New Formation Models for the Kepler-36 System Bodenheimer, Peter; Stevenson, David J.; Lissauer, Jack J. ...
Astrophysical journal/The Astrophysical journal,
12/2018, Letnik:
868, Številka:
2
Journal Article
Recenzirano
Odprti dostop
Formation of the planets in the Kepler-36 system is modeled by detailed numerical simulations according to the core-nucleated accretion scenario. The standard model is updated to include the ...dissolution of accreting rocky planetesimals in the gaseous envelope of the planet, leading to substantial enrichment of the envelope mass in heavy elements and a non-uniform composition with depth. For Kepler-36 c, models involving in situ formation and models involving orbital migration are considered. The results are compared with standard formation models. The calculations include the formation (accretion) phase as well as the subsequent cooling phase, up to the age of Kepler-36 (7 Gyr). During the latter phase, mass loss induced by stellar XUV radiation is included. In all cases, the results fit the measured mass, 7.84 M⊕, and radius, 3.68 R⊕, of Kepler-36 c. Two parameters are varied to obtain these fits: the disk solid surface density at the formation location and the "efficiency" factor in the XUV mass-loss rate. The updated models are hotter and therefore less dense in the silicate portion of the planet and in the overlying layers of H/He, as compared with standard models. The lower densities mean that only about half as much H/He is needed to be accreted to fit the present-day mass and radius constraints. For Kepler-36 b, an updated in situ calculation shows that the entire H/He envelope is lost, early in the cooling phase, in agreement with observation.
We describe the catalogs assembled and the algorithms used to populate the revised TESS Input Catalog (TIC), based on the incorporation of the Gaia second data release. We also describe a revised ...ranking system for prioritizing stars for 2 minute cadence observations, and we assemble a revised Candidate Target List (CTL) using that ranking. The TIC is available on the Mikulski Archive for Space Telescopes server, and an enhanced CTL is available through the Filtergraph data visualization portal system at http://filtergraph.vanderbilt.edu/tess_ctl.
Abstract
We perform long-term simulations, up to ten billion years, of closely spaced configurations of 2–6 planets, each as massive as the Earth, traveling on nested orbits about either stellar ...component in
α
Centauri AB. The innermost planet initially orbits at either the inner edge of its star’s empirical habitable zone (HZ) or the inner edge of its star’s conservative HZ. Although individual planets on low inclination, low eccentricity, orbits can survive throughout the HZs of both stars, perturbations from the companion star require that the minimum spacing of planets in multi-planet systems within the HZs of each star must be significantly larger than the spacing of similar multi-planet systems orbiting single stars in order to be long-lived. The binary companion induces a forced eccentricity upon the orbits of planets in orbit around either star. Planets on appropriately phased circumstellar orbits with initial eccentricities equal to their forced eccentricities can survive on more closely spaced orbits than those with initially circular orbits, although the required spacing remains higher than for planets orbiting single stars. A total of up to nine planets on nested prograde orbits can survive for the current age of the system within the empirical HZs of the two stars, with five of these orbiting
α
Centauri B and four orbiting
α
Centauri A.
Changes in planetary obliquity, or axial tilt, influence the climates on Earth-like planets. In the solar system, the Earth's obliquity is stabilized by interactions with our moon, and the resulting ...small amplitude variations (∼2 4) are beneficial for advanced life. Most Sun-like stars have at least one stellar companion, and the habitability of circumstellar exoplanets is shaped by their stellar companion. We show that a stellar companion can dramatically change whether Earth-like obliquity stability is possible through planetary orbital precession relative to the binary orbit or resonant pumping of the obliquity through spin-orbit interactions. We present a new formalism for the planetary spin precession that accounts for orbital misalignments between the planet and binary. Using numerical modeling in Centauri AB, we show the following: there is a stark contrast between the planetary obliquity variations depending on the host star, planetary neighbors limit the possible spin states for Earth-like obliquity stability, and the presence of a moon can destabilize the obliquity, defying our Earth-based expectations. An Earth-like rotator orbiting the primary star would experience small obliquity variations for 87%, 74%, or 54% of solar-type binaries, depending on the mass of the primary (0.8, 1.0, or 1.2 M , respectively). Thus, Earth-like planets likely experience much larger obliquity variations, with more extreme climates, unless they are in specific states, such as orbiting nearly planar with the binary and rotating retrograde (backward) like Venus.
