# Would it be possible to detect a magnetic field around an exoplanet?

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Of course, we can't fly a magnetometer next to an exoplanet to measure the magnetic field, but might it be possible to find indirect evidence (e.g. polar auroras) of a magnetic field generated by an exoplanet? Has this already been done?

You usually look for electronic synchrotron emission in the radio, which corresponds to the magnetic field strength expected for giant planets. There has been a single detection claimed with LOFAR by Turner et al., (2020) around $$au$$ Bootes, but it's very noisy and hence disputed.

Other effects like Zeeman-splitting have been considered for detection, but require clean, high-resolution signals from planets. So far this is too difficult to perform due to stellar contamination.

Magnetic field searches around brown dwarves have been more successful, but those are not planets, of course.

Magnetic fields can be measured via the Zeeman effect, thus a change of absorption and emission lines in spectra - a shift which depends on the direction and the strength of the magnetic field. This is a method commonly employed to measure and map the magnetic field of the Sun (also here).

The difficulty in doing so is the necessary precision in the spectral data of the source and the ambiguity if you sample over a broad area where this effect is also convoluted with the doppler broadening due to mapping areas with different relative speed towards the observer. Generally, getting high-resolution spectral data requires much light or long exposure, both of which can be difficult to obtain for exoplanet atmospheres. I'm not aware of magnetic field measurements on exoplanets so far, yet it might become viable in the future. It is done for stars.

## Astronomers detect possible radio emission from exoplanet

By monitoring the cosmos with a radio telescope array, an international team of scientists has detected radio bursts emanating from the constellation Boötes -- that could be the first radio emission collected from a planet beyond our solar system.

The team, led by Cornell postdoctoral researcher Jake D. Turner, Philippe Zarka of the Observatoire de Paris -- Paris Sciences et Lettres University and Jean-Mathias Griessmeier of the Université d'Orléans will publish their findings in the forthcoming research section of Astronomy & Astrophysics, on Dec. 16.

"We present one of the first hints of detecting an exoplanet in the radio realm," Turner said. "The signal is from the Tau Boötes system, which contains a binary star and an exoplanet. We make the case for an emission by the planet itself. From the strength and polarization of the radio signal and the planet's magnetic field, it is compatible with theoretical predictions."

Among the co-authors is Turner's postdoctoral advisor Ray Jayawardhana, the Harold Tanner Dean of the College of Arts and Sciences, and a professor of astronomy.

"If confirmed through follow-up observations," Jayawardhana said, "this radio detection opens up a new window on exoplanets, giving us a novel way to examine alien worlds that are tens of light-years away."

Using the Low Frequency Array (LOFAR), a radio telescope in the Netherlands, Turner and his colleagues uncovered emission bursts from a star-system hosting a so-called hot Jupiter, a gaseous giant planet that is very close to its own sun. The group also observed other potential exoplanetary radio-emission candidates in the 55 Cancri (in the constellation Cancer) and Upsilon Andromedae systems. Only the Tau Boötes exoplanet system -- about 51 light-years away -- exhibited a significant radio signature, a unique potential window on the planet's magnetic field.

Observing an exoplanet's magnetic field helps astronomers decipher a planet's interior and atmospheric properties, as well as the physics of star-planet interactions, said Turner, a member of Cornell's Carl Sagan Institute.

Earth's magnetic field protects it from solar wind dangers, keeping the planet habitable. "The magnetic field of Earth-like exoplanets may contribute to their possible habitability," Turner said, "by shielding their own atmospheres from solar wind and cosmic rays, and protecting the planet from atmospheric loss."

Two years ago, Turner and his colleagues examined the radio emission signature of Jupiter and scaled those emissions to mimic the possible signatures from a distant Jupiter-like exoplanet. Those results became the template for searching radio emission from exoplanets 40 to 100 light-years away.

