1,081 research outputs found
Habitable Climates: The Influence of Eccentricity
In the outer regions of the habitable zone, the risk of transitioning into a
globally frozen "snowball" state poses a threat to the habitability of planets
with the capacity to host water-based life. We use a one-dimensional energy
balance climate model (EBM) to examine how obliquity, spin rate, orbital
eccentricity, and ocean coverage might influence the onset of such a snowball
state. For an exoplanet, these parameters may be strikingly different from the
values observed for Earth. Since, for constant semimajor axis, the annual mean
stellar irradiation scales with (1-e^2)^(-1/2), one might expect the greatest
habitable semimajor axis (for fixed atmospheric composition) to scale as
(1-e^2)^(-1/4). We find that this standard ansatz provides a reasonable lower
bound on the outer boundary of the habitable zone, but the influence of
obliquity and ocean fraction can be profound in the context of planets on
eccentric orbits. For planets with eccentricity 0.5, our EBM suggests that the
greatest habitable semimajor axis can vary by more than 0.8 AU (78%!) depending
on obliquity, with higher obliquity worlds generally more stable against
snowball transitions. One might also expect that the long winter at an
eccentric planet's apoastron would render it more susceptible to global
freezing. Our models suggest that this is not a significant risk for Earth-like
planets around Sun-like stars since such planets are buffered by the thermal
inertia provided by oceans covering at least 10% of their surface. Since
planets on eccentric orbits spend much of their year particularly far from the
star, such worlds might turn out to be especially good targets for direct
observations with missions such as TPF-Darwin. Nevertheless, the extreme
temperature variations achieved on highly eccentric exo-Earths raise questions
about the adaptability of life to marginally or transiently habitable
conditions.Comment: References added, text and figures updated, accepted by Ap
Terrestrial, Habitable-Zone Exoplanet Frequency from Kepler
Data from Kepler's first 136 days of operation are analyzed to determine the
distribution of exoplanets with respect to radius, period, and host-star
spectral type. The analysis is extrapolated to estimate the percentage of
terrestrial, habitable-zone exoplanets. The Kepler census is assumed to be
complete for bright stars (magnitude 0.5
Earth radius and periods <42 days. It is also assumed that the size
distribution of planets is independent of orbital period, and that there are no
hidden biases in the data. Six significant statistical results are found: there
is a paucity of small planet detections around faint target stars, probably an
instrumental effect; the frequency of mid-size planet detections is independent
of whether the host star is bright or faint; there are significantly fewer
planets detected with periods <3 days, compared to longer periods, almost
certainly an astrophysical effect; the frequency of all planets in the
population with periods <42 days is 29%, broken down as terrestrials 9%, ice
giants 18%, and gas giants 3%; the population has a planet frequency with
respect to period which follows a power-law relation dN/dP ~ P^{\beta - 1},
with \beta = 0.71 +/- 0.08; and an extrapolation to longer periods gives the
frequency of terrestrial planets in the habitable zones of FGK stars as
\eta_\oplus = (34 +/- 14)%. Thus about one-third of FGK stars are predicted to
have at least one terrestrial, habitable-zone planet.Comment: 27 pages, 5 figure
On the Apparent Orbital Inclination Change of the Extrasolar Transiting Planet TrES-2b
On June 15, 2009 UT the transit of TrES-2b was detected using the University
of Arizona's 1.55 meter Kuiper Telescope with 2.0-2.5 millimag RMS accuracy in
the I-band. We find a central transit time of
HJD, an orbital period of days, and an
inclination angle of , which is consistent with our
re-fit of the original I-band light curve of O'Donovan et al. (2006) where we
find . We calculate an insignificant inclination
change of over the last 3 years, and as
such, our observations rule out, at the level, the apparent
change of orbital inclination to as
predicted by Mislis and Schmitt (2009) and Mislis et al. (2010) for our epoch.
Moreover, our analysis of a recently published Kepler Space Telescope light
curve (Gilliland et al. 2010) for TrES-2b finds an inclination of for a similar epoch. These Kepler results definitively
rule out change in as a function of time. Indeed, we detect no significant
changes in any of the orbital parameters of TrES-2b.Comment: 19 pages, 1 table, 7 figures. Re-submitted to ApJ, January 14, 201
Adaptive Optics Images of Kepler Objects of Interest
All transiting planets are at risk of contamination by blends with nearby,
unresolved stars. Blends dilute the transit signal, causing the planet to
appear smaller than it really is, or produce a false positive detection when
the target star is blended with eclipsing binary stars. This paper reports on
high spatial-resolution adaptive optics images of 90 Kepler planetary
candidates. Companion stars are detected as close as 0.1 arcsec from the target
star. Images were taken in the near-infrared (J and Ks bands) with ARIES on the
MMT and PHARO on the Palomar Hale 200-inch. Most objects (60%) have at least
one star within 6 arcsec separation and a magnitude difference of 9. Eighteen
objects (20%) have at least one companion within 2 arcsec of the target star; 6
companions (7%) are closer than 0.5 arcsec. Most of these companions were
previously unknown, and the associated planetary candidates should receive
additional scrutiny. Limits are placed on the presence of additional companions
for every system observed, which can be used to validate planets statistically
using the BLENDER method. Validation is particularly critical for low-mass,
potentially Earth-like worlds, which are not detectable with current-generation
radial velocity techniques. High-resolution images are thus a crucial component
of any transit follow-up program.Comment: 9 pages, 4 figures, accepted to A
Planet Hunters: Assessing the Kepler Inventory of Short Period Planets
We present the results from a search of data from the first 33.5 days of the
Kepler science mission (Quarter 1) for exoplanet transits by the Planet Hunters
citizen science project. Planet Hunters enlists members of the general public
to visually identify transits in the publicly released Kepler light curves via
the World Wide Web. Over 24,000 volunteers reviewed the Kepler Quarter 1 data
set. We examine the abundance of \geq 2 R\oplus planets on short period (< 15
days) orbits based on Planet Hunters detections. We present these results along
with an analysis of the detection efficiency of human classifiers to identify
planetary transits including a comparison to the Kepler inventory of planet
candidates. Although performance drops rapidly for smaller radii, \geq 4
R\oplus Planet Hunters \geq 85% efficient at identifying transit signals for
planets with periods less than 15 days for the Kepler sample of target stars.
