The Rio Scale

Nearly everyone is familiar with the Richter Scale for quantifying earthquake severity. Can we similarly quantify the importance of a candidate SETI signal? The Rio Scale is an attempt to do just that. It is an ordinal scale between zero and ten, used to quantify the impact of any public announcement regarding evidence of extraterrestrial intelligence.

The concept was first proposed in Rio de Janeiro, Brazil (hence its name) by Iván Almár and Jill Tarter in a paper presented to the 51st International Astronautical Congress, 29th Review Meeting on the Search for Extraterrestrial Intelligence, in October, 2000. Under their leadership, members of the IAA SETI Committee (and its successor, the IAA SETI Permanent Study Group) officially adopted the Rio Scale in 2002, and have continued working to refine and perfect it, in order to bring some objectivity to the otherwise subjective interpretation of any claimed ETI detection.

10 - Extraordinary

9 - Outstanding

8 - Far-reaching

7 - High

6 - Noteworthy

5 - Intermediate

4 - Moderate

3 - Minor

2 - Low

1 - Insignificant

Scale is based on:

Class of Phenomenon:

• Earth-specific message, or an ET artifact, capable of contact, or a physical encounter
• Omnidirectional message with decipherable information, or a functioning ET artifact or space probe
• Earth-specific beacon to draw our attention, or an ET artifact with a message to mankind
• Omnidirectional beacon designed to draw attention, or an ET artifact with a message of a general character
• Leakage radiation, without possible interpretation, or an ET artifact the purpose of which is understandable
• Traces of astroengineering, or any indication of technological activity by an extant or extinct civilization at any distance, or an ET artifact, the purpose of which is unknown

Type of Discovery:

• SETI/SETA observation; steady phenomenon verifiable by repeated observation or investigation
• Non-SETI/SETA observation; steady phenomenon verifiable by repeated observation or investigation
• SETI/SETA observation; transient phenomenon that has been verified but never repeated
• Non-SETI/SETA observation; transient phenomenon that is reliable but never repeated
• From archival data; a posteriori discovery without possiblity of verification

Apparent Distance:

• Within the solar system
• Within a distance which allows communication (at lightspeed) within a human lifetime
• Within the Galaxy
• Extragalactic

Credibility of report:

• Absolutely reliable, without any doubt
• Very probable, with verification already carried out
• Possible, but should be verified before taken seriously
• Very uncertain, but worthy of verification efforts
• Obviously fake or fraudulent

As originally proposed and subsequently refined, the Rio Scale is mathematically defined as:

where Q, an estimated level of consequences, is the sum of three parameters (class of the phenomenon, discovery type, and distance), and d represents the assessed credibility of a claimed discovery. The value for Q is easily quantified as a function of the class of the reported phenomenon, the type of discovery, and the estimated distance to the source of the phenomenon detected. The value assigned for d is somewhat more subjective, and is likely to vary over time and between observers.

It should be noted that the Rio Scale is a tool for dynamic, rather than static, analysis. Throughout the life of any unfolding SETI event, as research is conducted and verification measures pursued, new information is constantly being made available which will impact our perceptions as to the significance and credibility of the claimed detection. Thus, the Rio Scale value assigned to any SETI detection can be expected to change significantly (either upward or downward) over time.

Rio Scale Calculator