After poring over nearly 100-hours of radio observations, the researchers were able to find the expected hot Jupiter signature in Tau Boötes. "We learned from our own Jupiter what this kind of detection looks like. We went searching for it and we found it," Turner said.

The signature, though, is weak. "There remains some uncertainty that the detected radio signal is from the planet. The need for follow-up observations is critical," he said.

Turner and his team have already begun a campaign using multiple radio telescopes to follow up on the signal from Tau Boötes.

In addition to Turner, Jayawardhana, Griessmeier and Zarka, the co-authors are Laurent Lamy and Baptiste Cecconi of the Observatoire de Paris, France Joseph Lazio from NASA's Jet Propulsion Laboratory J. Emilio Enriquez and Imke de Pater from the University of California, Berkeley Julien N. Girard from Rhodes University, Grahamstown, South Africa and Jonathan D. Nichols from the University of Leicester, United Kingdom.

Turner, who laid the groundwork for this research while earning his doctorate at the University of Virginia, received funding from the National Science Foundation.

## Astronomers detect First Potential Radio Signal from an exoplanet

Using the Low-Frequency Array (LOFAR), a radio telescope in the Netherlands, researchers discovered emissions from the Tau Bootes star-system housing the so-called hot Jupiter, a gaseous giant planet that is very close to its own sun. By observing the universe with a radio telescope array, a multinational team of scientists has observed radio bursts originating from the Boötes constellation-which may be the first radio signal collected from a planet outside our solar system.

The first possible radio signal from a world outside our solar system, originating from an exoplanet system about 51 light-years away, was obtained by a multinational team of scientists. The team, led by Cornell postdoctoral researcher Jake D. Turner, Philippe Zarka of the Paris Observatory—Paris Sciences et Lettres University and Jean-Mathias Griessmeier of the Université d’Orléans, will report their findings in the upcoming Astronomy & Astrophysics research section on Dec. 16.

An international team of scientists has picked up the first radio waves emitted by an exoplanet. The planet is a “Hot Jupiter” orbiting a star system 40 light-years from Earth.

“We present one of the first hints of detecting an exoplanet in the radio realm,” Turner said. “The signal is from the Tau Boötes system, which contains a binary star and an exoplanet. We make the case for emission by the planet itself. From the strength and polarization of the radio signal and the planet’s magnetic field, it is compatible with theoretical predictions.”

Among the co-authors are Ray Jayawardhana, a postdoctoral adviser to Turner, Harold Tanner Dean of the College of Arts and Sciences, and Professor of Astronomy. “If confirmed through follow-up observations,” Jayawardhana said, “this radio detection opens up a new window on exoplanets, giving us a novel way to examine alien worlds that are tens of light-years away.”

Using the Low-Frequency Array (LOFAR), a radio telescope in the Netherlands, Turner and his collaborators discovered emissions from a star-system containing the so-called hot Jupiter, a gaseous giant planet that is very close to its own sun. The group has found other possible exoplanetary radio-emission targets in the 55 Cancri (cancer constellation) and Upsilon Andromedae systems. Only the exoplanet system of Tau Boötes—around 51 light-years away—has a significant radio signature, a rare possible window on the magnetic field of the world.

Observing the magnetic field of the exoplanet allows astronomers to discern the inner and atmospheric properties of the planet, as well as the mechanics of star-planet interactions, said Turner, a member of the Carl Sagan Institute of Cornell. Earth’s magnetic field protects it from the hazards of the solar wind, making the earth habitable. “The magnetic field of Earth-like exoplanets may contribute to their possible habitability,” Turner said, “by shielding their own atmospheres from solar wind and cosmic rays, and protecting the planet from atmospheric loss.”

A few years ago, Turner and his colleagues examined the radio emission signature of Jupiter and scaled those emissions to mimic the possible signatures from a distant Jupiter-like exoplanet. Those results became the template for searching radio emission from exoplanets 40 to 100 light-years away.

