THE SNOW LINE IN VISCOUS DISKS AROUND LOW-MASS STARS: IMPLICATIONS FOR WATER DELIVERY TO TERRESTRIAL PLANETS IN THE HABITABLE ZONE

AbstractTerrestrial planets form in a series of dynamical steps from the solid component of circumstellar disks. First, km-sized planetesimals form likely via a combination of sticky collisions, turbulent concentration of solids, and gravitational collapse from micron-sized dust grains in the thin disk midplane. Second, planetesimals coalesce to form Moon- to Mars-sized protoplanets, also called “planetary embryos”. Finally, full-sized terrestrial planets accrete from protoplanets and planetesimals. This final stage of accretion lasts about 10-100 Myr and is strongly affected by gravitational perturbations from any gas giant planets, which are constrained to form more quickly, during the 1-10 Myr lifetime of the gaseous component of the disk. It is during this final stage that the bulk compositions and volatile (e.g., water) contents of terrestrial planets are set, depending on their feeding zones and the amount of radial mixing that occurs. The main factors that influence terrestrial planet formation are the mass and surface density profile of the disk, and the perturbations from giant planets and binary companions if they exist. Simple accretion models predicts that low-mass stars should form small, dry planets in their habitable zones. The migration of a giant planet through a disk of rocky bodies does not completely impede terrestrial planet growth. Rather, “hot Jupiter” systems are likely to also contain exterior, very water-rich Earth-like planets, and also “hot Earths”, very close-in rocky planets. Roughly one third of the known systems of extra-solar (giant) planets could allow a terrestrial planet to form in the habitable zone.

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Erosion of an exoplanetary atmosphere caused by stellar winds

Astronomy and Astrophysics ◽

10.1051/0004-6361/201935543 ◽

2019 ◽

Vol 630 ◽

pp. A52 ◽

Cited By ~ 3

Author(s):

J. M. Rodríguez-Mozos ◽

A. Moya

Keyword(s):

Stellar Wind ◽

Extreme Environment ◽

Stellar Winds ◽

Habitable Zone ◽

Low Mass Stars ◽

Zone Versus ◽

Low Mass ◽

The Difference ◽

Interaction Regions ◽

The Impact

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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Triaxial deformation and asynchronous rotation of rocky planets in the habitable zone of low-mass stars

Monthly Notices of the Royal Astronomical Society ◽

10.1093/mnras/stx1076 ◽

2017 ◽

Vol 469 (3) ◽

pp. 2879-2885 ◽

Cited By ~ 4

Author(s):

J. J. Zanazzi ◽

Dong Lai

Keyword(s):

Habitable Zone ◽

Low Mass Stars ◽

Triaxial Deformation ◽

Low Mass ◽

Rocky Planets

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Detecting Planets Around Low Mass Stars: The Gateway to Terrestrial Planets

10.1063/1.3099104 ◽

2009 ◽

Author(s):

A. Tanner ◽

E. W. Geunther ◽

E. Martin ◽

M. R. Zapatero Osorio ◽

Eric Stempels

Keyword(s):

Terrestrial Planets ◽

Low Mass Stars ◽

Low Mass

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Physical properties of terrestrial planets and water delivery in the habitable zone using N-body simulations with fragmentation

Astronomy and Astrophysics ◽

10.1051/0004-6361/201936061 ◽

2019 ◽

Vol 632 ◽

pp. A14 ◽

Cited By ~ 3

Author(s):

A. Dugaro ◽

G. C. de Elía ◽

L. A. Darriba

Keyword(s):

