K. G. Kislyakova

Origin and loss of nebula-captured hydrogen envelopes from ‘sub’- to ‘super-Earths’ in the habitable zone of Sun-like stars

We investigate the origin and loss of captured hydrogen envelopes from protoplanets having masses in a range between ‘sub-Earth’-like bodies of 0.1 M⊕ and ‘super-Earths’ with 5 M⊕ in the habitable zone at 1 au of a Sun-like G star, assuming that their rocky cores had formed before the nebula gas dissipated. We model the gravitational attraction and accumulation of nebula gas around a planet’s core as a function of protoplanetary luminosity during accretion and calculate the resulting surface temperature by solving the hydrostatic structure equations for the protoplanetary nebula. Depending on nebular properties, such as the dust grain depletion factor, planetesimal accretion rates, and resulting luminosities, for planetary bodies of 0.1– 1M ⊕ we obtain hydrogen envelopes with masses between ∼2.5 × 10 19 and 1.5 × 10 26 g. For ‘super-Earths’ with masses between 2 and 5 M ⊕ more massive hydrogen envelopes within the mass range of ∼7.5 × 10 23 –1.5 × 10 28 g can be captured from the nebula. For studying the escape of these accumulated hydrogen-dominated protoatmospheres, we apply a hydrodynamic upper atmosphere model and calculate the loss rates due to the heating by the high soft-X-ray and extreme ultraviolet (XUV) flux of the young Sun/star. The results of our study indicate that under most nebula conditions ‘sub-Earth’ and Earth-mass planets can lose their captured hydrogen envelopes by thermal escape during the first ∼100 Myr after the disc dissipated. However, if a nebula has a low dust depletion factor or low accretion rates resulting in low protoplanetary luminosities, it is possible that even protoplanets with Earth-mass cores may keep their hydrogen envelopes during their whole lifetime. In contrast to lower mass protoplanets, more massive ‘super-Earths’, which can accumulate a huge amount of nebula gas, lose only tiny fractions of their primordial hydrogen envelopes. Our results agree with the fact that Venus, Earth, and Mars are not surrounded by dense hydrogen envelopes, as well as with the recent discoveries of low density ‘super-Earths’ that most likely could not get rid of their dense protoatmospheres.

Magnetic moment and plasma environment of HD 209458b as determined from Lyα observations

Transit observations of HD 209458b in the stellar Lyman-α(Lyα) line revealed strong absorption in both blue and red wings of the line interpreted as hydrogen atoms escaping from the planet’s exosphere at high velocities. The following sources for the absorption were suggested: acceleration by the stellar radiation pressure, natural spectral line broadening, or charge exchange with the stellar wind. We reproduced the observation by means of modeling that includes all aforementioned processes. Our results support a stellar wind with a velocity of ≈400 kilometers per second at the time of the observation and a planetary magnetic moment of ≈1.6 × 1026 amperes per square meter. An exoplanet’s magnetic field is manifested as a particular absorption pattern in the transmitted spectrum of the host star. Transit marked by magnetosphere effects Life on Earth exists under the protective sheath of our magnetosphere that deflects charged particles blown out by the Sun. Kislyakova et al. calculated the strength of the magnetic field of a well-studied hot-Jupiter–type exoplanet that produces similar effects. During the planets transit in front of its host star, HD 209458, hydrogen atoms leave a peculiar asymmetric signature in the transmitted spectrum. Science, this issue p. 981

Variability of solar/stellar activity and magnetic field and its influence on planetary atmosphere evolution

It is shown that the evolution of planetary atmospheres can only be understood if one recognizes the fact that the radiation and particle environment of the Sun or a planet’s host star were not always on the same level as at present. New insights and the latest observations and research regarding the evolution of the solar radiation, plasma environment and solar/stellar magnetic field derived from the observations of solar proxies with different ages will be given. We show that the extreme radiation and plasma environments of the young Sun/stars have important implications for the evolution of planetary atmospheres and may be responsible for the fact that planets with low gravity like early Mars most likely never build up a dense atmosphere during the first few 100 Myr after their origin. Finally we present an innovative new idea on how hydrogen clouds and energetic neutral atom (ENA) observations around transiting Earth-like exoplanets by space observatories such as the WSO-UV, can be used for validating the addressed atmospheric evolution studies. Such observations would enhance our understanding on the impact on the activity of the young Sun on the early atmospheres of Venus, Earth, Mars and other Solar System bodies as well as exoplanets.