After porting almost 100 hours of radio observations, the researchers were able to locate the predicted hot Jupiter signature in Tau Boötes. “We learned from our own Jupiter what this kind of detection looks like. We went searching for it and we found it,” Turner said. The signature, however, is poor. “There remains some uncertainty that the detected radio signal is from the planet. The need for follow-up observations is critical,” he said.

Turner and his colleagues have already launched a campaign using several radio telescopes to map the signal from Tau Boötes. In addition to Turner, Jayawardhana, Griessmeier, and Zarka, co-authors are Laurent Lamy and Baptiste Cecconi of the Paris Observatory, France Joseph Lazio of NASA’s Jet Propulsion Laboratory J. Emilio Enriquez and Imke de Pater of the University of California, Berkeley Julien N. Girard of Rhodes University, Grahamstown, South Africa and Jonathan D. Nichols of the University of California.

Turner, who laid the basis for this study when pursuing his doctorate at the University of Virginia, obtained funds from the National Science Foundation. Two years ago, Turner and his collaborators looked at the radio emission signature of Jupiter and scaled those emissions to imitate potential signatures from a distant Jupiter-like exoplanet. These observations have been a model for the quest for radio emissions from exoplanets 40 to 100 light-years distant.

## Magnetic Fields Around Red Dwarf Planets Might Not Be Strong Enough To Support Life

Low-mass stars create a hostile environment for planets that orbit them, probably stripping their atmospheres and preventing life, new modeling of stellar outbursts suggests. This possibility has been raised before, but further evidence has been presented. The findings mean the recent findings of Earth-like planets around nearby red dwarfs might not be as exciting as we thought.

Most stars are M-type, also known as red dwarfs. Consequently, they illuminate the majority of the planets where we might look for life. In our own galactic neighborhood, we have found abundant planets in the so-called “habitable zone” around red dwarfs within about 40 light-years. Existing telescopes can't get a good view of these, but the next generation, being built at this moment, might well be able to detect atmospheres around these worlds.

Nevertheless, there has been one fly in the exoplanet cornucopia ointment: The fear that coronal mass ejections (CME), to which M-type stars are particularly prone, could strip the atmospheres from planets orbiting close enough to these dim stars not to freeze. Without an atmosphere, liquid water can't be sustained – ice sublimes directly to water vapor, which gets blown away by the next CME.

Planetary magnetic fields provide protection, as Earth's does for us, but astronomers have been troubled by the question of how strong a field would need to be to make life possible around a typical red dwarf. Too strong, according to Boston University PhD student Christina Kay.

Kay picked V374 Pegasi, an M-type star 29 light-years away, and not much more than half as hot. It's magnetic field, flares and CMEs have been particularly heavily studied, but Kay told the UK National Astronomy Meeting she'd found something new.

"We figured that the CMEs would be more powerful and more frequent than solar CMEs, but what was unexpected was where the CMEs ended up," Kay said in a statement. She found CMEs get pushed into an area known as the Astrospheric Current Sheet, roughly equivalent to the plane of the solar equator – and where most planets orbit.

Kay reported in the Astrophysical Journal such regular CME exposure would blast the atmospheres from nearby planets with magnetic fields similar to Earth's. Unless a planet orbited so far out it would be covered in ice anyway, it would need a magnetic field at least 10, and often several thousand, times as strong as the Earth's to hold onto its air.

Without exceptionally strong magnetic fields, planets like Proxima b, and the multiple members of the TRAPPIST-1 system, are likely to be blasted wastelands. Efforts to find life elsewhere might need to go back to focusing on rarer mid-mass stars, where CMEs are rarer, and not focussed into the planetary plane.

There are certainly many ways to go about detecting a magnetic field on an exoplanet. Many involve studying interactions with the home star. You may have heard of the case of HD 209458 b. Here, Kislyakova et al. (2014) looked at Lyman- $alpha$ absorption around the planet, which was indicative of neutral atoms moving at high velocities. The only model that explained the behavior was a magnetic field with a magnetic moment about that of Jupiter's, interacting with the stellar wind.

