7. A system comprising: one or more computers and one or more storage devices storing instructions that are operable, when executed by the one or more computers, to cause the one or more computers to perform operations comprising: maintaining a mapping from keywords to respective authoritative sites, wherein a site is a collection of resources on the Internet, and wherein the mapping identifies, for each keyword, one or more authoritative sites on the Internet for queries having the keyword, and wherein the authoritative sites for a keyword are sites that a search system comprising the one or more computers has determined are authoritative for the keyword; receiving a first search query; obtaining first search results responsive to the first search query; determining, after obtaining the first search results, that the search system should obtain a search result from a site that is authoritative for the first search query, and that a first authoritative site is authoritative for the first search query; in response to the determining, performing a second search limited to resources occurring on the first authoritative site, using a second search query different from the first search query, to obtain an authoritative search result, the authoritative search result being a search result that identifies a resource that is responsive to the second search query and that is on the first authoritative site; ranking search results including the authoritative search result and the first search results; and providing the ranked search results including the authoritative search result and one or more of the first search results in response to the first search query.
9. The system of claim 7 , wherein the mapping includes one or more topic keywords that identify respective topics, and wherein the mapping identifies one or more authoritative sites for each topic keyword.