Earth and Planetary Astrophysics (astro-ph.EP)

Abstract: The present study aims at characterizing the habitability conditions of exoplanets with an Earth-like magnetosphere inside the habitable zone of M stars and F stars like tau Boo, caused by the direct deposition of the stellar wind on the exoplanet surface if the magnetosphere shielding is inefficient. In addition, the radio emission generated by exoplanets with a Earth-like magnetosphere is calculated for different space weather conditions. The study is based on a set of MHD simulations performed by the code PLUTO reproducing the space weather conditions expected for exoplanets orbiting the habitable zone of M stars and F stars type tau Boo. Exoplanets hosted by M stars at 0.2 au are protected from the stellar wind during regular and CME-like space weather conditions if the star rotation period is slower than 3 days, that is to say, faster rotators generate stellar winds and interplanetary magnetic fields large enough to endanger the exoplanet habitability. Exoplanets hosted by a F stars type tau Boo at >= 2.5 au are protected during regular space weather conditions, but a stronger magnetic field compared to the Earth is mandatory if the exoplanet is close to the inner edge of the star habitable zone (2.5 au) to shield the exoplanet surface during CME-like space weather conditions. The range of radio emission values calculated in the simulations are consistent with the scaling proposed by [Zarka 2018] during regular and common CME-like space weather conditions. If the radio telescopes measure a relative low radio emission signal with small variability from an exoplanet, that may indicate favorable exoplanet habitability conditions with respect to the space weather states considered and the intrinsic magnetic field of the exoplanet.

Abstract: We introduce a quasi-periodic restricted Hamiltonian to describe the secular motion of a small-mass planet in a multi-planetary system. In particular, we refer to the motion of $\upsilon$-And $b$ which is the innermost planet among those discovered in the extrasolar system orbiting around the $\upsilon$-Andromedae A star. We preassign the orbits of the Super-Jupiter exoplanets $\upsilon$-And $c$ and $\upsilon$-And $d$ in a stable configuration. The Fourier decompositions of their secular motions are reconstructed by using the Frequency Analysis and are injected in the equations describing the orbital dynamics of $\upsilon$-And $b$ under the gravitational effects exerted by those two external exoplanets (expected to be major ones in such an extrasolar system). We end up with a $2+3/2$ degrees of freedom Hamiltonian model; its validity is confirmed by the comparison with several numerical integrations of the complete $4$-body problem. Furthermore, the model is enriched by taking into account also the relativistic effects on the secular motion of the innermost exoplanet. We focus on the problem of the stability of $\upsilon$-And $b$ as a function of the parameters that mostly impact on its orbit, i.e. the initial values of its inclination and the longitude of its node. We study the evolution of its eccentricity, crucial to exclude orbital configurations with high probability of (quasi)collision with the central star in the long-time evolution of the system. Moreover, we also introduce a normal form approach, that further reduces our Hamiltonian model to a system with $2$ degrees of freedom, which is integrable because it admits a constant of motion related to the total angular momentum. This allows us to quickly preselect the domains of stability for $\upsilon$-And $b$, with respect to the set of the initial orbital configurations that are compatible with the observations.

Abstract: In-situ plasma measurements as well as remote mapping of energetic neutral atoms around Jupiter provide indirect evidence that an enhancement of neutral gas is present near the orbit of the moon Europa. Simulations suggest that such a neutral gas torus can be sustained by escape from Europa's atmosphere and consists primarily of molecular hydrogen, but the neutral gas torus has not yet been measured directly through emissions or in-situ. Here we present observations by the Cosmic Origins Spectrograph of the Hubble Space Telescope (HST/COS) from 2020 and 2021, which scanned the equatorial plane between 8 and 10 planetary radii west of Jupiter. No neutral gas emissions are detected. We derive upper limits on the emissions and compare these to modelled emissions from electron impact and resonant scattering using a Europa torus Monte Carlo model for the neutral gases. The comparison supports the previous findings that the torus is dilute and primarily consists of molecular hydrogen. A detection of sulfur ion emissions radially inward of the Europa orbit is consistent with emissions from the extended Io torus and with sulfur ion fractional abundances as previously detected.