There are other signs of a planet's magnetosphere. Electrons traveling along field lines in the magnetosphere can lead to radio emissions from electron cyclotron radiation, which has been observed in the gas giants in the solar system. Detecting these emissions from exoplanets is a lot trickier, as the signals will be fainter. However, at distances of less than 20 parsecs, this method might be feasible in the near future.

Yet another possible technique would be to try to study bow shocks (see Vidotto et al. (2010)) caused by the interaction of the planet's magnetosphere with the stellar wind. This - as well as the other methods - can work extremely well when the planet is close to its home star, as is the case with Hot Jupiters. You can also study atmosphere loss on these planets (which would be influenced by the presence of a magnetic field) - which is similar to what was done with HD 209458.

There's also the curious case of HD 179949 b. In 2004, astronomers noticed a hot spot on the surface of HD 179949, the host star. The spot has the same period as the planets, and appears to be the result of an interaction between the magnetic fields of the exoplanet and the star. I am not aware of newer observations of the spot, however.

I'd strongly recommend taking a look at Chapter four of this book for a brief summary of the different detection techniques, as well as a discussion of theoretical scaling laws.

It's also worth noting that Barnes et al. (2016) created models that found that it's possible for Proxima Centauri b to have a magnetic field, although there's not yet data to confirm or refute the possibility. Barnes himself explains that, as well as other habitability issues, on the Pale Red Dot website.

I'm aware of two recent papers (Vedantham et al. 2020, Pope et al. 2020) that claim an indirect detection of an exoplanet orbiting the star GJ 1151 by 1) detecting radio emission at

150 MHz and 2) ruling out companions of $Msin i>5.6M_$ , theorizing that the signal is from auroral emissions to due an exoplanet's magnetosphere. I can say more about that once I've taken a good look at the papers - and perhaps after anyone is able to make independent observations of the system.

## Estimating the magnetic field of an exoplanet

An artist's illustration of an evaporating hot giant exoplanet. Credit: NASA's Goddard Space Flight Center

Scientists developed a new method which allows to estimate the magnetic field of a distant exoplanet, i.e., a planet, which is located outside the Solar system and orbits a different star. Moreover, they managed to estimate the value of the magnetic moment of the planet HD 209458b.The group of scientists including one of the researchers of the Lomonosov State University published their article in the Science magazine.

In the two decades which passed since the discovery of the first planet outside the Solar system, astronomers have made a great progress in the study of these objects. While 20 years ago a big event was even the discovery of a new planet, nowadays astronomers are able to consider their moons, atmosphere and climate and other characteristics similar to the ones of the planets in the Solar system. One of the important properties of both solid and gaseous planets is their possible magnetic field and its magnitude. On the Earth it protects all the living creatures from the dangerous cosmic rays and helps animals to navigate in space.

Kristina Kislyakova of the Space Research Institute of the Austrian Academy of Sciences in Graz together with an international group of physicists for the first time ever was able to estimate the value of the magnetic moment and the shape of the magnetosphere of the exoplanet HD 209458b. Maxim Khodachenko, a researcher at the Department of Radiation and computational methods of the Skobeltsyn Institute of Nuclear Physics of the Lomonosov Moscow State University, is also one of the authors of the article. He also works at the Space Research Institute of the Austrian Academy of Sciences.

Planet HD 209458b (Osiris) is a hot Jupiter, approximately one third larger and lighter than Jupiter. It is a hot gaseous giant orbiting very close to the host star HD 209458. HD 209458b accomplishes one revolution around the host star for only 3.5 Earth days. It has been known to astronomers for a long time and is relatively well studied. In particular, it is the first planet where the atmosphere was detected. Therefore, for many scientists it has become a model object for the development of their hypotheses.

