Summary
A research project led by Project Assistant Professor Yasunori Hori from the Astrobiology Center reveals that short-period gas giants outside the solar system hold magnetic fields tens to hundreds of Gauss, which is stronger than Earth or Jupiter, and the generation and strength of these magnetic fields are closely related to the presence of a central core. This finding suggests that detecting radio emissions from gas giants in the near future could provide insights into the unknown interiors of exoplanets through their magnetic field properties. This research achievement was published in the Astrophysical Journal on February 16, 2021.
Research Background
It is believed that planetary magnetic fields are generated and maintained via electric currents generated by the motion of conductive fluids in planetary interiors. In the solar system, Mercury, Earth, Jupiter, Saturn, Uranus, and Neptune (Ganymede, Jupiter’s third satellite, is also suggested to have a dynamo-driven intrinsic magnetic field1), the iron-nickel alloy in the outer core of Earth, the metallic hydrogen2 layer in gas planets, and the ionic water3 region in ice ionized water3 regions on planets play a role in the generation of magnetic fields. Thus, the presence and strength of planetary magnetic fields are closely related to the thermal and physical conditions of planetary interiors.
Observational attempts to detect magnetic fields in exoplanets have also been made; in 2020, a circularly polarized observation of Tau in the constellation Ursa Major using the radio telescope LOFAR4 reported the detection of signs of radio emission from a short-period gas planet (Tau b in the constellation Ursa Major)5 (Turner et al. 2020). 2020). The temporal variations in the surface activity of four stars (HD 179949, HD 189733, Tau b in Leo, and Upsilon Andromeda) with short-period gaseous planets (Hot Jupiters) were synchronized with the orbital motion of the Hot Jupiters, and the existence of magnetic fields of extrasolar gas planets was indirectly detected (Cauley et al. 2020). (Cauley et al. 2019). These observations suggest that short-period gas planets may have magnetic fields of several 10 G to several 100 G (Gauss), which are stronger than those of Jupiter.
For exoplanets that cannot be directly measured in situ by spacecraft, the information (mass, radius, upper atmosphere composition, and atmospheric flow) obtained from ground-based and space telescope observations is limited. However, radio emissions*6 from short-period gaseous planets, which are likely to have strong intrinsic magnetic fields, can be used to make observations with future radio telescopes (e.g. SKA: Square Kilometre Array) is expected to detect them. Therefore, more information on the magnetic fields of exoplanets is expected to be obtained in the future.
Research Findings
In this study, we investigated a method to probe the interior of mysterious exoplanets from information on planetary magnetic fields, with a view to detecting radio waves from exoplanets. In this study, we focused on short-period gas giants in the vicinity of their central stars. We investigated the generation of magnetic fields and changes in the strength of the magnetic field inside the planet during the 10 billion years of thermal history since the formation of the short-period gas planet. As a result, we found that the generation and intensity of planetary magnetic fields are not affected much by the internal structure of the planet (e.g., the size of the central core), and that short-period gas giants (hot Jupiters) with masses greater than 50% of Jupiter’s are likely to possess strong magnetic fields ranging from several tens to several hundred G (see Figure 1). (see Figure 1). In terms of expected planetary magnetic field strength, these short-period gas giants are good candidates for future radio emission observations.
On the other hand, for short-period gas giants (hot Saturns) with a mass less than Saturn (about 30% of Jupiter’s mass), if the core is too small, no magnetic field is generated for several tens of million years (up to several hundred million years) after their birth (right figure in Figure 1). In other words, the presence or absence of a magnetic field in a short-period gas planet below Saturn’s mass is a clue to the existence of a core inside the planet. However, due to the weak magnetic field strength, radio emissions from hot Saturn are likely to be shielded by plasma in the Earth’s ionosphere, and detection by ground-based radio observations may be difficult.
From the above, as observationally suggested, Hot Jupiter is likely to maintain a stronger magnetic field for a longer period of time than Earth or Jupiter in the solar system. Short-period gas giants outside the solar system, which are theoretically expected to possess strong intrinsic magnetic fields, are also exposed to intense high-energy particles (mainly electrons from stellar winds and coronal mass ejections) from the central star, which may cause much more intense auroral phenomena than have been observed on Earth, Jupiter, and Saturn. Auroral phenomena may be much more intense than those observed on Earth, Jupiter, and Saturn.
Publication Information:
*1 Jupiter’s magnetic field is about 7.766 G (Gauss), and the Earth’s maximum surface magnetic field (in the south pole region) is about 0.66 G.
*2 In the interior of gas giants such as Jupiter and Saturn, pressure-ionized hydrogen exhibits metallic properties under ultrahigh pressure and high temperature environments of 2000 K and pressure of 100 GPa or higher.
*3 Normally, H2O in the three states of vapor, ice, and liquid water changes its state to various phases such as high-pressure ice, supercritical water, superionic water, ionic water, and plasma state depending on pressure and temperature.
*4 LOFAR (LOw Frequency ARray) is a radio telescope operated by the Netherlands Institute for Radio Astronomy
*5 A short-period gas planet with an orbital period of about 3 days and about 6 times the mass of Jupiter was discovered in one of the binary star system Tau in the constellation Ursa Major (Tau A).
*6 The two main mechanisms of radio emission from planets are synchrotron radiation and cyclotron radiation. The former is non-thermal radio emission produced by the accelerated motion of electrons in the planetary magnetic field. The latter, also called auroral radio emission, is radio emission from electrons moving along a planetary magnetic field (= electron cyclotron instability) and is concentrated in the direction of electron motion. Future radio telescopes are expected to detect the latter type of auroral radio emissions.
Paper Information:
Journal：The Astrophysical Journal
Paper Title：The Linkage between the Core Mass and the Magnetic Field of an Extrasolar Giant Planet from Future Radio Observations
DOI：https://doi.org/10.3847/1538-4357/abd8d1
Author: Yasunori Hori (Astrobiology Center/National Astronomical Observatory)
