Siberian Federal University: Radio Signals Will Tell New Facts about Exoplanets

Researchers of Siberian Federal University and their partners from Austria and France have found out whether radio emission can escape from the magnetosphere of the planet Upsilon Andromedae b, and presented a new method for obtaining a lower estimate of the mass of hot Jupiters.

The researchers carried out mathematical modeling of atmosphere of Upsilon Andromedae b (a massive exoplanet in the constellation of Andromeda). In particular, experts were interested in under what conditions can directional radio emission based on the principle of a cyclotron maser be generated near the planet and whether it can leave the area of its source and be registered on Earth. To answer these questions, the experts ran calculations of the distributions of physical parameters in the atmosphere and ionosphere of the planet on the basis of the obtained model and checked the physical criterion of radio emission generation. It turned out that the mass of the planet is crucial for the occurrence and propagation of this radio emission. The mass should exceed a certain threshold value found as a result of model calculations.

Exoplanets have been in the focus of attention of astronomers and astrophysicists for several decades. Various methods of measuring the mass of these planets are being developed, in particular, to identify their suitability for life. Scientists believe that knowing the mass of an exoplanet, they can calculate the composition of its surface, study thermoregulation, plate tectonics, magnetic fields and gas flows in the atmosphere. As for radio emission, its registration from exoplanets is considered a promising way of searching for them.

“It is known that planets can emit coherent, polarized radio emission generated in the surrounding plasma during cyclotron maser instability. The essence of this effect is the amplification of electromagnetic waves by free electrons moving in a magnetic field. This happens if the frequency of electron oscillations in the plasma is significantly lower than their cyclotron frequency. By the way, it is the cyclotron maser instability that is the cause of the intense decameter radio emission from Jupiter,” said Nikolay Yerkaev, co-author of the study, professor of the Department of Applied Mechanics at Siberian Federal University. “These radio waves can, under certain conditions, escape from the magnetosphere surrounding the planet into open space and reach Earth, bringing important information about the source planet. In the case of massive exoplanets called hot Jupiters, there may be a situation of strong heating of the upper atmosphere due to the absorption of X-ray and extreme ultraviolet radiation of the host star, which leads to the expansion of the upper atmosphere, in which ionized gas can prevent the release of radio emissions into outer space”.

According to the scientist, the mass of Upsilon Andromedae b is still unknown, but in the case of detection of planetary radio emission, the proposed approach allows to unequivocally indicate the lower limit of the mass of this exoplanet. A foreign publication (.pdf) in 2021 informed about the preliminary experimental detection of radio emission from Upsilon Andromedae system.

Using the ground-based LOFAR system, a team of astronomers led by Jake D. Turner of Cornell University published radio observations of three planetary systems: 55 Cancer, Upsilon Andromedae, and Tau Boötes, which are within 50 light-years of the Sun. The scientists were looking for possible radio emission from exoplanets at frequencies of 10-90 megahertz with strong circular polarization, which would indicate a connection with the instability of the electron cyclotron maser. The researchers recorded one radio pulse from Upsilon Andromedae, although its statistical significance was only 2.2 sigma, which is not enough for a confirmed discovery.

In the case of Tau Boötis, the researchers found circularly polarized bursty emission in the range of 14-21 megahertz (3.2 sigma) and a stream of radio emission with circular polarization in the frequency range of 21-30 megahertz. According to the astronomers, the source of the radio emission is a hot Jupiter exoplanet in the system with a mass of about six Jupiter masses. The estimated magnetic field strength at the poles of the exoplanet near its surface is in the range of 5-11 gauss.

“I believe that further radio observations on several telescopes will confirm this significant discovery. If this signal is confirmed by subsequent observations using telescopes, then the ideas about the mass of this planet will be significantly adjusted,” added Mr Yerkaev.

According to the calculations made during the Russian-European study, radio emission that can be detected by interplanetary probes can be generated only if the planet has a mass of at least 2.25 Jupiter masses. Thus, the proposed method opens up new opportunities for determining and clarifying the masses of many large exoplanets.