Quick. What's the closest star after the sun? If you said Alpha Centauri you're one-third correct. Alpha is actually a triple system comprised of Rigil Kentaurus (Alpha Centauri A), Toliman (Alpha B) and Proxima Centauri. The first two form a close, bright pair of sun-like stars located east of the Southern Cross. Standoffish Proxima, a dim red dwarf, orbits 2.2° away and requires a 4-inch telescope to see. Despite its low profile, Proxima has two claims to fame: it's the closest star at 4.24 light-years and orbited by at least two planets, named Proxima Centauri b and c.
Most extrasolar planets are ginormous gas giants resembling Jupiter but orbiting their host suns at close distances, the reason they're referred to as "hot Jupiters". They don't seem like the sort of places you'd find life. Others like Proxima b lie within a star's habitable zone, where water can pool on the planet's surface. So much life-sustaining chemistry takes place in watery water it seems essential to life as we know it. Orbit too close to a star and water boils away. Too far and it turns to ice. Proxima's other planet, a gas giant orbiting at a much greater distance, is an example of the latter.
Multiple times in April and May 2020 the Parkes 210-foot radio telescope in Australia picked up an unexplained, narrow burst of radio waves at 982 MHz (megahertz) from Proxima Centauri. Astronomers at the Breakthrough Listen project, a privately-funded search for alien radio signals, made the find while sifting through the telescope's data. Sofia Sheikh, a graduate student at Penn State, is now leading a team studying the signal.
Natural sources typically emit radio waves across a broader range, so a narrow, strong signal makes astronomers sit up and take notice. This one spiked at 982 MHz (waves that wriggle at the rate of 982 million cycles per second) and even has a name — Breakthrough Listen Candidate-1 or BLC-1. It showed up during five 30-minute periods over several days and has not been heard from since. BLC-1 almost certainly a technological transmission but whether alien or human we can't say yet.
No known natural phenomenon would produce such a tight signal at that frequency. But it's not too far off the mark from 900 MHz, the frequency of the microwaves generated in an industrial microwave oven. Five years ago, another Parkes signal had everyone in a tizzy until astronomers realized it was just someone heating up lunch in the observatory break room oven.
To eliminate the possibility of a local, terrestrial source the telescope pointed to another part of the sky and then returned to Proxima — only to find the signal still there. The beam's frequency also shifted during the observations which is what you expect if it came from an orbiting planet. As the planet moved toward the Earth, the frequency would increase and then decrease as it moved away. A local radio antenna is bolted to the ground and broadcasts a single, steady signal.
So is it possible we just stumbled on a beacon broadcast by intelligent beings on Proxima b? Maybe, but there are many other possibilities, and these must be eliminated before considering the most exciting option.
According to Seth Shostak, senior astronomer at the SETI Institute, it "might just be a telemetry signal from an orbiting satellite. The orbital motion of satellites causes their transmissions to rise and fall in frequency." And with nearly 3,000 functioning satellites providing everything from GPS to weather photos, this seems a reasonable possibility.
Proxima may even be off the hook as the source. The beacon could have come from far behind the star but along the same line of sight. Remember how Jupiter and Saturn almost touched in December even though they were at vastly different distances?
Despite its favored location Proxima b may not be so habitable after all. Red dwarf stars like Proxima Centauri often unleash huge flares that can strip away a nearby planet's atmosphere, making it unlikely that advanced forms of life could develop there.
Magnetic fields protect against such brutish behavior, but a field's strength is linked to the planet's rotation speed. Many astronomers think that Proxima b may be tidally locked to its sun and rotate too slowly to generate a significant magnetic field.
Hopefully, another telescope will confirm the signal. If not, we'll be left with yet another tantalizing astronomical mystery like the famous "Wow!" signal, a strong, narrowband transmission recorded by Ohio State University's Big Ear radio telescope in 1977. While that was likely human-generated interference no one has yet to put their finger on the cause.
It's always tempting to jump to conclusions when evidence is lacking. That's why skepticism plays an important role in science. Astronomers involved in the discovery are investigating the data and will publish a paper on their results in early 2021.
"Astro" Bob King is a freelance writer for the Duluth News Tribune. Read more of his work at duluthnewstribune.com/astrobob.