Scientists used the observations of the Hubble Space Telescope of the HD 209458b in the hydrogen Lyman-alpha line at the time of transit, when the planet crosses the stellar disc as seen from the Earth. At first, the scientists studied the absorption of the star radiation by the atmosphere of the planet. Afterwards they were able to estimate the shape of the gas cloud surrounding the hot Jupiter, and, based on these results, the size and the configuration of the magnetosphere.

"We modeled the formation of the cloud of hot hydrogen around the planet and showed that only one configuration, which corresponds to specific values of the magnetic moment and the parameters of the stellar wind, allowed us to reproduce the observations" - explained Kristina Kislyakova.

To make the model more accurate, scientists accounted for many factors that define the interaction between the stellar wind and the atmosphere of the planet: so-called charge exchange between the stellar wind and the neutral atmospheric particles and their ionization, gravitational effects, pressure, radiation acceleration, and the spectral line broadening.

At present, scientists believe that the size of the atomic hydrogen envelope is defined by the interaction between the gas outflows from the planet and the incoming stellar wind protons. Similarly to the Earth, the interaction of the atmosphere with the stellar wind occurs above the magnetosphere. By knowing the parameters of an atomic hydrogen cloud, one can estimate the size of the magnetosphere by means of a specific model.

Since direct measurements of the magnetic field of exoplanets are currently impossible, the indirect methods are broadly used, for example, using the radio observations. There exist a number of attempts to detect the radio emission from the planet HD 209458b. However, because of the large distances the attempts to detect the radio emission from exoplanets have yet been unsuccessful.

"The planet's magnetosphere was relatively small beeing only 2.9 planetary radii corresponding to a magnetic moment of only 10% of the magnetic moment of Jupiter"—explained Kislyakova, a graduate of the Lobachevsky State University of Nizhny Novgorod. According to her, it is consistent with the estimates of the effectiveness of the planetary dynamo for this planet.

"This method can be used for every planet, including Earth-like planets, if there exist an extended high energetic hydrogen envelope around them" - summarized Maxim Khodachenko.

## Astronomers detect possible radio emission from exoplanet

In this artistic rendering of the Tau Boötes b system, the lines representing the invisible magnetic field are shown protecting the hot Jupiter planet from solar wind. Credit: Jack Madden/Cornell University

By monitoring the cosmos with a radio telescope array, a Cornell University-led international team of scientists has detected radio bursts emanating from the constellation Boötes. The signal could be the first radio emission collected from a planet beyond our solar system.

The team, led by Cornell postdoctoral researcher Jake D. Turner, Philippe Zarka of the Observatoire de Paris—Paris Sciences et Lettres University and Jean-Mathias Griessmeier of the Université d'Orléans published their findings in the forthcoming research section of the journal Astronomy & Astrophysics, on Dec. 16.

"We present one of the first hints of detecting an exoplanet in the radio realm," Turner said. "The signal is from the Tau Boötes system, which contains a binary star and an exoplanet. We make the case for an emission by the planet itself. From the strength and polarization of the radio signal and the planet's magnetic field, it is compatible with theoretical predictions."

Among the co-authors is Turner's postdoctoral advisor Ray Jayawardhana, the Harold Tanner Dean of the College of Arts and Sciences at Cornell, and a professor of astronomy.

"If confirmed through follow-up observations," Jayawardhana said, "this radio detection opens up a new window on exoplanets, giving us a novel way to examine alien worlds that are tens of light-years away."

Using the Low Frequency Array (LOFAR), a radio telescope in the Netherlands, Turner and his colleagues uncovered emission bursts from a star-system hosting a so-called hot Jupiter, a gaseous giant planet that is very close to its own sun. The group also observed other potential exoplanetary radio-emission candidates in the 55 Cancri (in the constellation Cancer) and Upsilon Andromedae systems. Only the Tau Boötes exoplanet system—about 51 light-years away—exhibited a significant radio signature, a unique potential window on the planet's magnetic field.

