Detecting Magnetic Fields in Exoplanets with Spectropolarimetry of the Helium Line at 1083 nm

Abstract

The magnetic fields of the Solar System planets provide valuable insights into the planets' interiors and can have dramatic consequences for the evolution of their atmospheres and interaction with the solar wind. However, we have little direct knowledge of magnetic fields in exoplanets. Here we present a new method for detecting magnetic fields in the atmospheres of close-in exoplanets, based on spectropolarimetric transit observations at the wavelength of the helium line at 1083 nm. Strong absorption signatures (transit depths on the order of a few percent) in the 1083 nm line have recently been observed for several close-in exoplanets. We show that in the conditions in these escaping atmospheres, metastable helium atoms should be optically pumped by the starlight, and for field strengths >few×10 −4 G, should align with the magnetic field. This results in linearly polarized absorption at 1083 nm that traces the field direction (the Hanle effect), which we explore by both analytic computation and with the Hazel numerical code. The linear polarization Q 2 + U 2 /I ranges from ∼ 10 −3 in optimistic cases down to a few×10 −5 for particularly unfavorable cases, with very weak dependence on field strength. The line of sight component of the field results in a slight circular polarization (the Zeeman effect), also reaching V /I ∼ few × 10 −5 (B /10 G). We discuss the detectability of these signals with current (SPIRou) and future (extremely large telescope) high-resolution infrared spectropolarimeters, and briefly comment on possible sources of astrophysical contamination.

The GAPS Programme at TNG

“…The interesting possibility of the PTS being a magnetic excitation of pulsations given by the interaction with the planetary magnetic field could be further explored with spectro-polarimetric measurements, since magnetic fields affect not only the spectral line profiles but also polarisation properties of stellar radiation (e.g. Landi Degl 'Innocenti & Landolfi 2004;Oklopčić et al 2020).…”

Symmetries of Magnetic Fields Driven by Spherical Dynamos of Exoplanets and Their Host Stars

“…This phenomenon can not be directly related to planet-host star magnetic interactions, but magnetic fields can influence and accelerate the outflows of ionized gas [55]. The exact method was recently proposed [56] for direct detecting of the presence of magnetic fields in the atmospheres of transiting exoplanets. This method is similar to one that has been initially introduced in solar physics by [57] based on detecting radiation polarization in the helium line triplet at 1083 nm during transits of close-in exoplanets with extended or escaping atmospheres.…”

Hot Jupiters: Origins, Structure, Atmospheres

“…This helium is presumably a part of the bulk flow from the planets (Oklopčić & Hirata, 2018). The combination of more detailed modeling studies (Oklopčić et al., 2020) and high‐spectral resolution observations from ground‐based telescopes that resolve velocity information for the He triplet (Allart et al., 2019) make this an important area for growth.…”