The Rio Scale

31 October 2002

By Seth Shostak

Sometimes being close counts – in horseshoes and romance, for example. For SETI, however, it’s hard to argue that a close call is any better than no call. After all, even if a signal mimics ET’s expected emission, what good is it if we eventually learn that it was only the carrier from a telecommunications satellite, ricocheting off the backup structure of our antenna? Ergo, SETI is often described as a one-bit experiment: there are no partial discoveries.

This sounds simple and good, but as in most things, the real world is neither. Sure, there were both white-hatted and black-hatted cowboys, but most cowpokes were somewhere in the land of gray chapeaux. The wild west wasn’t binary.

In astronomy, a situation that mimics a discovery – that’s "close" – frequently confuses both the public and the pro’s. Perhaps the most renowned of these in recent times is the matter of the martian meteorite, ALH 84001. Are there fossilized remnants of red planet residents in that meteorite – or not? The evidence is uncertain and so is the answer.

A more dramatic example of contentious claims occurs when someone makes a fresh discovery of an asteroid that seems to be on a collision course for terra firma. Unlike the martian meteorite, this bit of research news could seriously impact your backyard and your lifestyle. Consequently, in the late 1990s MIT professor Richard Binzel proposed an index for ranking the hazard of near-earth objects. Since his index was unveiled in the northern Italian city of Turin, it’s known as the Torino Scale.

A SETI detection could have important consequences for society too. So at the International Astronautics Federation’s annual get-together in Rio de Janeiro two years ago, Hungarian astronomer Ivan Almar and SETI Institute researcher Jill Tarter proposed the Rio Scale for ranking both the importance and credibility of claims that evidence for extraterrestrial intelligence has been found.

The Rio Scale runs from 0 (forget it) to 10 (this is definitely IT!) It’s composed of two multiplicative factors: a term Q that measures the importance of the discovery (for example, if a signal comes from within the solar system, it’s adjudged to be more important than one coming from the other side of the Galaxy), and a term δ that estimates the credibility of the claim (is there hard data, or only an anecdotal report?) The Rio index is imply Q times δ.

For the curious and the quantitatively inclined, the components of both Q and δ are listed in the accompanying table. You can also find a handy Rio Scale calculator at

So OK, picture this future scenario: You log onto a computer for your daily dose of hot stories, and find that someone with a backyard antenna is claiming to have picked up a "wow" signal from 47 Upsilon Andromedae. The papers don’t know whether to run this story in 72 point type above the fold, or bury it with the news of the weird. Fortunately, an international team of SETI cognoscenti has looked at the evidence and given the claim a Rio Scale ranking, thus helping the media help the masses. As new information about the claim becomes available, the Rio ranking adjusts appropriately.

It’s a good idea, and one that’s applicable to both signals and artifacts. But like a foreign language or a significant other, it’s only useful if it’s familiar. At October’s World Space Congress, Almar and I presented a paper in which we applied the Rio Scale to a sample of extraterrestrial detections known to everyone – the aliens of Hollywood films.

As example, consider the 1997 sci-fi blockbuster, "Contact." When Jodie Foster, monitoring cosmic static at the Very Large Array, hears strange sounds in her earphones, how does her discovery rank on the Rio Scale? (The answer is 4-8). Once the signal is confirmed by radio telescopes elsewhere, the ranking edges up to 6-10. And when a transponded 1936 German TV broadcast is found buried in the signal, we’ve learned something even more profound; the transmission is for us, and comes from only a few dozen light-years away. The final Rio Scale ranking is 9.

We applied the Rio Scale to a half-dozen popular film and book scenarios. We also ranked the 1998 internet hoax claiming a signal from the star EQ Pegasi as well as that endlessly contentious case of planetary physiognomy, the face on Mars. Our intent, once again, is to make the Rio Scale familiar and useful.

Of course, unlike the Richter Scale for earthquakes, the Rio Scale involves factors that are hard to quantify, such as those which govern the credibility of a claim. This latter quantity (δ) is ultimately a matter of the experts’ considered opinion. And while experts can be wrong, they are more often right (which is what qualifies them for the sobriquet "expert" in the first place.) The media and the public invariably want to know what the experts have to say regarding discoveries of potentially great import. The Rio Scale, we hope, will be one method of concisely addressing their request. In a field where certainty is sometimes elusive, it’s good to have a scheme for gauging the color of the hats.


Components of the revised Rio Scale (note: Rio Scale ranking = Q • δ)

Level of Importance Q

(Sum the three terms to get Q)



Class of Phenomenon


Discovery Type




Traces of astroengineering at any distance, or any indication of technological activity by extant or extinct civilization


From archival data; a posteriori discovery without possibility of verification




Leakage radiation, without possible interpretation


Non-SETI/SETA observation; transient phenomenon that is reliable but never repeated


Within the Galaxy


Omnidirectional beacon designed to draw attention


SETI/SETA observation; transient phenomenon that has been verified but never repeated


Within a distance which allows communication (at light speed) within a human lifetime


Earth-specific beacon to draw our attention


Non-SETI/SETA observation; steady phenomenon verifiable by repeated observation or investigation


Within the solar system


Omnidirectional message with decipherable information


Same as 4, but result of SETI/SETA observation




Earth-specific message or physical encounter






Credibility δ




Obviously fake or fraudulent


Very uncertain, but worthy of verification effort


Possible, but should be verified before taken seriously


Very probable with verification already carried out


Absolutely reliable, without any doubt