Observing an exoplanet's magnetic field helps astronomers decipher a planet's interior and atmospheric properties, as well as the physics of star-planet interactions, said Turner, a member of Cornell's Carl Sagan Institute.

Earth's magnetic field protects it from solar wind dangers, keeping the planet habitable. "The magnetic field of Earth-like exoplanets may contribute to their possible habitability," Turner said, "by shielding their own atmospheres from solar wind and cosmic rays, and protecting the planet from atmospheric loss."

Two years ago, Turner and his colleagues examined the radio emission signature of Jupiter and scaled those emissions to mimic the possible signatures from a distant Jupiter-like exoplanet. Those results became the template for searching radio emission from exoplanets 40 to 100 light-years away.

After poring over nearly 100-hours of radio observations, the researchers were able to find the expected hot Jupiter signature in Tau Boötes. "We learned from our own Jupiter what this kind of detection looks like. We went searching for it and we found it," Turner said.

The signature, though, is weak. "There remains some uncertainty that the detected radio signal is from the planet. The need for follow-up observations is critical," he said.

Turner and his team have already begun a campaign using multiple radio telescopes to follow up on the signal from Tau Boötes.

## The First Radio Signal From An Exoplanet May Have Just Been Detected

Of the thousands of planets beyond our Solar System (exoplanets) found since 1992, only a handful have been directly observed. In the rare cases where this has occurred, it has been in visible, or near-visible, wavelengths. Now, however, astronomers have picked up what appear to be radio waves coming from an exoplanet. Before you make an interplanetary-sized jump to conclusions, no this isn't a sign of extraterrestrial intelligence. Still, it could be a significant milestone in the quest to find life beyond the Earth.

To the naked eye, Tau Boötis looks like a very ordinary star one needs to get away from city lights to even see. However, at 51 light-years away and a spectral classification of F, the larger component of this two-star system is one of our nearer neighbors with a strong resemblance to the Sun, although somewhat on the brighter and more massive size. Besides a red dwarf, the system also includes one of the first exoplanets to be discovered, Tau Boötis b, located in 1996.

It is from the direction of this system that Dr Jake Turner of Cornell University helped detect a 14-21 megahertz radio signal using the Low-Frequency Array in the Netherlands. “We make the case for an emission by the planet itself. From the strength and polarization of the radio signal and the planet's magnetic field, it is compatible with theoretical predictions." Turner said in a statement.

Turner and co-authors present their findings, and the reasons they think the signal is from Tau Boötis b, in Astronomy and Astrophysics. They argue the circularly polarized nature of the signal, the lack of flare events on Tau Boötis A, and the known nature of the planet all make it much more likely the signal came from the planet rather than the star. If so, it indicates a very strong magnetic field being bombarded by Tau Boötis A's strong stellar wind.

Turner studied Jupiter's radio emissions and scaled what he saw for a planet 40-100 light-years away, in the hope of knowing what to look for. A brief signal was detected from the Upsilon Andromedae system, and nothing at all from 55 Cancri, both potential candidates studied at the same time.

We can be very confident Tau Boötis b does not host life. It's a classic “hot Jupiter” – gas giant exoplanets similar to Jupiter that orbit much closer to their star – six times as massive as our Solar System's largest planet, with a temperature estimated at 1,400ºC (2,600ºF). However, the Earth's magnetic field is very important for life on Earth, having enabled our planet to hold onto its atmosphere by shielding against the solar wind. Without a magnetic field life might not be impossible, but would certainly be constrained. If Turner's work is the introduction to detecting magnetic fields around planets more promising than Tau Boötis b, it could be a big step towards identifying habitable worlds.

The SETI Institute wants to make quite sure you're not misinterpreting this.

