Mysterious radio wave pulses from deep in space have been hitting Earth for decades, but the scientists who recently discovered them have no concrete explanation for the origin of the signals.

For 35 years, the strange blasts of energy in varying levels of brightness have occurred like clockwork approximately every 20 minutes, sometimes lasting for five minute intervals. That's what Curtin University astronomers from the International Centre for Radio Astronomy Research (ICRAR) concluded in research published last week in the journal Nature.

The discovery of the signal, which researchers named GPMJ1839-10, has the scientists baffled. Believed to be coming from around 15,000 light years away from Earth, the signal has been occurring at intervals and for a period of time previously thought to be impossible.

“This remarkable object challenges our understanding of neutron stars and magnetars, which are some of the most exotic and extreme objects in the universe,” lead author Dr. Natasha Hurley-Walker said in a statement on ICRAR's website.

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First signal detected from 2018 data

Using data gathered in 2018, astronomers first detected another magnetar spinning much slower than usual and sending similar signals every 18 minutes. But by the time they analyzed the data in 2020, it was no longer producing radio waves, according to Hurley-Walker.

So they looked again, knowing that the chance was high they would find another long-term radio source.

The team of astronomers used the Murchison Widefield Array radio telescope in Western Australia to scan the Milky Way galaxy every three nights for several months. They didn't have to wait long to find what they were looking for.

Within no time, a new source was discovered in a different part of the sky, this time repeating every 22 minutes with five-minute pulses.

Studying records at the Very Large Array in New Mexico, which maintains the longest-running archive of data, the researchers discovered that the source's pulse was first observed in 1988.

What's even more alarming than that this radio signal was able to go undetected for more than three decades is that scientists have not determined with confidence what it could be.

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But before you go assuming that E.T. is trying to phone our planet, the researchers do have other theories about what may be causing it.

Even Hurley-Walker noted in an article she penned on The Conversation — a media outlet with articles written by academics and researchers — that it can be tempting to include extraterrestrial intelligence as a possible source of the signal. In fact, that's what happened when the first pulsar was discovered and astrophysicists nicknamed it "LGM 1" for "Little Green Men 1" before additional observations caused them to rule the possibility out.

The most likely culprit, researchers say, is pulsars, neutron stars that blink and rotate like lighthouses emitting energetic beams as they rotate toward and away from Earth. But pulsars slow down as time passes, their pulses growing fainter with age until they eventually stop producing radio signals.

What's more confounding: the object that the researches detected resembles a pulsar, but spins 1,000 times slower.

Another explanation researchers offer is that the object could be an ultra-long period magnetar, a rare type of neutron star with extremely strong magnetic fields that can produce powerful bursts of energy. But until recently, all known magnetars released energy at intervals ranging from a few seconds to a few minutes — far more often than the 22-minute intervals that this object emits radio waves, according to the study.

Magnetars also generate radio waves for several months before stopping, not for 35 years and counting, according to researchers. The radio emissions should be slowing down, but as observations show, it is not.

In fact, researchers note that it shouldn't be possible for it to produce radio waves at all. The object is spinning so slow as to fall below the "death line," a critical threshold where a star’s magnetic field becomes too weak to produce radio emissions.

To determine what's behind the mysterious pulsing, the astronomers said that additional observations and study are needed.

“Whatever mechanism is behind this," Hurley-Walker said in the statement, "is extraordinary.”

The discovery joins a list of mysterious finds this year beyond Earth's gravitational pull.

In May, researchers at Sandia National Laboratories unveiled strange findings after recording unidentified sounds in the stratosphere using solar-powered balloons.

And in January, NASA's James Webb Space Telescope discovered an exoplanet outside our solar system that shares similar qualities with Earth.

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Eric Lagatta covers breaking and trending news for USA TODAY. Reach him at [email protected] and follow him on Twitter @EricLagatta.