Terrestrial Planets ◽

Habitable Zone ◽

Water Delivery ◽

Class A ◽

Class B ◽

Solar Type ◽

Formation And Evolution ◽

Final Fraction ◽

The Masses ◽

Water Contents

Aims. The goal of this research is to study how the fragmentation of planetary embryos can affect the physical and dynamical properties of terrestrial planets around solar-type stars. Our study focuses on the formation and evolution of planets and water delivery in the habitable zone (HZ). We distinguish class A and class B HZ planets, which have an accretion seed initially located inside and beyond the snow line, respectively. Methods. We developed an N-body integrator that incorporates fragmentation and hit-and-run collisions, which is called D3 N-body code. From this, we performed 46 numerical simulations of planetary accretion in systems that host two gaseous giants similar to Jupiter and Saturn. We compared two sets of 23 N-body simulations, one of which includes a realistic collisional treatment and the other one models all impacts as perfect mergers. Results. The final masses of the HZ planets formed in runs with fragmentation are about 15–20% lower than those obtained without fragmentation. As for the class A HZ planets, those formed in simulations without fragmentation experience very significant increases in mass with respect to their initial values, while the growth of those produced in runs with fragmentation is less relevant. We remark that the fragments play a secondary role in the masses of the class A HZ planets, providing less than 30% of their final values. In runs without fragmentation, the final fraction of water of the class A HZ planets keeps the initial value since they do not accrete water-rich embryos. In runs with fragmentation, the final fraction of water of such planets strongly depends on the model used to distribute the water after each collision. The class B HZ planets do not show significant differences concerning their final water contents in runs with and without fragmentation. From this, we find that the collisional fragmentation is not a barrier to the survival of water worlds in the HZ.

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Water delivery to dry protoplanets by hit-and-run collisions

Proceedings of the International Astronomical Union ◽

10.1017/s1743921318008621 ◽

2018 ◽

Vol 14 (S345) ◽

pp. 287-288

Author(s):

Christoph Burger ◽

Thomas I. Maindl ◽

Christoph Schäfer

Keyword(s):

Terrestrial Planets ◽

Habitable Zone ◽

Water Delivery ◽

Smooth Particle Hydrodynamics ◽

Particle Hydrodynamics ◽

Hit And Run

AbstractFinal water inventories of newly formed terrestrial planets are shaped by their collision history. A setting where volatiles are transported from beyond the snowline to habitable-zone planets suggests collisions of very dry with water-rich bodies. By means of smooth particle hydrodynamics (SPH) simulations we study water delivery in scenarios where a dry target is hit by a water-rich projectile, focusing on hit-and-run encounters with two large surviving bodies, which probably comprise about half of all similar-sized collisions (Genda et al. 2017).

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The high-energy radiation environment of the habitable-zone super-Earth LHS 1140b

Astronomy and Astrophysics ◽

10.1051/0004-6361/201935636 ◽

2019 ◽

Vol 627 ◽

pp. A144 ◽

Cited By ~ 4

Author(s):

R. Spinelli ◽

F. Borsa ◽

G. Ghirlanda ◽

G. Ghisellini ◽

S. Campana ◽

...

Keyword(s):

Spectral Energy Distribution ◽

High Energy ◽

Radiation Environment ◽

Habitable Zone ◽

Low Mass Stars ◽

High Energy Radiation ◽

Energy Radiation ◽

X Ray ◽

Near Ultraviolet ◽

Low Mass

Context. In the last few years many exoplanets in the habitable zone (HZ) of M-dwarfs have been discovered, but the X-ray/UV activity of cool stars is very different from that of our Sun. The high-energy radiation environment influences the habitability, plays a crucial role for abiogenesis, and impacts the chemistry and evolution of planetary atmospheres. LHS 1140b is one of the most interesting exoplanets discovered. It is a super-Earth-size planet orbiting in the HZ of LHS 1140, an M4.5 dwarf at ~15 parsecs. Aims. In this work, we present the results of the analysis of a Swift X-ray/UV observing campaign. We characterize for the first time the X-ray/UV radiation environment of LHS 1140b. Methods. We measure the variability of the near ultraviolet (NUV) flux and estimate the far ultraviolet (FUV) flux with a correlation between FUV1344−1786Å and NUV1771−2831Å flux obtained using the sample of low-mass stars in the GALEX archive. We highlight the presence of a dominating X-ray source close to the J2000 coordinates of LHS 1140, characterize its spectrum, and derive an X-ray flux upper limit for LHS 1140. We find that this contaminant source could have influenced the previously estimated spectral energy distribution. Results. No significant variation of the NUV1771−2831Å flux of LHS 1140 is found over 3 months, and we do not observe any flare during the 38 ks on the target. LHS 1140 is in the 25th percentile of least variable M4-M5 dwarfs of the GALEX sample. Analyzing the UV flux experienced by the HZ planet LHS 1140b, we find that outside the atmosphere it receives a NUV1771−2831Å flux <2% with respect to that of the present-day Earth, while the FUV1344−1786Å/NUV1771−2831Å ratio is ~100–200 times higher. This represents a lower limit to the true FUV/NUV ratio since the FUV1344−1786Å band does not include Lyman-alpha, which dominates the FUV output of low-mass stars. This is a warning for future searches for biomarkers, which must take into account this high ratio. Conclusions. The relatively low level and stability of UV flux experienced by LHS 1140b should be favorable for its present-day habitability.

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