Our high efficiency rate for simulated transits along with recovery of the
majority of Kepler \geq 4 R\oplus planets suggest suggests the Kepler inventory
of \geq 4 R\oplus short period planets is nearly complete.Comment: 41 pages,13 figures, 8 tables, accepted to Ap
Proceedings of the Workshop on Improvements to Photometry
The purposes of the workshop were to determine what astronomical problems would benefit by increased photometric precision, determine the current level of precision, identify the processes limiting the precision, and recommend approaches to improving photometric precision. Twenty representatives of the university, industry, and government communities participated. Results and recommendations are discussed
The Occurrence Rate of Earth Analog Planets Orbiting Sunlike Stars
Kepler is a space telescope that searches Sun-like stars for planets. Its
major goal is to determine {\eta}_Earth, the fraction of Sunlike stars that
have planets like Earth. When a planet 'transits' or moves in front of a star,
Kepler can measure the concomitant dimming of the starlight. From analysis of
the first four months of those measurements for over 150,000 stars, Kepler's
science team has determined sizes, surface temperatures, orbit sizes and
periods for over a thousand new planet candidates. In this paper, we
characterize the period probability distribution function of the super-Earth
and Neptune planet candidates with periods up to 132 days, and find three
distinct period regimes. For candidates with periods below 3 days the density
increases sharply with increasing period; for periods between 3 and 30 days the
density rises more gradually with increasing period, and for periods longer
than 30 days, the density drops gradually with increasing period. We estimate
that 1% to 3% of stars like the Sun are expected to have Earth analog planets,
based on the Kepler data release of Feb 2011. This estimate of is based on
extrapolation from a fiducial subsample of the Kepler planet candidates that we
chose to be nominally 'complete' (i.e., no missed detections) to the realm of
the Earth-like planets, by means of simple power law models. The accuracy of
the extrapolation will improve as more data from the Kepler mission is folded
in. Accurate knowledge of {\eta}_Earth is essential for the planning of future
missions that will image and take spectra of Earthlike planets. Our result that
Earths are relatively scarce means that a substantial effort will be needed to
identify suitable target stars prior to these future missions.Comment: Accepted for publication in the Astrophysical Journal. 19 pages, 8
figures. Minor text revisions, as requested by the scientific editor.
Included an additional figure. No changes in the scientific result
Environmental effects of SPS: The middle atmosphere
The heavy lift launch vehicle associated with the solar power satellite (SPS) would deposit in the upper atmosphere exhaust and reentry products which could modify the composition of the stratosphere, mesosphere, and lower ionosphere. In order to assess such effects, atmospheric model simulations were performed, especially considering a geographic zone centered at the launch and reentry latitudes
Two-dimensional model studies of the effect of supersonic aircraft operations on the stratospheric ozone content
For a fleet of 250 aircraft, the change in the ozone column is predicted to be very close to zero; in fact, the ozone overburden may actually increase as a result of show that above 25 to 30 km the ozone abundance decreases via catalytic destruction, but at lower heights it increases, mainly as a result of coupling with odd hydrogen species. Water vapor released in the engine exhaust is predicted to cause ozone decreases; for the hypothetical engines used in the study, the total column ozone changes due to water vapor emission largely offset the predicted ozone increases due to NOx emission. The actual effect of water vapor may be less than calculated because present models do not include thermal feedback. Feedback refers to the cooling effect of additional water vapor that would tend to slow the NOx reactions which destroy ozone
Tests of a multichannel photometer based on silicon diode detectors
A breadboard photometer was constructed that demonstrates a precision of 2 times 10 to the 4th power in the laboratory and scintillation-limited performance when used with an 0.5 m aperture telescope. Because the detectors and preamps are not cooled, only stars with m sub v approx. less than 4 are bright enough to allow the photometer to attain a precision of 1 times 10 to the 3rd power for three minute observations with an 0.5 m aperature telescope. Cooling the telescope should allow much fainter stars to be observed. Increasing the aperture of the telescope will allow higher precision and the observation of fainter stars
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