Researchers have, by the way, used radio waves to detect an exoplanet before, but that was very different. GJ1151, a red dwarf 26 light-years away, has a strong magnetic field, which gets disturbed by a planet passing through it, producing radio waves as a result. We know the exoplanet is there because the radio waves can't really be explained any other way, they don't come from the exoplanet itself.

## By Blaine Friedlander |

By monitoring the cosmos with a radio telescope array, an international team of scientists has detected radio bursts emanating from the constellation Boötes – that could be the first radio emission collected from a planet beyond our solar system.

The team, led by Cornell postdoctoral researcher Jake D. Turner, Philippe Zarka of the Observatoire de Paris - Paris Sciences et Lettres University and Jean-Mathias Griessmeier of the Université d’Orléans will publish their findings in the forthcoming research section of Astronomy & Astrophysics, on Dec. 16.

“We present one of the first hints of detecting an exoplanet in the radio realm,” Turner said. “The signal is from the Tau Boötes system, which contains a binary star and an exoplanet. We make the case for an emission by the planet itself. From the strength and polarization of the radio signal and the planet’s magnetic field, it is compatible with theoretical predictions.”

Among the co-authors is Turner’s postdoctoral advisor Ray Jayawardhana, the Harold Tanner Dean of the College of Arts and Sciences, and a professor of astronomy.

“If confirmed through follow-up observations,” Jayawardhana said, “this radio detection opens up a new window on exoplanets, giving us a novel way to examine alien worlds that are tens of light-years away.”

Using the Low Frequency Array (LOFAR), a radio telescope in the Netherlands, Turner and his colleagues uncovered emission bursts from a star-system hosting a so-called hot Jupiter, a gaseous giant planet that is very close to its own sun. The group also observed other potential exoplanetary radio-emission candidates in the 55 Cancri (in the constellation Cancer) and Upsilon Andromedae systems. Only the Tau Boötes exoplanet system – about 51 light-years away – exhibited a significant radio signature, a unique potential window on the planet’s magnetic field.

Observing an exoplanet’s magnetic field helps astronomers decipher a planet’s interior and atmospheric properties, as well as the physics of star-planet interactions, said Turner, a member of Cornell’s Carl Sagan Institute.

Earth’s magnetic field protects it from solar wind dangers, keeping the planet habitable. “The magnetic field of Earth-like exoplanets may contribute to their possible habitability,” Turner said, “by shielding their own atmospheres from solar wind and cosmic rays, and protecting the planet from atmospheric loss.”

Two years ago, Turner and his colleagues examined the radio emission signature of Jupiter and scaled those emissions to mimic the possible signatures from a distant Jupiter-like exoplanet. Those results became the template for searching radio emission from exoplanets 40 to 100 light-years away.

After poring over nearly 100-hours of radio observations, the researchers were able to find the expected hot Jupiter signature in Tau Boötes. “We learned from our own Jupiter what this kind of detection looks like. We went searching for it and we found it,” Turner said.

The signature, though, is weak. “There remains some uncertainty that the detected radio signal is from the planet. The need for follow-up observations is critical,” he said.

Turner and his team have already begun a campaign using multiple radio telescopes to follow up on the signal from Tau Boötes.

In addition to Turner, Jayawardhana, Griessmeier and Zarka, the co-authors are Laurent Lamy and Baptiste Cecconi of the Observatoire de Paris, France Joseph Lazio from NASA’s Jet Propulsion Laboratory J. Emilio Enriquez and Imke de Pater from the University of California, Berkeley Julien N. Girard from Rhodes University, Grahamstown, South Africa and Jonathan D. Nichols from the University of Leicester, United Kingdom.

Turner, who laid the groundwork for this research while earning his doctorate at the University of Virginia, received funding from the National Science Foundation.

By monitoring the cosmos with a radio telescope array, an international team of scientists has detected radio bursts emanating from the constellation Boötes – that could be the first radio emission collected from a planet beyond our solar system. Cornell postdoctoral researcher Jake D. Turner explains the research.

