718 research outputs found
Luminosity of young Jupiters revisited. Massive cores make hot planets
The intrinsic luminosity of young Jupiters is of high interest for planet
formation theory. It is an observable quantity that is determined by important
physical mechanisms during formation, namely the accretion shock structure, and
even more fundamentally, the basic formation mechanism (core accretion or
gravitational instability). We study the impact of the core mass on the
post-formation entropy and luminosity of young giant planets forming via core
accretion with a supercritical shock (cold accretion). For this, we conduct
self-consistently coupled formation and evolution calculations of giant planets
with masses between 1 and 12 Jovian masses and core masses between 20 and 120
Earth masses. We find that the post-formation luminosity of massive giant
planets is very sensitive to the core mass. An increase of the core mass by a
factor 6 results in an increase of the post-formation luminosity of a 10 Jovian
mass planet by a factor 120. Due to this dependency, there is no single well
defined post-formation luminosity for core accretion, but a wide range. For
massive cores (~100 Earth masses), the post-formation luminosities of core
accretion planets become so high that they approach those in the hot start
scenario that is often associated with gravitational instability. For the
mechanism to work, it is necessary that the solids are accreted before or
during gas runaway accretion, and that they sink deep into the planet. We make
no claims whether or not such massive cores can actually form in giant planets.
But if yes, it becomes difficult to rule out core accretion as formation
mechanism based solely on luminosity for directly imaged planets that are more
luminous than predicted for low core masses. Instead of invoking gravitational
instability as the consequently necessary formation mode, the high luminosity
could also be caused simply by a more massive core.Comment: 11 pages, 6 figures. A&A accepte
Compositional imprints in density-distance-time: a rocky composition for close-in low-mass exoplanets from the location of the valley of evaporation
We use an end-to-end model of planet formation, thermodynamic evolution, and
atmospheric escape to investigate how the statistical imprints of evaporation
depend on the bulk composition of planetary cores (rocky vs. icy). We find that
the population-wide imprints like the location of the "evaporation valley" in
the distance-radius plane and the corresponding bimodal radius distribution
clearly differ depending on the bulk composition of the cores. Comparison with
the observed position of the valley (Fulton et al. 2017) suggests that close-in
low-mass Kepler planets have a predominately Earth-like rocky composition.
Combined with the excess of period ratios outside of MMR, this suggests that
low-mass Kepler planets formed inside of the water iceline, but still
undergoing orbital migration. The core radius becomes visible for planets
losing all primordial H/He. For planets in this "triangle of evaporation" in
the distance-radius plane, the degeneracy in compositions is reduced. In the
observed diagram, we identify a trend to more volatile-rich compositions with
increasing radius (R/R_Earth3: H/He).
The mass-density diagram contains important information about formation and
evolution. Its characteristic broken V-shape reveals the transitions from solid
planets to low-mass core-dominated planets with H/He and finally to
gas-dominated giants. Evaporation causes density and orbital distance to be
anti-correlated for low-mass planets, in contrast to giants, where closer-in
planets are less dense, likely due to inflation. The temporal evolution of the
statistical properties reported here will be of interest for the PLATO 2.0
mission which will observe the temporal dimension.Comment: 24 pages, 12 figures. Accepted in ApJ. Minor changes relative to v
Deuterium burning in objects forming via the core accretion scenario - Brown dwarfs or planets?
Aims. Our aim is to study deuterium burning in objects forming according to
the core accretion scenario in the hot and cold start assumption and what
minimum deuterium burning mass limit is found for these objects. We also study
how the burning process influences the structure and luminosity of the objects.
Furthermore we want to test and verify our results by comparing them to already
existing hot start simulations which did not consider, however, the formation
process.
Methods. We present a new method to calculate deuterium burning of objects in
a self-consistently coupled model of planet formation and evolution. We discuss
which theory is used to describe the process of deuterium burning and how it
was implemented.
