The Outsiders are a fictional alien race in Larry Niven's Known Space series. They are many-limbed beings that are invariably described as a cat o'nine tails with a fattened handle. Their body composition includes ultra-cold superfluid helium.
Outsiders are estimated to be the most advanced species in Known Space, possibly the Galaxy, but the extent of their development remains unknown. Though they have the technology to produce advanced faster-than-light drives, they rarely use them, preferring to travel the "slow" way, just below the speed of light. They do possess a "reactionless drive" technology that allows them to reach this speed almost instantaneously. In Ringworld's Children Louis Wu says that the Outsiders have "something better" than hyperdrive but this is not elaborated on.
They spend all of their time following starseeds and acting as information brokers to space-faring sentient races throughout the Milky Way. Their prices can be very high and scaled to the estimated impact the information will have on the civilization of the client race. Their most common wares are interstellar propulsion systems of various types. The Outsiders maintain a strict commercial ethos regarding any form of knowledge, which shrouds them in a secrecy only wealth can penetrate. They do not haggle. They will answer any question, even those about themselves, if you are willing to pay the price. Personal questions about the Outsiders have been priced beyond the ability of any individual or government to pay (on the order of a trillion credits). In "Peace and Freedom", it is revealed that starseeds are in fact packages of microorganisms designed to seed new planets with life, thus creating new customers for the Outsiders. They are reluctant to reveal this information because they are ashamed, since one of their starseeds created the Thrint, which destroyed nearly all intelligent life in the galaxy several billion years ago.
The Outsiders are thought to have evolved on a cold world with no atmosphere, similar to Neptune's moon Nereid, which they lease from the Earth government. They live on thermoelectricity by lying with their heads in sunlight and their tails in shadow; the temperature difference sets up a current. The novel A Darker Geometry by Gregory Benford and Mark O. Martin revealed that the Outsiders were created by a race of extra-dimensional aliens seeking to escape the heat death of their own universe. Edward M. Lerner revealed in an online chat (as 'EML') that Larry Niven had ruled A Darker Geometry as definitely non-canonical and incompatible with the then forthcoming Juggler of Worlds, which was co-authored by Niven and Lerner. Juggler of Worlds introduces a number of possible retcons to established Outsider history.
While in most of the Known Space Series, the name "Outsiders" refers to the aforementioned species, in stories that happen before the discovery of aliens the term "Outsider" refers to any alien that might make contact with mankind.
The Outsiders may have inspired the Melnorme, from Star Control 2, another highly advanced and very mysterious species. The Investor species in Shaper/Mechanist stories by Bruce Sterling have a similar ethos but an altogether different biology.