Aliens Might Have Their Radio Signals Blurred By Their Star's Solar Wind

Back in the early 2000s, my computer screen, like that of many other space enthusiasts, was typically covered in a series of rainbow-colored spectral signals. As my computer crunched through thousands of data points of radio signals collected by the SETI@Home initiative, I was hoping I was in some small way contributing to one of humanity’s greatest scientific endeavours - the search for extraterrestrial life. But, according to a new paper published in The Astrophysical Journal by Vishal Gajjar and Grayce Brown of the SETI Institute, it seems unlikely that the signals SETI@Home was tailored to look for actually exist. That doesn’t mean there weren’t aliens yelling into the void at the top of their electronic lungs, but simply that the space weather from their local star might have changed the signal to make it unrecognizable by the time it reached us.

Signals are a tricky business in both astronomy and engineering. One of the baseline assumptions of SETI is that aliens were intentionally sending a signal if they want to make contact with other species. Intentional signaling, at least to humans, typically takes us a relatively small bandwidth. Around 1 Hz is average for our intentional signals, as spreading the energy out across a wider spectrum would lessen its detected power at the receiver.

With that assumption, SETI researchers originally thought that aliens would be doing the same, and set a threshold of around a 1 Hz “band” for any given signal. That’s part of the reason SETI@Home was set up in the first place - there were a lot of frequencies to check in 1 Hz chunks. Historically, SETI focused on frequencies in the “water hole” between 1.4 GHz and 1.6 GHz, which are relatively quiet by galactic background noise standards. But analyzing 1 Hz slices of a 200 MHz bandwidth is time intensive to say the least - and even more so when trying to analyze data from literally all over the sky.

Fraser discusses the difficulty in searching for extraterrestrial life.

Maintaining a 1 Hz signal over interstellar distances is no mean feat, and according to the new paper it’s likely not even possible. That is due to the interference from the stellar weather of a civilization’s local Sun. Turbulence from the civilization’s host star can broaden the signal from an “intentional” 1 Hz out to 10 Hz, or even 100 Hz even just during its journey out of the immediate solar system. Since signal power drops drastically as it is spread over wider frequencies, a signal that is spread from 1 Hz to 10 Hz in width only retains about 6% of its original power. It would also fall through the gaps of normal SETI filtering algorithms.

To prove their point, the authors looked at signals sent within our own solar system. They empirically studied the signal distortion of a signal from an interplanetary spacecraft traveling through our Sun’s interplanetary medium. Using data from a wide variety of deep-space probes, from Mariner IV to Rosetta, they calculated exactly how our star’s plasma distorts a radio signal. Then, they used those numbers to extrapolate out to other stars with different stellar characteristics.

One of the biggest factors was distance - the closer the source of a radio signal was to its star, the more spreading occurs. This is particularly important for one of the most interesting types of stars - red dwarfs. They make up 75% of the stars in the Milky Way, and also, as of now, the vast majority of potentially habitable exoplanet candidates. But, since their habitable zone is so close to their star, they are also very susceptible to this signal corruption since the source for the radio waves (i.e. their home planet) would likely be very close to the star. Red dwarfs are also notoriously violent, with major space weather events happening much more frequently than they do on our own Sun, further spreading any potential signals trying to escape.

For those who want some nostalgia - here’s a loop of an old SETI@Home screensaver. Credit - Alex Piskun YouTube Channel

So what does all this mean for the future of SETI astronomy? Given that the authors are SETI Institute researchers it might not be a surprise that their suggestion is simple - collect more data - or in this case, simply filter less. Future observatories, such as the Square Kilometer Array, could use signal processing techniques like matching filters and multi-resolution channelization to detect smeared signals more easily. But that still requires a ton of extra processing power to analyze - so maybe we’ll see a resurgence in those rainbow spectral graphs on home computers (or AI clusters) sometime again soon.

Learn More:

SETI Institute - Why SETI Might Have Been Missing Alien Signals

V. Gajjar & G. C. Brown - Exo–IPM Scattering as a Hidden Gatekeeper of Narrowband Technosignatures

UT - Could it be We've Received Alien Signals in the Past and Didn't Notice? Not Bloody Likely, According to New Study

UT - Setting Bounds On SETI