## How to estimate the magnetic field of an exoplanet?

Scientists developed a new method which allows to estimate the magnetic field of a distant exoplanet, i.e., a planet, which is located outside the Solar system and orbits a different star. Moreover, they managed to estimate the value of the magnetic moment of the planet HD 209458b.The group of scientists including one of the researchers of the Lomonosov Moscow State University (Russia) published their article in the Science magazine.

In the two decades which passed since the discovery of the first planet outside the Solar system, astronomers have made a great progress in the study of these objects. While 20 years ago a big event was even the discovery of a new planet, nowadays astronomers are able to consider their moons, atmosphere and climate and other characteristics similar to the ones of the planets in the Solar system. One of the important properties of both solid and gaseous planets is their possible magnetic field and its magnitude. On the Earth it protects all the living creatures from the dangerous cosmic rays and helps animals to navigate in space.

Kristina Kislyakova of the Space Research Institute of the Austrian Academy of Sciences in Graz together with an international group of physicists for the first time ever was able to estimate the value of the magnetic moment and the shape of the magnetosphere of the exoplanet HD 209458b. Maxim Khodachenko, a researcher at the Department of Radiation and computational methods of the Skobeltsyn Institute of Nuclear Physics of the Lomonosov Moscow State University, is also one of the authors of the article. He also works at the Space Research Institute of the Austrian Academy of Sciences.

Planet HD 209458b (Osiris) is a hot Jupiter, approximately one third larger and lighter than Jupiter. It is a hot gaseous giant orbiting very close to the host star HD 209458. HD 209458b accomplishes one revolution around the host star for only 3.5 Earth days. It has been known to astronomers for a long time and is relatively well studied. In particular, it is the first planet where the atmosphere was detected. Therefore, for many scientists it has become a model object for the development of their hypotheses.

Scientists used the observations of the Hubble Space Telescope of the HD 209458b in the hydrogen Lyman-alpha line at the time of transit, when the planet crosses the stellar disc as seen from the Earth. At first, the scientists studied the absorption of the star radiation by the atmosphere of the planet. Afterwards they were able to estimate the shape of the gas cloud surrounding the hot Jupiter, and, based on these results, the size and the configuration of the magnetosphere.

"We modeled the formation of the cloud of hot hydrogen around the planet and showed that only one configuration, which corresponds to specific values of the magnetic moment and the parameters of the stellar wind, allowed us to reproduce the observations" - explained Kristina Kislyakova.

To make the model more accurate, scientists accounted for many factors that define the interaction between the stellar wind and the atmosphere of the planet: so-called charge exchange between the stellar wind and the neutral atmospheric particles and their ionization, gravitational effects, pressure, radiation acceleration, and the spectral line broadening.

At present, scientists believe that the size of the atomic hydrogen envelope is defined by the interaction between the gas outflows from the planet and the incoming stellar wind protons. Similarly to the Earth, the interaction of the atmosphere with the stellar wind occurs above the magnetosphere. By knowing the parameters of an atomic hydrogen cloud, one can estimate the size of the magnetosphere by means of a specific model.

Since direct measurements of the magnetic field of exoplanets are currently impossible, the indirect methods are broadly used, for example, using the radio observations. There exist a number of attempts to detect the radio emission from the planet HD 209458b. However, because of the large distances the attempts to detect the radio emission from exoplanets have yet been unsuccessful.

"The planet's magnetosphere was relatively small beeing only 2.9 planetary radii corresponding to a magnetic moment of only 10% of the magnetic moment of Jupiter" -- explained Kislyakova, a graduate of the Lobachevsky State University of Nizhny Novgorod. According to her, it is consistent with the estimates of the effectiveness of the planetary dynamo for this planet.

"This method can be used for every planet, including Earth-like planets, if there exist an extended high energetic hydrogen envelope around them" - summarized Maxim Khodachenko.

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