  Messaging to extraterrestrial intelligence (METI) is a branch of study
concerned with constructing and broadcasting a message toward habitable
planets. Since the Arecibo message of 1974, the handful of METI broadcasts have
increased in content and complexity, but the lack of an established protocol
has produced unorganized or cryptic messages that could be difficult to
interpret. Here we outline the development of a self-consistent protocol for
messaging to extraterrestrial intelligence that provides constraints and
guidelines for the construction of a message in order to maximize the
probability that the message effectively communicates. A METI protocol
considers several factors including signal encoding, message length,
information content, anthropocentrism, transmission method, and transmission
periodicity. Once developed, the protocol will be released for testing on
different human groups worldwide and across cultural boundaries. An effective
message to extraterrestrials should at least be understandable by humans, and
releasing the protocol for testing will allow us to improve the protocol and
develop potential messages. Through an interactive website, users across the
world will be able to create and exchange messages that follow the protocol in
order to discover the types of messages better suited for cross-cultural
communication. The development of a METI protocol will serve to improve the
quality of messages to extraterrestrials, foster international collaboration,
and extend astrobiology outreach to the public.
