12. 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: receiving, from a client device and by a document system, a request for one or more documents; obtaining, by the document system and from a corpus of documents, a set of documents responsive to the request; obtaining, from a user profile associated with a source of the request, representations of one or more topics of interest to a user; selecting, by the document system and using an index, at least one document from the set of documents that is associated with a particular topic that matches at least one of the one or more topics of interest to the user; for the at least one selected document, obtaining a value corresponding to an inverse document frequency of the particular topic in the corpus of documents, comprising: identifying a first number of documents in the corpus of documents that have been created during a limited time period that ranges from a prior time to a current time, wherein the first number of documents is less than a total number of documents in the corpus of documents; and identifying a second number of documents in the corpus of documents that have been created during the limited time period and that reference the particular topic, wherein the second number of documents is less than the first number of documents, wherein the value corresponding to the inverse document frequency is based on a ratio of the first number of documents and the second number of documents; generating a score for the at least one document based at least in part on the value corresponding to the inverse document frequency that is based on the ratio of the first number of documents and the second number of documents; determining that the score for the at least one document satisfies a threshold score that indicates that the particular topic of the at least one document is an infrequent topic in the corpus of documents; and responsive to determining that the score for the at least one document satisfies the threshold score that indicates that the particular topic of the at least one document is an infrequent topic in the corpus of documents, transmitting information associated with the at least one document from the document system to the client device in response to the request, wherein the transmitted information includes information for rendering an interface that provides access to the at least one document.
12. 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: receiving, from a client device and by a document system, a request for one or more documents; obtaining, by the document system and from a corpus of documents, a set of documents responsive to the request; obtaining, from a user profile associated with a source of the request, representations of one or more topics of interest to a user; selecting, by the document system and using an index, at least one document from the set of documents that is associated with a particular topic that matches at least one of the one or more topics of interest to the user; for the at least one selected document, obtaining a value corresponding to an inverse document frequency of the particular topic in the corpus of documents, comprising: identifying a first number of documents in the corpus of documents that have been created during a limited time period that ranges from a prior time to a current time, wherein the first number of documents is less than a total number of documents in the corpus of documents; and identifying a second number of documents in the corpus of documents that have been created during the limited time period and that reference the particular topic, wherein the second number of documents is less than the first number of documents, wherein the value corresponding to the inverse document frequency is based on a ratio of the first number of documents and the second number of documents; generating a score for the at least one document based at least in part on the value corresponding to the inverse document frequency that is based on the ratio of the first number of documents and the second number of documents; determining that the score for the at least one document satisfies a threshold score that indicates that the particular topic of the at least one document is an infrequent topic in the corpus of documents; and responsive to determining that the score for the at least one document satisfies the threshold score that indicates that the particular topic of the at least one document is an infrequent topic in the corpus of documents, transmitting information associated with the at least one document from the document system to the client device in response to the request, wherein the transmitted information includes information for rendering an interface that provides access to the at least one document. 16. The system of claim 12 , wherein the score for the at least one document is generated based on a function of the value corresponding to the inverse document frequency.