Results. We find that the objects forming according to a hot start scenario
behave approximately in the same way as found in previous works of evolutionary
calculations, which did not consider the formation. However, for cold start
objects one finds that the objects expand during deuterium burning instead of
being partially stabilized against contraction. In both cases, hot and cold
start, the mass of the solid core has an influence on the minimum mass limit of
deuterium burning. The general position of the mass limit, 13 MJ, stays however
approximately the same. None of the investigated parameters was able to change
this mass limit by more than 0.8 MJ. Due to deuterium burning, the luminosity
of hot and cold start objects becomes comparable after ~ 200 Myrs.Comment: Accepted to A&A. Identical as v1 except for corrected typos. 22
pages, 15 figure
Evolution and Magnitudes of Candidate Planet Nine
The recently renewed interest in a possible additional major body in the
outer solar system prompted us to study the thermodynamic evolution of such an
object. We assumed that it is a smaller version of Uranus and Neptune. We
modeled the temporal evolution of the radius, temperature, intrinsic
luminosity, and the blackbody spectrum of distant ice giant planets. The aim is
also to provide estimates of the magnitudes in different bands to assess
whether the object might be detectable. Simulations of the cooling and
contraction were conducted for ice giants with masses of 5, 10, 20, and 50
Mearth that are located at 280, 700, and 1120 AU from the Sun. The core
composition, the fraction of H/He, the efficiency of energy transport, and the
initial luminosity were varied. The atmospheric opacity was set to 1, 50, and
100 times solar metallicity. We find for a nominal 10 Mearth planet at 700 AU
at the current age of the solar system an effective temperature of 47 K, much
higher than the equilibrium temperature of about 10 K, a radius of 3.7 Rearth,
and an intrinsic luminosity of 0.006 Ljupiter. It has estimated apparent
magnitudes of Johnson V, R, I, L, N, Q of 21.7, 21.4, 21.0, 20.1, 19.9, and
10.7, and WISE W1-W4 magnitudes of 20.1, 20.1, 18.6, and 10.2. The Q and W4
band and other observations longward of about 13 microns pick up the intrinsic
flux. If candidate Planet 9 has a significant H/He layer and an efficient
energy transport in the interior, then its luminosity is dominated by the
intrinsic contribution, making it a self-luminous planet. At a likely position
on its orbit near aphelion, we estimate for a mass of 5, 10, 20, and 50 Mearth
a V magnitude from the reflected light of 24.3, 23.7, 23.3, and 22.6 and a Q
magnitude from the intrinsic radiation of 14.6, 11.7, 9.2, and 5.8. The latter
would probably have been detected by past surveys.Comment: 6 pages, 3 figures, accepted to A&
Planetary evolution with atmospheric photoevaporation I. Analytical derivation and numerical study of the evaporation valley and transition from super-Earths to sub-Neptunes
Observations have revealed in the Kepler data a depleted region separating
smaller super-Earths from larger sub-Neptunes. This can be explained as an
evaporation valley between planets with and without H/He that is caused by
atmospheric escape. First, we conduct numerical simulations of the evolution of
close-in low-mass planets with H/He undergoing escape. Second, we develop an
analytical model for the valley locus. We find that the bottom of the valley
quantified by the radius of the largest stripped core at a given
orbital distance depends only weakly on post-formation H/He mass. The reason is
that a high initial H/He mass means that there is more gas to evaporate, but
also that the planet density is lower, increasing loss. Regarding stellar
, scales as . The same weak
dependency applies to the efficiency factor of energy-limited
evaporation. As found numerically and analytically, varies as
function of orbital period for a constant as where is the mass-radius
relation of solid cores. is about 1.7 at a 10-day
orbit for an Earth-like composition, increasing linearly with ice mass
fraction. The numerical results are explained very well with the analytical
model where complete evaporation occurs if the temporal integral over the
stellar XUV irradiation absorbed by the planet is larger than binding energy of
the envelope in the gravitational potential of the core. The weak dependency on
primordial H/He mass, and explains why
observationally the valley is visible, and why theoretically models find
similar results. At the same time, given the large observed spread of , the dependency on it is still strong enough to explain why the valley is
not completely empty.Comment: 32 pages, 16 figures. Accepted to A&
Two Empirical Regimes of the Planetary Mass-Radius Relation
Today, with the large number of detected exoplanets and improved
measurements, we can reach the next step of planetary characterization.
Classifying different populations of planets is not only important for our
understanding of the demographics of various planetary types in the galaxy, but
also for our understanding of planet formation. We explore the nature of two
regimes in the planetary mass-radius (M-R) relation. We suggest that the
transition between the two regimes of "small" and "large" planets, occurs at a
mass of 124 \pm 7, M_Earth and a radius of 12.1 \pm 0.5, R_Earth. Furthermore,
the M-R relation is R \propto M^{0.55\pm 0.02} and R \propto M^{0.01\pm0.02}
for small and large planets, respectively. We suggest that the location of the
breakpoint is linked to the onset of electron degeneracy in hydrogen, and
therefore, to the planetary bulk composition. Specifically, it is the
characteristic minimal mass of a planet which consists of mostly hydrogen and
helium, and therefore its M-R relation is determined by the equation of state
of these materials. We compare the M-R relation from observational data with
the one derived by population synthesis calculations and show that there is a
good qualitative agreement between the two samples.Comment: accepted for publication in A&
Giant Planet Formation by Core Accretion
We present a review of the standard paradigm for giant planet formation, the
