SETI may not have succeeded in finding alien life yet because space weather around other stars can interfere with the aliens’ attempts to send out radio messages, according to a new study that tries to understand why the universe seems so quiet.
“Space weather” describes the electromagnetic disturbances produced by gusts of radiation in a stellar wind or coronal mass ejections (CME) from a star. These events spew out a lot of plasma and electrons into interplanetary space around a star, and plasma and electrons are like kryptonite for coherent radio signals.
The other reason SETI is looking for narrowband signals, with bandwidths of only a few hertz, is that nothing known in nature produces such a tightly confined radio signal. So if we discovered one, we would know it was more than likely artificial.
But until now, no one had quantified the effect of plasma and electrons spewed out of activity on stars. If a technological species on a distant the exoplanet wanted to send a message into space, the space weather in the home system could adversely affect the characteristics of that signal.
“SETI searches are often optimized for extremely narrow signals,” said Vishal Gajjar, of the SETI Institute in Mountain View, California, in a statement. “If a signal is broadened by its own star’s environment, it could slip below our detection limits, even if it is there, potentially helping to explain some of the radio silence we’ve seen in technosignature search.”
The most likely effect of space weather on narrowband radio signals is something called diffractive scintillation. This can cause a signal to be spread over a much wider frequency range when it interacts with plasma from a star. While the initial narrowband signal may have strong power over only a few frequencies, the smearing spreads this power over several frequencies, reducing the strength of the signal.
However, identifying the problem was only the first step. Gajjar and his SETI Institute colleague Grayce Brown wanted to take it one step further and quantify the effects of space weather so that it might be easier to mitigate during SETI searches.
To do so, the duo first had to quantify the effect in our own neighbourhood, by analyzing radio signals between Earth and space missions that explore our the solar system. Gajjar and Brown calibrated how fluctuations in solar wind and bursts from CMEs can affect narrowband signals, and average it over time. They then used our example sun as a basis for calibrating the broadening effect of space weather on signals around two main types of stars: Sun-like stars, and red dwarfswhich is the smallest, coolest type of star, and makes up three quarters of all the stars in Milky way galaxy.
Stars much more massive than the Sun were left out of the study, as their lifetimes are likely too short for technological life to develop on any orbiting planets.
To emphasize their point, Gajjar and Brown simulated a SETI search of the million nearest Sun-like and red dwarf stars and incorporated the effects of space weather based on the known activity of such stars.
The simulation depicted a search for alien signals in the area around 1 GHz, which is the most common frequency band to search in. Radio emissions from interstellar hydrogen, for example, are at 1.42 GHz.
According to the simulation, 70% of the stars result in the signals broadening in frequency by more than 1 Hz, and 30% of the stars produce a broadening of more than 10 Hz, especially red dwarf stars, which are known for their strong stellar activity.
Even more seriously, if a CME were to occur at the time a signal is transmitted, it could introduce a broadening above 1000 Hz, rendering a signal completely invisible to detectors focused on very narrowband signals.
But now that we know this can happen, efforts can be made to minimize the effect – just like how we can estimate the degree of dispersion of the interstellar medium, or how algorithms can remove Doppler operation in frequency caused by the motion of a transmitter on a planet orbiting its star.
“By quantifying how stellar activity can reshape narrowband signals, we can design searches that are better suited to what actually reaches Earth, not just what can be transmitted,” Brown said.
For over 66 years, SETI has searched for evidence of technological life in the the universe but so far haven’t found anything. For example, the citizen science project SETI@home, which started in 1999, is down to its size last 100 candidate signals and hopes are not high that any of them will turn out to be ET.
Some researchers refer to this inability to find technological aliens as “Great silence,” but could this space weather effect quantified by Gajjar and Brown be the cause? It’s possible that it has at least contributed to the great silence, depending on how many transmitting species are out there. But just as we monitor the sun and space weather in our solar system, it would seem fair to expect aliens to be technologically proficient enough to send messages into space and the wait in space and the time of the stars before sending.
However, this cannot be guaranteed, especially if the transmitter is always on (which will draw a lot of power), or if it is an automated transmitter. Gajjar and Brown suggest that far from a “Great Silence,” the universe may be overflowing with noisy messages, and we just haven’t been tuned in enough to hear them.
The research was published on 5 March i The Astrophysical Journal.
