111 research outputs found
Effect of Core Cooling on the Radius of Sub-Neptune Planets
Sub-Neptune planets are very common in our galaxy and show a large diversity
in their mass-radius relation. In sub-Neptunes most of the planet mass is in
the rocky part (hereafter core) which is surrounded by a modest hydrogen-helium
envelope. As a result, the total initial heat content of such a planet is
dominated by that of the core. Nonetheless, most studies contend that the core
cooling will only have a minor effect on the radius evolution of the gaseous
envelope, because the core's cooling is in sync with the envelope, i.e., most
of the initial heat is released early on timescales of about 10-100 Myr. In
this Letter we examine the importance of the core cooling rate for the thermal
evolution of the envelope. Thus, we relax the early core cooling assumption and
present a model where the core is characterized by two parameters: the initial
temperature and the cooling time. We find that core cooling can significantly
enhance the radius of the planet when it operates on a timescale similar to the
observed age, i.e. several Gyr. Consequently, the interpretation of
sub-Neptunes' mass-radius observations depends on the assumed core thermal
properties and the uncertainty therein. The degeneracy of composition and core
thermal properties can be reduced by obtaining better estimates of the planet
ages (in addition to their radii and masses) as envisioned by future
observations.Comment: Accepted for publication in A&A Letter
The Evolution and Internal Structure of Jupiter and Saturn with Compositional Gradients
The internal structure of gas giant planets may be more complex than the
commonly assumed core-envelope structure with an adiabatic temperature profile.
Different primordial internal structures as well as various physical processes
can lead to non-homogenous compositional distributions. A non-homogenous
internal structure has a significant impact on the thermal evolution and final
structure of the planets. In this paper, we present alternative structure and
evolution models for Jupiter and Saturn allowing for non-adiabatic primordial
structures and the mixing of heavy elements by convection as these planets
evolve. We present the evolution of the planets accounting for various initial
composition gradients, and in the case of Saturn, include the formation of a
helium-rich region as a result of helium rain. We investigate the stability of
regions with composition gradients against convection, and find that the helium
shell in Saturn remains stable and does not mix with the rest of the envelope.
In other cases, convection mixes the planetary interior despite the existence
of compositional gradients, leading to the enrichment of the envelope with
heavy elements. We show that non-adiabatic structures (and cooling histories)
for both Jupiter and Saturn are feasible. The interior temperatures in that
case are much higher that for standard adiabatic models. We conclude that the
internal structure is directly linked to the formation and evolution history of
the planet. These alternative internal structures of Jupiter and Saturn should
be considered when interpreting the upcoming Juno and Cassini data.Comment: accepted for publication in Ap
Explaining the low luminosity of Uranus: A self-consistent thermal and structural evolution
The low luminosity of Uranus is a long-standing challenge in planetary
science. Simple adiabatic models are inconsistent with the measured luminosity,
which indicates that Uranus is non-adiabatic because it has thermal boundary
layers and/or conductive regions. A gradual composition distribution acts as a
thermal boundary to suppress convection and slow down the internal cooling.
Here we investigate whether composition gradients in the deep interior of
Uranus can explain its low luminosity, the required composition gradient, and
whether it is stable for convective mixing on a timescale of some billion
years. We varied the primordial composition distribution and the initial energy
budget of the planet, and chose the models that fit the currently measured
properties (radius, luminosity, and moment of inertia) of Uranus. We present
several alternative non-adiabatic internal structures that fit the Uranus
measurements. We found that convective mixing is limited to the interior of
Uranus, and a composition gradient is stable and sufficient to explain its
current luminosity. As a result, the interior of Uranus might still be very
hot, in spite of its low luminosity. The stable composition gradient also
indicates that the current internal structure of Uranus is similar to its
primordial structure. Moreover, we suggest that the initial energy content of
Uranus cannot be greater than 20% of its formation (accretion) energy. We also
find that an interior with a mixture of ice and rock, rather than separated ice
and rock shells, is consistent with measurements, suggesting that Uranus might
not be "differentiated". Our models can explain the luminosity of Uranus, and
they are also consistent with its metal-rich atmosphere and with the
predictions for the location where its magnetic field is generated.Comment: 10 pages, 7 figures, accepted for publication in A&
The Nuclear Power Plant Environment Monitoring System through Mobile Units
This article describes the information system for measuring and evaluating the dose rate in the environment of nuclear power plants Mochovce and Bohunice in Slovakia. The article presents the results achieved in the implementation of the EU project–Research of monitoring and evaluation of non-standard conditions in the area of nuclear power plants. The objectives included improving the system of acquisition, measuring and evaluating data with mobile and autonomous units applying new knowledge from research. The article provides basic and specific features of the system and compared to the previous version of the system, also new functions
Contribution of the core to the thermal evolution of sub-Neptunes
Sub-Neptune planets are a very common type of planets. They are inferred to
harbour a primordial (H/He) envelope, on top of a (rocky) core, which dominates
the mass. Here, we investigate the long-term consequences of the core
properties on the planet mass-radius relation. We consider the role of various
core energy sources resulting from core formation, its differentiation, its
solidification (latent heat), core contraction and radioactive decay. We divide
the evolution of the rocky core into three phases: the formation phase, which
sets the initial conditions, the magma ocean phase, characterized by rapid heat
transport, and the solid state phase, where cooling is inefficient. We find
that for typical sub-Neptune planets of ~2-10 Earth masses and envelope mass
fractions of 0.5-10% the magma ocean phase lasts several Gyrs, much longer than
for terrestrial planets. The magma ocean phase effectively erases any signs of
the initial core thermodynamic state. After solidification, the reduced heat
flux from the rocky core causes a significant drop in the rocky core surface
temperature, but its effect on the planet radius is limited. In the long run,
radioactive heating is the most significant core energy source in our model.
Overall, the long term radius uncertainty by core thermal effects is up to 15%.Comment: ApJ Publishe
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