Abstract
Kepler-33 hosts five validated transiting planets ranging in period from 5 to 41 days. The planets are in nearly coplanar orbits and exhibit remarkably similar (appropriately scaled) transit ...durations indicative of similar impact parameters. The outer three planets have a radius of 3.5 ≲
R
p
/
R
⊕
≲ 4.7 and are closely packed dynamically, and thus transit timing variations can be observed. Photodynamical analysis of transit timing variations provide 2
σ
upper bounds on the eccentricity of the orbiting planets (ranging from <0.02 to <0.2) and the mean density of the host star (
0.39
−
0.02
+
0.01
g
cm
−
3
). We combine Gaia Early Data Release 3 parallax observations, the previously reported host-star effective temperature and metallicity, and our photodynamical model to refine properties of the host star and the transiting planets. Our analysis yields well-constrained masses for Kepler-33 e (
6.6
−
1.0
+
1.1
M
⊕
) and f (
8.2
−
1.2
+
1.6
M
⊕
) along with 2
σ
upper limits for planets c (<19
M
⊕
) and d (<8.2
M
⊕
). We confirm the reported low bulk densities of planet d (<0.4 g cm
−3
), e (0.8 ± 0.1 g cm
−3
), and f (0.7 ± 0.1 g cm
−3
). Based on comparisons with planetary evolution models, we find that Kepler-33 e and f exhibit relatively high envelope mass fractions of
f
env
=
7.0
−
0.5
+
0.6
%
and
f
env
= 10.3% ± 0.6%, respectively. Assuming a mass for planet d ∼4
M
⊕
suggests that it has
f
env
≳ 12%.
We model the growth of Jupiter via core nucleated accretion, applying constraints from hydrodynamical processes that result from the disk–planet interaction. We compute the planet's internal ...structure using a well tested planetary formation code that is based upon a Henyey-type stellar evolution code. The planet's interactions with the protoplanetary disk are calculated using 3-D hydrodynamic simulations. Previous models of Jupiter's growth have taken the radius of the planet to be approximately one Hill sphere radius,
R
H
. However, 3-D hydrodynamic simulations show that only gas within
∼
0.25
R
H
remains bound to the planet, with the more distant gas eventually participating in the shear flow of the protoplanetary disk. Therefore in our new simulations, the planet's outer boundary is placed at the location where gas has the thermal energy to reach the portion of the flow not bound to the planet. We find that the smaller radius increases the time required for planetary growth by ∼5%. Thermal pressure limits the rate at which a planet less than a few dozen times as massive as Earth can accumulate gas from the protoplanetary disk, whereas hydrodynamics regulates the growth rate for more massive planets. Within a moderately viscous disk, the accretion rate peaks when the planet's mass is about equal to the mass of Saturn. In a less viscous disk hydrodynamical limits to accretion are smaller, and the accretion rate peaks at lower mass. Observations suggest that the typical lifetime of massive disks around young stellar objects is
∼
3
Myr
. To account for the dissipation of such disks, we perform some of our simulations of Jupiter's growth within a disk whose surface gas density decreases on this timescale. In all of the cases that we simulate, the planet's effective radiating temperature rises to well above
1000
K
soon after hydrodynamic limits begin to control the rate of gas accretion and the planet's distended envelope begins to contract. According to our simulations, proto-Jupiter's distended and thermally-supported envelope was too small to capture the planet's current retinue of irregular satellites as advocated by Pollack et al. Pollack, J.B., Burns, J.A., Tauber, M.E., 1979. Icarus 37, 587–611.
FORMATION AND STRUCTURE OF LOW-DENSITY EXO-NEPTUNES ROGERS, Leslie A; BODENHEIMER, Peter; LISSAUER, Jack J ...
Astrophysical journal/The Astrophysical journal,
09/2011, Letnik:
738, Številka:
1
Journal Article
Recenzirano
Odprti dostop
Kepler has found hundreds of Neptune-size (2-6 R {circled plus}) planet candidates within 0.5 AU of their stars. The nature of the vast majority of these planets is not known because their masses ...have not been measured. Using theoretical models of planet formation, evolution, and structure, we explore the range of minimum plausible masses for low-density exo-Neptunes. We focus on highly irradiated planets with T eq >= 500 K. We consider two separate formation pathways for low-mass planets with voluminous atmospheres of light gases: core-nucleated accretion and outgassing of hydrogen from dissociated ices. We show that Neptune-size planets at T eq = 500 K with masses as small as a few times that of Earth can plausibly be formed by core-nucleated accretion coupled with subsequent inward migration. We also derive a limiting low-density mass-radius relation for rocky planets with outgassed hydrogen envelopes but no surface water. Rocky planets with outgassed hydrogen envelopes typically have computed radii well below 3 R {circled plus}. For both planets with H/He envelopes from core-nucleated accretion and planets with outgassed hydrogen envelopes, we employ planet interior models to map the range of planet mass-envelope mass-equilibrium temperature parameter space that is consistent with Neptune-size planet radii. Atmospheric mass loss mediates which corners of this parameter space are populated by actual planets and ultimately governs the minimum plausible mass at a specified transit radius. We find that Kepler's 2-6 R {circled plus} planet candidates at T eq = 500-1000 K could potentially have masses 4 M {circled plus}. Although our quantitative results depend on several assumptions, our qualitative finding that warm Neptune-size planets can have masses substantially smaller than those given by interpolating the masses and radii of planets within our Solar System is robust.