core accretion theory. After an overview of the basic concepts of this model,
results of the original implementation are discussed. Then, recent improvements
and extensions, like the inclusion of planetary migration and the resulting
effects are discussed. It is shown that these improvement solve the timescale
problem. Finally, it is shown that by means of generating synthetic populations
of (extrasolar) planets, core accretion models are able to reproduce in a
statistically significant way the actually observed planetary population.Comment: 8 pages, 3 figures, invited review, to appear in "Extreme Solar
Systems" ASP Conference Series, eds. Debra Fischer, Fred Rasio, Steve
Thorsett and Alex Wolszcza
Formation, Orbital and Internal Evolutions of Young Planetary Systems
The growing body of observational data on extrasolar planets and
protoplanetary disks has stimulated intense research on planet formation and
evolution in the past few years. The extremely diverse, sometimes unexpected
physical and orbital characteristics of exoplanets lead to frequent updates on
the mainstream scenarios for planet formation and evolution, but also to the
exploration of alternative avenues. The aim of this review is to bring together
classical pictures and new ideas on the formation, orbital and internal
evolutions of planets, highlighting the key role of the protoplanetary disk in
the various parts of the theory. We begin by briefly reviewing the conventional
mechanism of core accretion by the growth of planetesimals, and discuss a
relatively recent model of core growth through the accretion of pebbles. We
review the basic physics of planet-disk interactions, recent progress in this
area, and discuss their role in observed planetary systems. We address the most
important effects of planets internal evolution, like cooling and contraction,
the mass-luminosity relation, and the bulk composition expressed in the
mass-radius and mass-mean density relations.Comment: 49 pages, 12 figures, accepted for publication in Space Science
Reviews. Chapter in International Space Science Institute (ISSI) Book on "The
Disk in Relation to the Formation of Planets and their Proto-atmospheres" to
be published in Space Science Reviews by Springe
Exploring the formation by core accretion and the luminosity evolution of directly imaged planets: The case of HIP 65426 b
A low-mass companion to the two-solar mass star HIP65426 has recently been
detected by SPHERE at around 100 au from its host. Explaining the presence of
super-Jovian planets at large separations, as revealed by direct imaging, is
currently an open question.
We want to derive statistical constraints on the mass and initial entropy of
HIP65426b and to explore possible formation pathways of directly imaged objects
within the core-accretion paradigm, focusing on HIP65426b.
Constraints on the planet's mass and post-formation entropy are derived from
its age and luminosity combined with cooling models. For the first time, the
results of population synthesis are also used to inform the results. Then, a
formation model that includes N-body dynamics with several embryos per disc is
used to study possible formation histories and the properties of possible
additional companions. Finally, the outcomes of two- and three-planet
scattering in the post-disc phase are analysed, taking tides into account.
The mass of HIP65426b is found to be Mp = 9.9 +1.1 -1.8 MJ using the hot
population and Mp = 10.9 +1.4 -2.0 MJ with the cold-nominal population. Core
formation at small separations from the star followed by outward scattering and
runaway accretion at a few hundred AU succeeds in reproducing the mass and
separation of HIP65426b. Alternatively, systems having two or more giant
planets close enough to be on an unstable orbit at disc dispersal are likely to
end up with one planet on a wide HIP65426b-like orbit with a relatively high
eccentricity (>~ 0.5).
If this scattering scenario explains its formation, HIP65426b is predicted to
have a high eccentricity and to be accompanied by one or several roughly
Jovian-mass planets at smaller semi-major axes, which also could have a high
eccentricity. This could be tested by further direct-imaging as well as
radial-velocity observations.Comment: 17 pages, 11 figures. A&A in press. Bern EXoplanet cooling curves
(BEX) available upon request. v2: Language and other minor changes; Fig. 4
now has labels summarising a possible formation pathway discussed in the tex
Model atmospheres of irradiated exoplanets: The influence of stellar parameters, metallicity, and the C/O ratio
Many parameters constraining the spectral appearance of exoplanets are still
poorly understood. We therefore study the properties of irradiated exoplanet
atmospheres over a wide parameter range including metallicity, C/O ratio and
host spectral type. We calculate a grid of 1-d radiative-convective atmospheres
and emission spectra. We perform the calculations with our new
Pressure-Temperature Iterator and Spectral Emission Calculator for Planetary
Atmospheres (PETIT) code, assuming chemical equilibrium. The atmospheric
structures and spectra are made available online. We find that atmospheres of
planets with C/O ratios 1 and 1500 K can exhibit
inversions due to heating by the alkalis because the main coolants CH,
HO and HCN are depleted. Therefore, temperature inversions possibly occur
without the presence of additional absorbers like TiO and VO. At low
temperatures we find that the pressure level of the photosphere strongly
influences whether the atmospheric opacity is dominated by either water (for
low C/O) or methane (for high C/O), or both (regardless of the C/O). For hot,
carbon-rich objects this pressure level governs whether the atmosphere is
dominated by methane or HCN. Further we find that host stars of late spectral
type lead to planetary atmospheres which have shallower, more isothermal
temperature profiles. In agreement with prior work we find that for planets
with 1750 K the transition between water or methane dominated
spectra occurs at C/O 0.7, instead of 1, because condensation
preferentially removes oxygen.Comment: 30 pages, 20 figures. Accepted for publication in Ap
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