the particular second entity co-occurs in a same previously submitted search query as a recognized reference to the first entity.