Astronomers have detected unusual radio emission from a quiet star, which is best explained by interactions with a nearby planet. In this case, the movement of electrons along the lines of the magnetic field generates powerful auroras in the radio range at the poles of the star. A similar mechanism is known in a pair of satellite planets in the solar system (Jupiter and Io), but this is the first time it has been registered for a pair of star-exoplanets. The discovery could form the basis of a new method for exploring extrasolar planets, the authors write in the journal Nature Astronomy.
As a rule, ordinary stars are not strong sources of radio waves with frequencies below 150 megahertz. It is believed that in the case of observing this type of radiation, it is generated in inhomogeneous regions of the corona at heights of at least one radius of the star. In particular, low-frequency radiation from the Sun can be used to determine the structure of the corona, mass ejections, and space weather.
All recorded cases of noticeable radio emission from stars at gigahertz frequencies are associated with nonthermal processes in the outer layers. Moreover, the vast majority of these sources belong to one of the types of objects with magnetic activity, such as flare stars (AD Leo), luminaries with fast rotation (FK Coma's Hair) or close binaries (Algol). At lower frequencies of hundreds of megahertz, the only known stellar source of radio emission is the flashing UV Ceti, a prototype of the corresponding class of variables.
Astronomers from five countries, led by Harish Vedantham of ASTRON, discovered a unique case of low-frequency radiation with the European low-frequency interferometer LOFAR from a single M-class red dwarf, called GJ 1151, located eight parsecs away. that this star has a calm atmosphere and weak rotation, that is, it is not able to independently generate such powerful radio waves.
The luminary was found within the framework of comparing objects from the LOFAR catalog with stars no further than 20 parsecs from the Earth according to the Gaia satellite. The maximum distance was chosen to increase the chances of detecting sources with low absolute luminosity and to reduce the likelihood of overlapping different sources. Radio emission from GJ 1151 was recorded in one observation session out of four conducted during the month. It possessed a high degree of polarization (64 ± 6 percent), which, together with high variability, excludes an accidental coincidence with an extragalactic object.
In addition to the parameters of GJ 1151 that are unsuitable for generating radio waves, this radiation turned out to be unlike the known star outbursts, which can be divided into two broad types. The first includes incoherent gyrosynchrotron radiation (similar to solar radio storms), which is characterized by low polarization, brightness temperature of no more than 1010 kelvin, wide spectral range and duration of many hours. The second class is coherent radiation (similar to bursts of solar radio emission) with high circular polarization, a narrow instantaneous band of radiation and a duration from seconds to minutes. In contrast to these two types, the radiation from GJ 1151 lasted more than eight hours, was practically independent of the frequency in the range from 120 to 167 megahertz and had a high circular polarization.
Astronomers concluded that this can be satisfactorily explained only by the assumption that there is an exoplanet in close orbit that makes one revolution in several days. In this case, the movement of the planet through the magnetosphere of the star (and M class dwarfs usually have strong magnetic fields), in fact, creates an electric motor, like a dynamo. As a result, strong currents of electrons arise, which, when approaching the magnetic poles of the star, generate powerful radio waves and auroras in its atmosphere.
A similar process is known in the solar system - this is how radio emission from Jupiter arises. This planet also has a noticeable magnetic field, and associated with constant volcanic activity, the atmosphere of the satellite Io, located close to the gas giant, plays the role of a source of charged particles. As a result, under suitable conditions, an electron cyclotron maser instability arises, which synchronizes the phases of the radiation of charged particles and leads to directional coherent radiation. It is fixed on Earth with a periodicity corresponding to the frequency of Io's revolution around Jupiter. It is noteworthy that at low frequencies, Jupiter even turns out to be brighter than the Sun.
A similar phenomenon was predicted for stars more than thirty years ago, but has never been observed before. The authors suggest that in this case the radiation is associated with polar "radio beams" on the star, but theoretically it can be associated with the planet's magnetosphere. However, for this, the magnetic field of the exoplanet must be very strong, which may be the case for a hot Jupiter, and Earth-like planets are much more often found in class M dwarfs, for which powerful magnetic fields are not predicted.
As the radio survey continues on the LOFAR interferometer, more such systems will be discovered - about a hundred, according to astronomers' estimates. Since they all belong to the solar environs, it is possible for them to be studied by other methods, including the method of radial velocities. This will make it possible to independently estimate the exoplanet's orbital period and its mass, so that the correctness of the model can be verified.
Earlier, astronomers found out that the magnetic fields of hot Jupiters are many times stronger than the predictions of the theory, proposed to search for exoplanets with a magnetic field using the FAST radio telescope and substantiated the protective role of the ancient Earth's magnetic field.