131 research outputs found
Thermal evolution and structure models of the transiting super-Earth GJ 1214b
The planet GJ 1214b is the second known super-Earth with a measured mass and
radius. Orbiting a quiet M-star, it receives considerably less mass-loss
driving X-ray and UV radiation than CoRoT-7b, so that the interior may be quite
dissimilar in composition, including the possibility of a large fraction of
water. We model the interior of GJ 1214b assuming a two-layer (envelope+rock
core) structure where the envelope material is either H/He, pure water, or a
mixture of H/He and H2O. Within this framework we perform models of the thermal
evolution and contraction of the planet. We discuss possible compositions that
are consistent with Mp=6.55 ME, Rp=2.678 RE, an age tau=3-10 Gyr, and the
irradiation level of the atmosphere. These conditions require that if water
exists in the interior, it must remain in a fluid state, with important
consequences for magnetic field generation. These conditions also require the
atmosphere to have a deep isothermal region extending down to 80-800 bar,
depending on composition. Our results bolster the suggestion of a
metal-enriched H/He atmosphere for the planet, as we find water-world models
that lack an H/He atmosphere to require an implausibly large water-to-rock
ratio of more than 6:1. We instead favor a H/He/H2O envelope with high water
mass fraction (~0.5-0.85), similar to recent models of the deep envelope of
Uranus and Neptune. Even with these high water mass fractions in the H/He
envelope, generally the bulk composition of the planet can have subsolar
water:rock ratios. Dry, water-enriched, and pure water envelope models differ
to an observationally significant level in their tidal Love numbers k2 of
respectively ~0.018, 0.15, and 0.7.Comment: 11 pages, 6 figures, 1 table, accepted to Ap
Jupiter models with improved ab initio hydrogen EOS (H-REOS.2)
The amount and distribution of heavy elements in Jupiter gives indications on
the process of its formation and evolution. Core mass and metallicity
predictions however depend on the equations of state used, and on model
assumptions. We present an improved ab initio hydrogen equation of state,
H-REOS.2 and compute the internal structure and thermal evolution of Jupiter
within the standard three-layer approach. The advance over our previous Jupiter
models with H-REOS.1 by Nettelmann et al.(2008) is that the new models are also
consistent with the observed 2 or more times solar heavy element abundances in
Jupiter's atmosphere. Such models have a rock core mass Mcore=0-8 ME, total
mass of heavy elements MZ=28-32 ME, a deep internal layer boundary at 4 or more
Mbar, and a cooling time of 4.4-5.0 Gyrs when assuming homogeneous evolution.
We also calculate two-layer models in the manner of Militzer et al.(2008) and
find a comparable large core of 16-21 ME, out of which ~11 ME is helium, but a
significantly higher envelope metallicity of 4.5 times solar. According to our
preferred three-layer models, neither the characteristic frequency (nu0 ~156
microHz) nor the normalized moment of inertia (~0.276) are sensitive to the
core mass but accurate measurements could well help to rule out some classes of
models.Comment: 7 figures, 1 table, accepted to Ap
Uranus evolution models with simple thermal boundary layers
The strikingly low luminosity of Uranus (Teff ~ Teq) constitutes a
long-standing challenge to our understanding of Ice Giant planets. Here we
present the first Uranus structure and evolution models that are constructed to
agree with both the observed low luminosity and the gravity field data. Our
models make use of modern ab initio equations of state at high pressures for
the icy components water, methane, and ammonia. Proceeding step by step, we
confirm that adiabatic models yield cooling times that are too long, even when
uncertainties in the ice:rock ratio (I:R) are taken into account. We then argue
that the transition between the ice/rock-rich interior and the H/He-rich outer
envelope should be stably stratified. Therefore, we introduce a simple thermal
boundary and adjust it to reproduce the low luminosity. Due to this thermal
boundary, the deep interior of the Uranus models are up to 2--3 warmer than
adiabatic models, necessitating the presence of rocks in the deep interior with
a possible I:R of solar. Finally, we allow for an equilibrium
evolution (Teff ~ Teq) that begun prior to the present day, which would
therefore no longer require the current era to be a "special time" in Uranus'
evolution. In this scenario, the thermal boundary leads to more rapid cooling
of the outer envelope. When Teff ~ Teq is reached, a shallow, subadiabatic zone
in the atmosphere begins to develop. Its depth is adjusted to meet the
luminosity constraint. This work provides a simple foundation for future Ice
Giant structure and evolution models, that can be improved by properly treating
the heat and particle fluxes in the diffusive zones.Comment: 13 pages, Accepted to Icaru
Correlations in Hot Dense Helium
Hot dense helium is studied with first-principles computer simulations. By
combining path integral Monte Carlo and density functional molecular dynamics,
a large temperature and density interval ranging from 1000 to 1000000 K and 0.4
to 5.4 g/cc becomes accessible to first-principles simulations and the changes
in the structure of dense hot fluids can be investigated. The focus of this
article are pair correlation functions between nuclei, between electrons, and
between electrons and nuclei. The density and temperature dependence of these
correlation functions is analyzed in order to describe the structure of the
dense fluid helium at extreme conditions.Comment: accepted for publication in Journal of Physics
Atmospheric Helium Abundances in the Giant Planets
Noble gases are accreted to the giant planets as part of the gas component of
the planet-forming disk. While heavier noble gases can separate from the
evolution of the hydrogen-rich gas, helium is thought to remain at the
protosolar H/He ratio Yproto~0.27-0.28. However, spacecraft observations
revealed a depletion in helium in the atmospheres of Jupiter, Saturn, and
Uranus. For the gas giants, this is commonly seen as indication of H/He phase
separation at greater depths. Here, we apply predictions of the H/He phase
diagram and three H/He-EOS to compute the atmospheric helium mass abundance
Yatm as a result of H/He phase separation. We obtain a strong depletion
Yatm<0.1 for the ice giants if they are adiabatic. Introducing a thermal
boundary layer at the Z-poor/Z-rich compositional transition with a temperature
increase of up to a few 1000 K, we obtain a weak depletion in Uranus as
observed. Our results suggest dissimilar internal structures between Uranus and
Neptune. An accurate in-situ determination of their atmospheric He/H ratio
would help to constrain their internal structures. This is even more true for
Saturn, where we find that any considered H/He phase diagram and H/He-EOS would
be consistent with any observed value. However, some H/He-EOS and phase diagram
combinations applied to both Jupiter and Saturn require an outer
stably-stratified layer at least in one of them.Comment: accepted to Space Science Review
Estimating the number of planets that PLATO can detect
The PLATO mission is scheduled for launch in 2026. This study aims to
estimate the number of exoplanets that PLATO can detect as a function of
planetary size and period, stellar brightness, and observing strategy options.
