2,226 research outputs found
Tidal Effects on the Habitability of Exoplanets: The Case of GJ 581 d
Tides may be crucial to the habitability of exoplanets. If such planets form
around low-mass stars, then those in the circumstellar habitable zone will be
close enough to their host stars to experience strong tidal forces. Tides may
result in orbital decay and circularization, evolution toward zero obliquity, a
fixed rotation rate (not necessarily synchronous), and substantial internal
heating. Due to tidal effects, the range of habitable orbital locations may be
quite different from that defined by the traditional concept of a habitable
zone (HZ) based on stellar insolation, atmospheric effects, and liquid water on
a planet's surface. Tidal heating may make locations within the traditional HZ
too hot, while planets outside the traditional zone could be rendered quite
habitable due to tides. Here we consider these effects on the exoplanet GJ 581
d.Comment: 2 pages, 1 figure
Tidal Constraints on Planetary Habitability
We review how tides may impact the habitability of terrestrial-like planets.
If such planets form around low-mass stars, then planets in the circumstellar
habitable zone will be close enough to their host stars to experience strong
tidal forces. We discuss 1) decay of semi-major axis, 2) circularization of
eccentric orbits, 3) evolution toward zero obliquity, 4) fixed rotation rates
(not necessarily synchronous), and 5) internal heating. We briefly describe
these effects using the example of a 0.25 solar mass star with a 10 Earth-mass
companion. We suggest that the concept of a habitable zone should be modified
to include the effects of tides.Comment: 6 pages, 3 figures. Proceedings submitted to "Pathways Towards
Habitable Planets" Symposium (eds.: D. Gelino, V. Coude du Foresto, I. Ribas
Secular Behavior of Exoplanets: Self-Consistency and Comparisons with the Planet-Planet Scattering Hypothesis
If mutual gravitational scattering among exoplanets occurs, then it may
produce unique orbital properties. For example, two-planet systems that lie
near the boundary between circulation and libration of their periapses could
result if planet-planet scattering ejected a former third planet quickly,
leaving one planet on an eccentric orbit and the other on a circular orbit. We
first improve upon previous work that examined the apsidal behavior of known
multiplanet systems by doubling the sample size and including observational
uncertainties. This analysis recovers previous results that demonstrated that
many systems lay on the apsidal boundary between libration and circulation. We
then performed over 12,000 three-dimensional N-body simulations of hypothetical
three-body systems that are unstable, but stabilize to two-body systems after
an ejection. Using these synthetic two-planet systems, we test the
planet-planet scattering hypothesis by comparing their apsidal behavior, over a
range of viewing angles, to that of the observed systems and find that they are
statistically consistent regardless of the multiplicity of the observed
systems. Finally, we combine our results with previous studies to show that,
from the sampled cases, the most likely planetary mass function prior to
planet-planet scattering follows a power law with index -1.1. We find that this
pre-scattering mass function predicts a mutual inclination frequency
distribution that follows an exponential function with an index between -0.06
and -0.1.Comment: 29 pages, 3 figures, accepted for publication in A
Tides and the Evolution of Planetary Habitability
Tides raised on a planet by its host star's gravity can reduce a planet's
orbital semi-major axis and eccentricity. This effect is only relevant for
planets orbiting very close to their host stars. The habitable zones of
low-mass stars are also close-in and tides can alter the orbits of planets in
these locations. We calculate the tidal evolution of hypothetical terrestrial
planets around low-mass stars and show that tides can evolve planets past the
inner edge of the habitable zone, sometimes in less than 1 billion years. This
migration requires large eccentricities (>0.5) and low-mass stars (<0.35
M_Sun). Such migration may have important implications for the evolution of the
atmosphere, internal heating and the Gaia hypothesis. Similarly, a planet
detected interior to the habitable zone could have been habitable in the past.
We consider the past habitability of the recently-discovered, ~5 M_Earth
planet, Gliese 581 c. We find that it could have been habitable for reasonable
choices of orbital and physical properties as recently as 2 Gyr ago. However,
when we include constraints derived from the additional companions, we see that
most parameter choices that predict past habitability require the two inner
planets of the system to have crossed their mutual 3:1 mean motion resonance.
