The Threatening Magnetic and Plasma Environment of the TRAPPIST-1 Planets

Garraffo, Cecilia; Drake, J. J.; Cohen, Ofer; Alvarado‐Gómez, Julián D.; Moschou, S. P.

doi:10.3847/2041-8213/aa79ed

Cited by 133 publications

(124 citation statements)

References 36 publications

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“…As a background medium for studying the propagation of EPs within the TRAPPIST-1 system, we adopt Figure 1. Three dimensional stellar wind solution for GJ 3622 used here and in Garraffo et al (2017) as a proxy for TRAPPIST-1. The Z-axis is aligned with the stellar rotation axis.…”

Section: Trappist-1 Magnetospheric Modelmentioning

confidence: 99%

Stellar Energetic Particles in the Magnetically Turbulent Habitable Zones of TRAPPIST-1-like Planetary Systems

Fraschetti

Drake

Alvarado‐Gómez

et al. 2019

ApJ

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Planets in close proximity to their parent star, such as those in the habitable zones around M dwarfs, could be subject to particularly high doses of particle radiation. We have carried out testparticle simulations of ∼GeV protons to investigate the propagation of energetic particles accelerated by flares or travelling shock waves within the stellar wind and magnetic field of a TRAPPIST-1-like system. Turbulence was simulated with small-scale magnetostatic perturbations with an isotropic power spectrum. We find that only a few percent of particles injected within half a stellar radius from the stellar surface escape, and that the escaping fraction increases strongly with increasing injection radius. Escaping particles are increasingly deflected and focused by the ambient spiralling magnetic field as the superimposed turbulence amplitude is increased. In our TRAPPIST-1-like simulations, regardless of the angular region of injection, particles are strongly focused onto two caps within the fast wind regions and centered on the equatorial planetary orbital plane. Based on a scaling relation between far-UV emission and energetic protons for solar flares applied to M dwarfs, the innermost putative habitable planet, TRAPPIST-1e, is bombarded by a proton flux up to 6 orders of magnitude larger than experienced by the present-day Earth. We note two mechanisms that could strongly limit EP fluxes from active stars: EPs from flares are contained by the stellar magnetic field; and potential CMEs that might generate EPs at larger distances also fail to escape.

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Section: Trappist-1 Magnetospheric Modelmentioning

confidence: 99%

Stellar Energetic Particles in the Magnetically Turbulent Habitable Zones of TRAPPIST-1-like Planetary Systems

Fraschetti

Drake

Alvarado‐Gómez

et al. 2019

ApJ

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“…Zahnle & Catling 2017;Catling & Kasting 2017). These data are particularly valuable to inform the loss rate of terrestrial atmospheres for planets orbiting M-dwarfs that are known to be strongly magnetic (Garraffo et al 2017). MAVEN recently detected an oxygen escape rate of around 10 3 g/s at Mars (Jakosky et al 2018), which when sputtered for 4.5 Gyr corresponds to a loss of approximately 8 bar of CO 2 (Luhmann et al 1992).…”

Section: Atmospheric Escapementioning

confidence: 99%

Linking the evolution of terrestrial interiors and an early outgassed atmosphere to astrophysical observations

Bower¹,

Kitzmann²,

Wolf³

et al. 2019

Preprint

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Context.A terrestrial planet is molten during formation and may remain molten due to intense insolation or tidal forces. Observations favour the detection and characterisation of hot planets, potentially with large outgassed atmospheres. Aims. We aim to determine the radius of hot Earth-like planets with large outgassing atmospheres. Our goal is to explore the differences between molten and solid silicate planets on the mass-radius relationship and transmission and emission spectra.Methods. An interior-atmosphere model was combined with static structure calculations to track the evolving radius of a hot rocky planet that outgasses CO 2 and H 2 O. We generated synthetic emission and transmission spectra for CO 2 and H 2 O dominated atmospheres. Results. Atmospheres dominated by CO 2 suppress the outgassing of H 2 O to a greater extent than previously realised since previous studies applied an erroneous relationship between volatile mass and partial pressure. We therefore predict that more H 2 O can be retained by the interior during the later stages of magma ocean crystallisation. Formation of a surface lid can tie the outgassing of H 2 O to the efficiency of heat transport through the lid, rather than the radiative timescale of the atmosphere. Contraction of the mantle, as it cools from molten to solid, reduces the radius by around 5%, which can partly be offset by the addition of a relatively light species (e.g. H 2 O versus CO 2 ) to the atmosphere. Conclusions. A molten silicate mantle can increase the radius of a terrestrial planet by around 5% compared to its solid counterpart, or equivalently account for a 13% decrease in bulk density. An outgassing atmosphere can perturb the total radius, according to its composition, notably the abundance of light versus heavy volatile species. Atmospheres of terrestrial planets around M-stars that are dominated by CO 2 or H 2 O can be distinguished by observing facilities with extended wavelength coverage (e.g. JWST).

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“…To evaluate the effect of using this formalism to estimate the Alfvén radius, we compared its predictions for the Sun, Proxima Cen., and TRAPPIST-1 (30, 25, and 63 R * , respectively) with 24 R for the Sun and the results of Garraffo et al (2016Garraffo et al ( , 2017 using 3D models for Proxima Cen. (33R * with a B o =600 G), and for TRAPPIST-1 (53 R * for B o =300G and 75 R * for B o =600G).…”

Section: Alfvén Radiusmentioning

confidence: 99%

Erosion of an exoplanetary atmosphere caused by stellar winds

Rodríguez-Mozos

Moya

2019

A&A

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Aims. We present a formalism for a first-order estimation of the magnetosphere radius of exoplanets orbiting stars in the range from 0.08 to 1.3 M . With this radius, we estimate the atmospheric surface that is not protected from stellar winds. We have analyzed this unprotected surface for the most extreme environment for exoplanets: GKM-type and very low-mass stars at the two limits of the habitable zone. The estimated unprotected surface makes it possible to define a likelihood for an exoplanet to retain its atmosphere. This function can be incorporated into the new habitability index SEPHI. Methods. Using different formulations in the literature in addition to stellar and exoplanet physical characteristics, we estimated the stellar magnetic induction, the main characteristics of the stellar wind, and the different star-planet interaction regions (sub-and super-Alfvénic, sub-and supersonic). With this information, we can estimate the radius of the exoplanet magnetopause and thus the exoplanet unprotected surface. Results. We have conducted a study of the auroral aperture angles for Earth-like exoplanets orbiting the habitable zone of its star, and found different behaviors depending on whether the star is in rotational saturated or unsaturated regimes, with angles of aperture of the auroral ring above or below 36 • , respectively, and with different slopes for the linear relation between the auroral aperture angle at the inner edge of the habitable zone versus the difference between auroral aperture angles at the two boundaries of the habitable zone. When the planet is tidally locked, the unprotected angle increases dramatically to values higher than 40 • with a low likelihood of keeping its atmosphere. When the impact of stellar wind is produced in the sub-Alfvénic regime, the likelihood of keeping the atmosphere is almost zero for exoplanets orbiting very close to their star, regardless of whether they are saturated or not.

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The Threatening Magnetic and Plasma Environment of the TRAPPIST-1 Planets

Cited by 133 publications

References 36 publications

Stellar Energetic Particles in the Magnetically Turbulent Habitable Zones of TRAPPIST-1-like Planetary Systems

Stellar Energetic Particles in the Magnetically Turbulent Habitable Zones of TRAPPIST-1-like Planetary Systems

Linking the evolution of terrestrial interiors and an early outgassed atmosphere to astrophysical observations

Erosion of an exoplanetary atmosphere caused by stellar winds

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