A team of astronomers have found 19 bizarre radio signals from red dwarf stars, four of which could be from orbiting exoplanets, potentially marking the first time exoplanets have been discovered using radio frequencies.
Detecting these stars was not a big deal – they were all relatively close to Earth and the team compared the detections with existing optical observations – but “finding them on radio was a big deal” because they shouldn’t. be brilliant in radio frequencies. says Joe Callingham, radio astronomer at Leiden University in the Netherlands and lead author of the study. He and his colleagues used a huge radio telescope called a Low Frequency Array or LOFAR to observe nearby red dwarfs at radio frequencies, and published their findings in the journal. Nature astronomy.
The stars are not very bright in radio frequencies. If you could turn your eyes into a radio antenna, when you looked at the sky, “you wouldn’t see stars, in general,” Callingham said, “you would see the sun a little, you would see Jupiter very bright, and you would mostly see galaxies.
The team didn’t prove that any of these signals came from exoplanets, but after weighing the possible explanations for the weird radio signals, they see exoplanets as a good bet for four of the stars, Callingham says.
The exoplanet hypothesis is “definitely a possibility,” agrees Jake Turner, a Cornell University radio astronomer who was not involved in the study, and who last year measured a radio signal that could also have been generated by an exoplanet. “There is so much on [red] dwarves that we don’t understand, ”he says, so these readings could also be explained by stellar physics that we don’t yet understand.
To make sense of the 19 signals, the team focused on what Callingham calls the “boring stars.”
Although stars are generally radio-dark, the most active, those with numerous solar flares and coronal mass ejections, often produce radio signals. There is also a correlation between the speed of a star’s rotation and the activity of its corona, the shroud of plasma that envelops the star. The slower and more boring a star, the less likely it is to emit radio signals, and the more likely the signal will come from an exoplanet, says Callinghan.
As to how an exoplanet would produce a radio signal, we have a great analog for the process in our own solar system.
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Jupiter is the loudest pirate radio station in the solar system because it interacts with Io, one of its largest moons, in a way that produces tons of radio waves. Looking at Jupiter, scientists know that this type of interaction produces a distinct type of light called circularly polarized light. The four most promising radio signals had 60 to 100 percent of their light polarized this way, Callingham says. For comparison, he says, an active star alone shouldn’t exceed 50%.
It’s hard for a star to produce those kinds of radio signals, says Callingham, “that’s how we knew we were on to something really special.”
Jupiter and Io make their luminous radio emission by two means. One is through the solar winds. Much like the Earth, the solar winds project electrons at Jupiter, and the magnetic field that envelops the planet carries the electrons towards the poles, says Callingham. The electron shower makes beautiful auroras and emits radio waves.
While striking, it is the least contributor to Jupiter’s radio broadcasts. The main method is the movement of Io around the planet, which creates a kind of huge electric generator.
Any electric generator works by moving a conductor in a magnetic field. The magnetic field pushes the electric charges in the conductor and makes them circulate. In our solar system, Jupiter is the magnet and Io (with its cloud of volcanic particles) is the conductor that moves around it. This movement accelerates nearby electrons, which then release their excess energy in the form of radio waves that become brighter or darker depending on the angle at which we see them.
Astronomers believe exoplanets and their host stars could play this Jupiter-Io interaction to generate radio signals that should cycle in time like Jupiter’s.
Callingham and his collaborators are now trying to get more data from the most promising red dwarfs to see if and how their radio signals change over time, which could determine if they are exoplanets.
Astronomers will only be able to verify the existence of these planets with more observations. The next LOFAR telescope upgrade, LOFAR2.0, and possibly the Square Kilometer Array project, will allow much higher resolution data to help solve these kinds of astronomical puzzles, Turner says.
For now, Callingham thinks exoplanets aren’t too far-fetched an explanation. “As our optical colleagues have shown us, most stars have exoplanets … so it’s actually not that wild,” Callingham says. With thousands of exoplanets discovered in a few decades, the landscape is changing rapidly. “In 1996, if I tried to do that,” he says, “I would have been laughed at from the audience.”