New numerical simulations of the formation and evolution of Jupiter are presented. The formation model assumes that first a solid core of several
M
⊕
accretes from the planetesimals in the ...protoplanetary disk, and then the core captures a massive gaseous envelope from the protoplanetary disk. Earlier studies of the core accretion–gas capture model Pollack, J.B., Hubickyj, O., Bodenheimer, P., Lissauer, J.J., Podolak, M., Greenzweig, Y., 1996. Icarus 124, 62–85 demonstrated that it was possible for Jupiter to accrete with a solid core of 10–30
M
⊕
in a total formation time comparable to the observed lifetime of protoplanetary disks. Recent interior models of Jupiter and Saturn that agree with all observational constraints suggest that Jupiter's core mass is 0–11
M
⊕
and Saturn's is 9–22
M
⊕
Saumon, G., Guillot, T., 2004. Astrophys. J. 609, 1170–1180. We have computed simulations of the growth of Jupiter using various values for the opacity produced by grains in the protoplanet's atmosphere and for the initial planetesimal surface density,
σ
init
,
Z
, in the protoplanetary disk. We also explore the implications of halting the solid accretion at selected core mass values during the protoplanet's growth. Halting planetesimal accretion at low core mass simulates the presence of a competing embryo, and decreasing the atmospheric opacity due to grains emulates the settling and coagulation of grains within the protoplanet's atmosphere. We examine the effects of adjusting these parameters to determine whether or not gas runaway can occur for small mass cores on a reasonable timescale. We compute four series of simulations with the latest version of our code, which contains updated equation of state and opacity tables as well as other improvements. Each series consists of a run without a cutoff in planetesimal accretion, plus up to three runs with a cutoff at a particular core mass. The first series of runs is computed with an atmospheric opacity due to grains (hereafter referred to as ‘grain opacity’) that is 2% of the interstellar value and
σ
init
,
Z
=
10
g
/
cm
2
. Cutoff runs are computed for core masses of 10, 5, and 3
M
⊕
. The second series of Jupiter models is computed with the grain opacity at the full interstellar value and
σ
init
,
Z
=
10
g
/
cm
2
. Cutoff runs are computed for core masses of 10 and 5
M
⊕
. The third series of runs is computed with the grain opacity at 2% of the interstellar value and
σ
init
,
Z
=
6
g
/
cm
2
. One cutoff run is computed with a core mass of 5
M
⊕
. The final series consists of one run, without a cutoff, which is computed with a temperature dependent grain opacity (i.e., 2% of the interstellar value for
T
<
350
K
ramping up to the full interstellar value for
T
>
500
K
) and
σ
init
,
Z
=
10
g
/
cm
2
. Our results demonstrate that reducing grain opacities results in formation times less than half of those for models computed with full interstellar grain opacity values. The reduction of opacity due to grains in the upper portion of the envelope with
T
⩽
500
K
has the largest effect on the lowering of the formation time. If the accretion of planetesimals is not cut off prior to the accretion of gas, then decreasing the surface density of planetesimals lowers the final core mass of the protoplanet, but increases the formation timescale considerably. Finally, a core mass cutoff results in a reduction of the time needed for a protoplanet to evolve to the stage of runaway gas accretion, provided the cutoff mass is sufficiently large. The overall results indicate that, with reasonable parameters, it is possible that Jupiter formed at 5 AU via the core accretion process in 1 Myr with a core of 10
M
⊕
or in 5 Myr with a core of 5
M
⊕
.
ABSTRACT Kepler has discovered hundreds of systems with multiple transiting exoplanets which hold tremendous potential both individually and collectively for understanding the formation and evolution ...of planetary systems. Many of these systems consist of multiple small planets with periods less than ∼50 days known as Systems with Tightly spaced Inner Planets, or STIPs. One especially intriguing STIP, Kepler-80 (KOI-500), contains five transiting planets: f, d, e, b, and c with periods of 1.0, 3.1, 4.6, 7.1, and 9.5 days, respectively. We provide measurements of transit times and a transit timing variation (TTV) dynamical analysis. We find that TTVs cannot reliably detect eccentricities for this system, though mass estimates are not affected. Restricting the eccentricity to a reasonable range, we infer masses for the outer four planets (d, e, b, and c) to be , , , and Earth masses, respectively. The similar masses but different radii are consistent with terrestrial compositions for d and e and ∼2% H/He envelopes for b and c. We confirm that the outer four planets are in a rare dynamical configuration with four interconnected three-body resonances that are librating with few degree amplitudes. We present a formation model that can reproduce the observed configuration by starting with a multi-resonant chain and introducing dissipation. Overall, the information-rich Kepler-80 planets provide an important perspective into exoplanetary systems.
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