THE EVOLUTION OF STELLAR ROTATION AND THE HYDROGEN ATMOSPHERES OF HABITABLE-ZONE TERRESTRIAL PLANETS

Terrestrial planets formed within gaseous protoplanetary disks can accumulate significant hydrogen envelopes. The evolution of such an atmosphere due to XUV driven evaporation depends on the activity evolution of the host star, which itself depends sensitively on its rotational evolution, and therefore on its initial rotation rate. In this letter, we derive an easily applicable method for calculating planetary atmosphere evaporation that combines models for a hydrostatic lower atmosphere and a hydrodynamic upper atmosphere. We show that the initial rotation rate of the central star is of critical importance for the evolution of planetary atmospheres and can determine if a planet keeps or loses its primordial hydrogen envelope. Our results highlight the need for a detailed treatment of stellar activity evolution when studying the evolution of planetary atmospheres.

EUV-driven mass-loss of protoplanetary cores with hydrogen-dominated atmospheres: the influences of ionization and orbital distance

We investigate the loss rates of the hydrogen atmospheres of terrestrial planets with a range of masses and orbital distances by assuming a stellar extreme ultraviolet (EUV) luminosity that is 100 times stronger than that of the current Sun. We apply a 1D upper atmosphere radiation absorption and hydrodynamic escape model that takes into account ionization, dissociation and recombination to calculate hydrogen mass loss rates. We study the effects of the ionization, dissociation and recombination on the thermal mass loss rates of hydrogen-dominated super-Earths and compare the results to those obtained by the energy-limited escape formula which is widely used for mass loss evolution studies. Our results indicate that the energy-limited formula can to a great extent over- or underestimate the hydrogen mass loss rates by amounts that depend on the stellar EUV flux and planetary parameters such as mass, size, effective temperature, and EUV absorption radius.

Identifying the “true” radius of the hot sub-Neptune CoRoT-24b by mass loss modelling

For the hot exoplanets CoRoT-24b and CoRoT-24c, observations have provided transit radii R-T of 3.7 +/- 0.4R(circle plus) and 4.9 +/- 0.5R(circle plus), and masses of = 5.7M(circle plus) and 28 +/- 11M(circle plus), respectively. We study their upper atmosphere structure and escape applying an hydrodynamic model. Assuming R-T +/- R-PL, where R-PL is the planetary radius at the pressure of 100 mbar, we obtained for CoRoT-24b unrealistically high thermally driven hydrodynamic escape rates. This is due to the planets high temperature and low gravity, independent of the stellar EUV flux. Such high escape rates could last only for< 100 Myr, while R-PL shrinks till the escape rate becomes less than or equal to the maximum possible EUV-driven escape rate. For CoRoT-24b, R-PL must be therefore located at approximate to 1.9-2.2R(circle plus) and high altitude hazes/clouds possibly extinct the light at R-T. Our analysis constraints also the planets mass to be 5-5.7M(circle plus). For CoRoT-24c, R-PL and R-T lie too close together to be distinguished in the same way. Similar differences between R-PL and R-T may be present also for other hot, low-density sub-Neptunes.

Extreme hydrodynamic atmospheric loss near the critical thermal escape regime

By considering martian-like planetary embryos inside the habitable zone of solar-like stars we study the behavior of the hydrodynamic atmospheric escape of hydrogen for small values of the Jeans escape parameter

Origin and Stability of Exomoon Atmospheres: Implications for Habitability

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Possible manifestation of large-scale transverse oscillations of coronal loops in solar microwave emission

, near the base of the thermosphere, that is defined as a ratio of the gravitational and thermal energy. Our study is based on a 1-D hydrodynamic upper atmosphere model that calculates the volume heating rate in a hydrogen dominated thermosphere due to the absorption of the stellar soft X-ray and extreme ultraviolet (XUV) flux. We find that when the

Impact induced surface heating by planetesimals on early Mars

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