The Ross 128 Signal

Summary

On 12 May 2017 the Arecibo telescope recorded ten minutes of broadband, quasi-periodic, unpolarised pulses while pointed at Ross 128, a red dwarf eleven light-years away in Virgo. The observation was part of a survey of red dwarf radiation environments, not a search for extraterrestrial intelligence.

The team published the detection on 12 July without an explanation, listing three candidates and ranking an intelligent source last. Nine days later, after follow-up observations at three facilities, they attributed the pulses to geostationary satellites. Ross 128 lies less than a degree from the celestial equator, in the same region of sky as many geostationary satellites.

What is documented

The observation. Ross 128 was observed on 12 May 2017 at 8:53 p.m. Atlantic Standard Time, which is 00:53:55 UTC on 13 May, within a campaign of observations carried out at Arecibo between April and May 2017. The detection appears in a ten-minute dynamic spectrum.

What the signal looked like. Abel Mendez, then director of the Planetary Habitability Laboratory, described it in the laboratory’s published note: "The signals consisted of broadband quasi-periodic non-polarized pulses with very strong dispersion-like features." Broadband is the important word. It is the opposite of what a search for intelligence looks for.

What the campaign was for. The survey was studying the radiation and magnetic environments of red dwarf stars and looking for undiscovered planets around them. It was not a SETI programme, and the team did not present the detection as one.

What the team said at the time. In the note of 12 July 2017 Mendez wrote "We believe that the signals are not local radio frequency interferences (RFI)", and, plainly, "We do not know the origin of these signals", adding that "the three main explanations are as good as any at this moment".

The three candidates, as published. Emissions from Ross 128 itself, similar to a Type II solar radio burst; emissions from some other object in the same field of view; and bursts from a satellite in high orbit. Mendez noted that Type II emission usually occurs at much lower frequencies and that the observed dispersion looked inconsistent with it, that no planets or satellites were known in that field, and that satellites in low orbit cross the field of view too quickly.

The follow-up. On 16 July 2017 Ross 128 was observed again at Arecibo, with the Green Bank Telescope observing simultaneously under Breakthrough Listen and the SETI Institute’s Allen Telescope Array also observing.

The attribution. On 21 July 2017, nine days after publishing the detection, the laboratory announced the result: "We are now confident about the source of the Weird! Signal", and "The best explanation is that the signals are transmissions from one or more geostationary satellites."

Why this target and not others. Ross 128 lies at a declination of about plus 0.8 degrees, less than a degree from the celestial equator. In the laboratory’s words, the satellite explanation "explains why the signals were within the satellite’s frequencies and only appeared and persisted for Ross 128; this star is close to the celestial equator where many geostationary satellites are located." Its accompanying figure plots "Geostationary satellites operating between 4-8 GHz in the same region of the sky as Ross 128", a band that overlaps the one the telescope was observing in.

What the SETI community said. Seth Shostak of the SETI Institute: "It does look like the kind of broadband interference that you get in SETI experiments", and "Following up is mandatory." Douglas Vakoch noted that the signal lacked the narrowband character such projects look for and had not been replicated. Mendez himself placed the intelligent-source reading last, writing that "the recurrent aliens hypothesis is at the bottom of many other better explanations" and that "Statistically, this is always the last consideration".

What was left open. The strong dispersion-like features were never accounted for. Mendez suggested multiple reflections as a possible cause and said it still needed study, adding that "Unexplained here does not mean inexplicable."

A note on the publication route. Neither the detection nor its resolution went through peer review. Both were published as laboratory notes and a press release. The Registry records the attribution as widely accepted rather than established for that reason.

Leading explanations

Geostationary satellite transmissions (Mendez and colleagues, 21 July 2017; widely accepted). The pulses were transmissions from one or more geostationary satellites. In the team’s account this is why they fell within satellite frequencies and appeared only toward Ross 128, a target lying close to the celestial equator, where many such satellites are located. This is the research team’s own conclusion and the one reported since. It is recorded as widely accepted rather than established because no peer-reviewed analysis of the data was ever published.

A Type II-like radio flare from the star (Mendez, 12 July 2017; rejected). Abandoned after the follow-up. Type II emission occurs at much lower frequencies, and the observed dispersion looked inconsistent with it.

Another object in the field of view (Mendez, 12 July 2017; rejected). No such object was known in that field.

Local interference at the telescope (rejected, then superseded). Explicitly excluded by Mendez in the original note. The final answer was interference after all, but orbital rather than local, which is why the original exclusion was correct and the conclusion still changed.

An intelligent transmission (rejected). Never advanced by the research team. Raised in public and press coverage, ranked last by Mendez, and noted by Vakoch as having the wrong character for such a signal.

Current status

Later explained. The source is identified and the identification has not been publicly contested. A ten-minute dynamic spectrum from Arecibo showed broadband quasi-periodic pulses toward Ross 128; a three-facility follow-up led the team to attribute them to geostationary satellite transmissions, which the target’s near-zero declination makes a natural candidate. Two caveats belong on a record of this kind. Neither the claim nor its resolution was peer-reviewed, both having appeared as laboratory notes and a press release; and the dispersion-like structure was never explained beyond a suggestion of multiple reflections. A peer-reviewed analysis of the May 2017 spectrum, identification of the specific satellites with their transmit frequencies matched to the pulses, or a model reproducing the dispersion from satellite reflections would close those gaps. Recurrence under geometry that excludes the geostationary belt can no longer be tested with the original instrument: the Arecibo platform collapsed on 1 December 2020.

Last reviewed: November 2026. Records are provisional. Where the evidence changes, the entry changes. Found an error? Tell us.