As this crossing would likely have resulted in resonance capture, which is not
observed, we conclude that Gl 581 c was probably never habitable.Comment: 31 pages, 10 figures, accepted to Astrobiology. A version with full
resolution figures is available at
http://www.lpl.arizona.edu/~rory/publications/brjg07.pd
Tidal Limits to Planetary Habitability
The habitable zones of main sequence stars have traditionally been defined as
the range of orbits that intercept the appropriate amount of stellar flux to
permit surface water on a planet. Terrestrial exoplanets discovered to orbit M
stars in these zones, which are close-in due to decreased stellar luminosity,
may also undergo significant tidal heating. Tidal heating may span a wide range
for terrestrial exoplanets and may significantly affect conditions near the
surface. For example, if heating rates on an exoplanet are near or greater than
that on Io (where tides drive volcanism that resurface the planet at least
every 1 Myr) and produce similar surface conditions, then the development of
life seems unlikely. On the other hand, if the tidal heating rate is less than
the minimum to initiate plate tectonics, then CO_2 may not be recycled through
subduction, leading to a runaway greenhouse that sterilizes the planet. These
two cases represent potential boundaries to habitability and are presented
along with the range of the traditional habitable zone for main sequence,
low-mass stars. We propose a revised habitable zone that incorporates both
stellar insolation and tidal heating. We apply these criteria to GJ 581 d and
find that it is in the traditional habitable zone, but its tidal heating alone
may be insufficient for plate tectonics.Comment: 13 pages, 2 figures, accepted to ApJ Letters. A version with full
resolution images is available at
http://www.astro.washington.edu/users/rory/publications/bjgr09.pd
Origin and Dynamics of the Mutually Inclined Orbits of Upsilon Andromedae c and d
We evaluate the orbital evolution and several plausible origins scenarios for
the mutually inclined orbits of Upsilon Andromedae c and d. These two planets
have orbital elements that oscillate with large amplitudes and lie close to the
stability boundary. This configuration, and in particular the observed mutual
inclination, demands an explanation. The planetary system may be influenced by
a nearby low-mass star, Upsilon And B, which could perturb the planetary
orbits, but we find it cannot modify two coplanar orbits into the observed
mutual inclination of ~30 deg. However, it could incite ejections or collisions
between planetary companions that subsequently raise the mutual inclination to
>30 deg. Our simulated systems with large mutual inclinations tend to be
further from the stability boundary than Upsilon And, but we are able to
produce similar systems. We conclude that scattering is a plausible mechanism
to explain the observed orbits of Upsilon And c and d, but we cannot determine
whether the scattering was caused by instabilities among the planets themselves
or by perturbations from Upsilon And B. We also develop a procedure to
quantitatively compare numerous properties of the observed system to our
numerical models. Although we only implement this procedure to Upsilon And, it
may be applied to any exoplanetary system.Comment: 19 pages, 5 figures, accepted to Astrophysical Journa
Characterizing Multi-planet Systems with Classical Secular Theory
Classical secular theory can be a powerful tool to describe the qualitative
character of multi-planet systems and offer insight into their histories. The
eigenmodes of the secular behavior, rather than current orbital elements, can
help identify tidal effects, early planet-planet scattering, and dynamical
coupling among the planets, for systems in which mean-motion resonances do not
play a role. Although tidal damping can result in aligned major axes after all
but one eigenmode have damped away, such alignment may simply be fortuitous. An
example of this is 55 Cancri (orbital solution of Fischer et al., 2008) where
multiple eigenmodes remain undamped. Various solutions for 55 Cancri are
compared, showing differing dynamical groupings, with implications for the
coupling of eccentricities and for the partitioning of damping among the
planets. Solutions for orbits that include expectations of past tidal evolution
with observational data, must take into account which eigenmodes should be
damped, rather than expecting particular eccentricities to be near zero.
Classical secular theory is only accurate for low eccentricity values, but
comparison with other results suggests that it can yield useful qualitative
descriptions of behavior even for moderately large eccentricity values, and may
have advantages for revealing underlying physical processes and, as large
numbers of new systems are discovered, for triage to identify where more
comprehensive dynamical studies should have priority.Comment: Published in Celestial Mechanics and Dynamical Astronomy, 25 pages,
10 figure
Covid-19 Severity Scale For Claims Data Research
OBJECTIVE: to create and validate a methodology to assign a severity level to an episode of COVID-19 for retrospective analysis in claims data.
DATA SOURCE: Secondary data obtained by license agreement from Optum provided claims records nationally for 19,761,754 persons, of which, 692,094 persons had COVID-19 in 2020.
STUDY DESIGN: The World Health Organization (WHO) COVID-19 Progression Scale was used as a model to identify endpoints as measures of episode severity within claims data. Endpoints used included symptoms, respiratory status, progression to levels of treatment and mortality.
DATA COLLECTION/EXTRACTION METHODS: The strategy for identification of cases relied upon the February 2020 guidance from the Centers for Disease Control and Prevention (CDC).
PRINCIPAL FINDINGS: A total of 709,846 persons (3.6%) met the criteria for one of the nine severity levels based on diagnosis codes with 692,094 having confirmatory diagnoses. The rates for each level varied considerably by age groups, with the older age groups reaching higher severity levels at a higher rate. Mean and median costs increased as severity level increased. Statistical validation of the severity scales revealed that the rates for each level varied considerably by age group, with the older ages reaching higher severity levels (p \u3c 0.001). Other demographic factors such as race and ethnicity, geographic region, and comorbidity count had statistically significant associations with severity level of COVID-19.
CONCLUSION: A standardized severity scale for use with claims data will allow researchers to evaluate episodes so that analyses can be conducted on the processes of intervention, effectiveness, efficiencies, costs and outcomes related to COVID-19
Asteroids Were Born Big
How big were the first planetesimals? We attempt to answer this question by
conducting coagulation simulations in which the planetesimals grow by mutual
collisions and form larger bodies and planetary embryos. The size frequency
distribution (SFD) of the initial planetesimals is considered a free parameter
in these simulations, and we search for the one that produces at the end
objects with a SFD that is consistent with asteroid belt constraints. We find
that, if the initial planetesimals were small (e.g. km-sized), the final SFD
fails to fulfill these constraints. In particular, reproducing the bump
observed at diameter D~100km in the current SFD of the asteroids requires that
the minimal size of the initial planetesimals was also ~100km. This supports
the idea that planetesimals formed big, namely that the size of solids in the
proto-planetary disk ``jumped'' from sub-meter scale to multi-kilometer scale,
without passing through intermediate values. Moreover, we find evidence that
the initial planetesimals had to have sizes ranging from 100 to several 100km,
probably even 1,000km, and that their SFD had to have a slope over this
interval that was similar to the one characterizing the current asteroids in
the same size-range. This result sets a new constraint on planetesimal
formation models and opens new perspectives for the investigation of the
collisional evolution in the asteroid and Kuiper belts as well as of the
accretion of the cores of the giant planets.Comment: Icarus (2009) in pres
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