Silence of the Red Dwarfs express
The RATAN-600 radio telescope is a ring of hundreds of aluminum panels on a mountainside in Karachay-Cherkessia. In the spring of 2018, its mirror caught radio waves from sixteen red dwarfs — small, dim, but very long-lived stars. Each of them has planets similar to Earth in size and located in the "habitable zone": not too hot, not too cold, where water can remain liquid.
Scientists turned on receivers at four frequencies, like tuning a radio to different stations. The range from 2.3 to 22 gigahertz covers both natural cosmic noise and possible artificial signals. Each star was listened to from three to twenty-seven times, a few seconds per pass. The result: silence. Not a single flare, not a single steady signal.
Silence does not mean emptiness. It means a limit: if these stars do harbor civilizations, their radio transmitters are either weaker than our radars or silent. The observations also showed that the red dwarfs were calm that month — no powerful flares that could disrupt life on nearby planets.
The radio telescope has gone quiet, but its dish is already turning toward other stars. Every hour of silence is not a waste of time but a new boundary on the map of the search.
Original abstract
as written by the authors arXiv:2303.01791
The paper presents the results of multi-wave radio observations of sixteen red dwarfs with Earth-like planets in the habitable zones on RATAN-600. In the passage mode, radiometers of four frequency ranges of 22, 11, 4.7 and 2.3 GHz (1.38, 2.68, 6.38, 13.3 cm) were used with a sampling step of quasi-simultaneous registration of about $$\sim 18$$ Hz and an exposure duration (the time of passage of the instrument's directivity pattern) of 1.5 -- 15 s. Observations were carried out from 3 to 27 times in April-May 2018, while radio emission was not detected. Upper limits were established for its intensity in flares with a duration of 0.05 -- 10 s at the level of 80 -- 800 mJy (corresponding to a luminosity of $$10^{23} - 10^{29}$$ erg/s) and in a steady state with a luminosity of $$10^{22} - 10^{27}$$ erg/s. During the observations, 11 anomalous flux outlier at the level of 100 mJy were registered, which apparently have an atmospheric (thunderstorms, airplanes) origin. Assuming an artificial origin of the sought radio emission (transmission from inhabited planets), the upper limits of the power of hypothetical transmitters were $$2\times 10^9 - 10^{15}$$ W, interestingly that the lower boundary of this interval close to the power of terrestrial planetary radars.
RATAN-600 is one of the world's largest radio telescopes: its ring's diameter is 576 meters, but it is not a solid dish; rather, it consists of 895 individual panels that can be tilted.