In December 2020, a team led by Jake D. Turner, then at Cornell University, reported a faint burst-like radio signal coming from the direction of Tau Boötis, a star system about 51 light-years away that hosts a giant planet. If the emission really came from the planet, it would be one of the first radio detections of a world beyond our solar system and a rare way to measure an exoplanet's magnetic field. The team itself called it a tentative result, and later follow-up observations, published in 2024, did not see the signal again. As of today it remains unconfirmed.
Here is what the astronomers did, why radio signals from planets matter, what happened next, and what such a detection could and could not say about life.
What the LOFAR team reported
The observations came from LOFAR, the Low Frequency Array, a network of radio antennas centred in the Netherlands and spread across Europe. Turner and colleagues from Cornell, the Paris Observatory, the University of Orléans, NASA's Jet Propulsion Laboratory, UC Berkeley and other institutions pointed it at three nearby stars already known to have planets: 55 Cancri, Upsilon Andromedae and Tau Boötis.
Only Tau Boötis produced a significant signal. A possible, much weaker hint was seen near Upsilon Andromedae, but it was not strong enough to claim. The results were published in Astronomy & Astrophysics in December 2020.
Turner described the finding carefully. "We present one of the first hints of detecting an exoplanet in the radio realm," he said, while stressing that there remained uncertainty about whether the planet was the source. His adviser at Cornell, Ray Jayawardhana, framed it the same way: "If confirmed through follow-up observations, this radio detection opens up a new window on exoplanets."
One small correction to the popular coverage: the signal was not the "first ever" radio signal from an exoplanet, but a possible first. And it came from the direction of the system; linking it to the planet required further reasoning.
Meet Tau Boötis and its scorching giant planet
Tau Boötis is a pair of stars in the constellation Boötes. The main star is a yellow-white F-type star a little hotter and larger than the Sun; a dim red dwarf companion circles it at a great distance. (You will often see the spelling "Tau Boötes"; Boötis is the correct Latin form of the star's name.)
Its planet, Tau Boötis b, is a "hot Jupiter": a gas giant several times the mass of Jupiter that races around its star in about 3.3 days, far closer than Mercury is to our Sun. The star and planet appear to be tidally locked to each other, and the star has another distinction: it was the first star other than the Sun seen to flip the polarity of its magnetic field, which it does on a cycle much shorter than the Sun's.
That combination of a massive planet, a close orbit and an active star makes the system one of the best places to look for planetary radio emission, which is why it was on the target list in the first place.
Why planets give off radio waves: lessons from Jupiter
Our own Jupiter is the brightest radio source in the sky at some low frequencies. Its powerful magnetic field funnels charged particles from the solar wind and from its volcanic moon Io toward its poles, where they spiral along magnetic field lines and emit intense, polarised radio bursts. The same process powers Jupiter's auroras.
The frequency of that emission depends on the strength of the planet's magnetic field. So if astronomers can detect radio bursts from an exoplanet and measure their frequency and polarisation, they gain a direct handle on the planet's magnetic field, something almost impossible to measure any other way at such distances.
Turner's team used Jupiter as a template. They took real Jupiter radio data, scaled them as if Jupiter sat 51 light-years away and orbited much closer to a brighter star, and checked that their methods could pick out such a signal. The emission they saw from Tau Boötis had the strength and polarisation that theory predicted for a planet like Tau Boötis b.
The follow-up: no signal the second time
Because the signal was faint, everyone involved knew it needed repeating. Turner's group organised new LOFAR observations covering about 70 percent of the planet's orbit, including the orbital positions at which the original bursts had appeared. In a paper released in March 2024, they reported no signs of either bursty or slowly varying emission from the system.
That does not prove the first result was wrong. The authors list two broad possibilities: the earlier signal may have been caused by an unrecognised instrumental effect, or the planet's radio output may genuinely vary as conditions on the restless host star change. Their recommendation was more radio monitoring from several observatories at once, combined with maps of the star's magnetic field.
Meanwhile the wider hunt has continued. In 2023 two teams reported repeating radio bursts from the nearby red dwarf YZ Ceti, about 12 light-years away, which they linked to interactions between the star and its innermost rocky planet. If that interpretation holds, it could be the first evidence of a magnetic field on a rocky exoplanet. It, too, awaits further confirmation.
Magnetic fields and habitability: what a signal could and could not show
Earth's magnetic field is generated by a dynamo: the churning of molten iron in the planet's liquid outer core. It deflects much of the solar wind and is often credited with helping Earth keep its atmosphere and water over billions of years.
That idea is popular but more nuanced than it sounds. Mars lost its global field long ago and much of its atmosphere with it, which fits the story. Venus has no strong internal field yet still holds an extremely thick atmosphere, which shows that a magnetic field is not the only factor. Scientists still debate how important a field is for keeping a planet habitable.
Tau Boötis b itself is a scorching gas giant with no solid surface, so nobody expects life there. The value of a confirmed radio detection would be scientific rather than biological: it would let astronomers measure magnetic fields on distant planets, compare them with Jupiter, and eventually apply the same techniques to smaller, rocky worlds where the habitability question really matters.
Common misconceptions about the "exoplanet radio signal"
- "Aliens sent a signal." No. Natural radio emission from planetary magnetic fields has nothing to do with technology or intelligent life.
- "The signal was confirmed." It was tentative from the start, and the 2024 follow-up did not detect it again.
- "A magnetic field means a planet can host life." A field may help, but it is neither necessary on its own nor sufficient.
- "The signal came from the planet's core." The radio waves would come from charged particles in the planet's magnetic environment, high above the planet, not from the core itself.
The Tau Boötis story is a good example of science working as it should: a careful team reported an exciting but uncertain result, said clearly that it needed checking, and then went back to check it.
Sources: Cornell Chronicle: Cornell postdoc detects possible exoplanet radio emission (December 2020); Turner et al., Follow-up LOFAR observations of the τ Boötis exoplanetary system (arXiv, 2024); Wikipedia: Tau Boötis; Wikipedia: YZ Ceti; earlier report: Space.com.
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