Deviations from these estimates will be informative of the true occurrence
rates of planets, which helps constraining planet formation models. For this
purpose, we developed the Planet Yield for PLATO estimator (PYPE), which adopts
a statistical approach. We apply given occurrence rates from planet formation
models and from different search and vetting pipelines for the Kepler data. We
estimate the stellar sample to be observed by PLATO using a fraction of the
all-sky PLATO stellar input catalog (PIC). PLATO detection efficiencies are
calculated under different assumptions that are presented in detail in the
text. The results presented here primarily consider the current baseline
observing duration of four years. We find that the expected PLATO planet yield
increases rapidly over the first year and begins to saturate after two years. A
nominal (2+2) four-year mission could yield about several thousand to several
tens of thousands of planets, depending on the assumed planet occurrence rates.
We estimate a minimum of 500 Earth-size (0.8-1.25 RE) planets, about a dozen of
which would reside in a 250-500d period bin around G stars. We find that
one-third of the detected planets are around stars bright enough (V )
for RV-follow-up observations. We find that a three-year-long observation
followed by 6 two-month short observations (3+1 years) yield roughly twice as
many planets as two long observations of two years (2+2 years). The former
strategy is dominated by short-period planets, while the latter is more
beneficial for detecting earths in the habitable zone.Comment: 14 pages, 11 figures, accepted by A&A (July 5, 2023
New indication for a dichotomy in the interior structure of Uranus and Neptune from the application of modified shape and rotation data
Since the Voyager fly-bys of Uranus and Neptune, improved gravity field data
have been derived from long-term observations of the planets' satellite
motions, and modified shape and solid-body rotation periods were suggested. A
faster rotation period (-40 min) for Uranus and a slower rotation period
(+1h20) of Neptune compared to the Voyager data were found to minimize the
dynamical heights and wind speeds. We apply the improved gravity data, the
modified shape and rotation data, and the physical LM-R equation of state to
compute adiabatic three-layer structure models, where rocks are confined to the
core, and homogeneous thermal evolution models of Uranus and Neptune. We
present the full range of structure models for both the Voyager and the
modified shape and rotation data. In contrast to previous studies based solely
on the Voyager data or on empirical EOS, we find that Uranus and Neptune may
differ to an observationally significant level in their atmospheric heavy
element mass fraction Z1 and nondimensional moment of inertia, nI. For Uranus,
we find Z1 < 8% and nI=0.2224(1), while for Neptune Z1 < 65% and nI=0.2555(2)
when applying the modified shape and rotation data, while for the unmodified
data we compute Z1 < 17% and nI=0.230(1) for Uranus and Z1 < 54% and
nI=0.2410(8) for Neptune. In each of these cases, solar metallicity models
(Z1=0.015) are still possible. The cooling times obtained for each planet are
similar to recent calculations with the Voyager rotation periods: Neptune's
luminosity can be explained by assuming an adiabatic interior while Uranus
cools far too slowly. More accurate determinations of these planets' gravity
fields, shapes, rotation periods, atmospheric heavy element abundances, and
intrinsic luminosities are essential for improving our understanding of the
internal structure and evolution of icy planets.Comment: accepted to Planet. Space Sci., special editio
Probing the interiors of the ice giants: Shock compression of water to 700 GPa and 3.8 g/ccm
Recently there has been tremendous increase in the number of identified
extra-solar planetary systems. Our understanding of their formation is tied to
exoplanet internal structure models, which rely upon equations of state of
light elements and compounds like water. Here we present shock compression data
for water with unprecedented accuracy that shows water equations of state
commonly used in planetary modeling significantly overestimate the
compressibility at conditions relevant to planetary interiors. Furthermore, we
show its behavior at these conditions, including reflectivity and isentropic
response, is well described by a recent first-principles based equation of
state. These findings advocate this water model be used as the standard for
modeling Neptune, Uranus, and "hot Neptune" exoplanets, and should improve our
understanding of these types of planets.Comment: Accepted to Phys. Rev. Lett.; supplementary material attached
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