238 research outputs found
The MEarth project: searching for transiting habitable super-Earths around nearby M-dwarfs
Due to their small radii, M-dwarfs are very promising targets to search for
transiting super-Earths, with a planet of 2 Earth radii orbiting an M5 dwarf in
the habitable zone giving rise to a 0.5% photometric signal, with a period of
two weeks. This can be detected from the ground using modest-aperture
telescopes by targeting samples of nearby M-dwarfs. Such planets would be very
amenable to follow-up studies due to the brightness of the parent stars, and
the favourable planet-star flux ratio. MEarth is such a transit survey of ~2000
nearby M-dwarfs. Since the targets are distributed over the entire (Northern)
sky, it is necessary to observe them individually, which will be done by using
8 independent 0.4m robotic telescopes, two of which have been in operation
since December 2007 at the Fred Lawrence Whipple Observatory (FLWO) located on
Mount Hopkins, Arizona. We discuss the survey design and hardware, and report
on the current status of the survey, and preliminary results obtained from the
commissioning data.Comment: 7 pages, 5 figures. To appear in the Proceedings of the 253rd IAU
Symposium: "Transiting Planets", May 2008, Cambridge, M
Design Considerations for a Ground-based Transit Search for Habitable Planets Orbiting M dwarfs
By targeting nearby M dwarfs, a transit search using modest equipment is
capable of discovering planets as small as 2 Earth radii in the habitable zones
of their host stars. The MEarth Project, a future transit search, aims to
employ a network of ground-based robotic telescopes to monitor M dwarfs in the
northern hemisphere with sufficient precision and cadence to detect such
planets. Here we investigate the design requirements for the MEarth Project. We
evaluate the optimal bandpass, and the necessary field of view, telescope
aperture, and telescope time allocation on a star-by-star basis, as is possible
for the well-characterized nearby M dwarfs. Through these considerations, 1,976
late M dwarfs (R < 0.33 Rsun) emerge as favorable targets for transit
monitoring. Based on an observational cadence and on total telescope time
allocation tailored to recover 90% of transit signals from planets in habitable
zone orbits, we find that a network of ten 30 cm telescopes could survey these
1,976 M dwarfs in less than 3 years. A null result from this survey would set
an upper limit (at 99% confidence) of 17% for the rate of occurrence of planets
larger than 2 Earth radii in the habitable zones of late M dwarfs, and even
stronger constraints for planets lying closer than the habitable zone. If the
true occurrence rate of habitable planets is 10%, the expected yield would be
2.6 planets.Comment: accepted to PAS
Where Dreams May Come: Incubation Sanctuaries in the Greco-Roman World, by Gil H. Renberg. Leiden; Boston : Brill 2017 (Book Review)
[First paragraph] Gil Renberg has done the field an incredible service with the publication of this monumental and far-reaching study. In the preface, Renberg states that one of his primary goals is to offer scholars a single resource for ancient incubation across the Near Eastern and Classical worlds (XVI). He has done precisely this with an exhaustive treatment of textual and archaeological evidence for incubation, including quotations of relevant texts in both the original language and in translation, alongside complete publication histories. [1
Extrasolar Asteroid Mining as Forensic Evidence for Extraterrestrial Intelligence
The development of civilisations like ours into spacefaring, multi-planet
entities requires significant raw materials to construct vehicles and habitats.
Interplanetary debris, including asteroids and comets, may provide such a
source of raw materials. In this article we present the hypothesis that
extraterrestrial intelligences (ETIs) engaged in asteroid mining may be
detectable from Earth. Considering the detected disc of debris around Vega as a
template, we explore the observational signatures of targeted asteroid mining
(TAM), such as unexplained deficits in chemical species, changes in the size
distribution of debris and other thermal signatures which may be detectable in
the spectral energy distribution (SED) of a debris disc. We find that
individual observational signatures of asteroid mining can be explained by
natural phenomena, and as such they cannot provide conclusive detections of
ETIs. But, it may be the case that several signatures appearing in the same
system will prove harder to model without extraterrestrial involvement.
Therefore signatures of TAM are not detections of ETI in their own right, but
as part of "piggy-back" studies carried out in tandem with conventional debris
disc research, they could provide a means of identifying unusual candidate
systems for further study using other SETI techniques.Comment: 15 pages, 2 figures, accepted for publication in the International
Journal of Astrobiolog
A Precise Estimate of the Radius of HD 149026b
We present Spitzer 8 micron transit observations of the extrasolar planet
system HD 149026. At this wavelength, transit light curves are weakly affected
by stellar limb-darkening, allowing for a simpler and more accurate
determination of planetary parameters. We measure a planet-star radius ratio of
R_p/R_s = 0.05158 +/- 0.00077, and in combination with ground-based data and
independent constraints on the stellar mass and radius, we derive an orbital
inclination of i = 85.4 +0.9/-0.8 deg. and a planet radius of 0.755 +/- 0.040
Jupiter radii. These measurements further support models in which the planet is
greatly enriched in heavy elements.Comment: To appear in the Proceedings of the 253rd IAU Symposium: "Transiting
Planets", May 2008, Cambridge, M
Asteroseismology of the Transiting Exoplanet Host HD 17156 with HST FGS
Observations conducted with the Fine Guidance Sensor on Hubble Space
Telescope (HST) providing high cadence and precision time-series photometry
were obtained over 10 consecutive days in December 2008 on the host star of the
transiting exoplanet HD 17156b. During this time 10^12 photons (corrected for
detector deadtime) were collected in which a noise level of 163 parts per
million per 30 second sum resulted, thus providing excellent sensitivity to
detection of the analog of the solar 5-minute p-mode oscillations. For HD 17156
robust detection of p-modes supports determination of the stellar mean density
of 0.5301 +/- 0.0044 g/cm^3 from a detailed fit to the observed frequencies of
modes of degree l = 0, 1, and 2. This is the first star for which direct
determination of the mean stellar density has been possible using both
asteroseismology and detailed analysis of a transiting planet light curve.
Using the density constraint from asteroseismology, and stellar evolution
modeling results in M_star = 1.285 +/- 0.026 solar, R_star = 1.507 +/- 0.012
solar, and a stellar age of 3.2 +/- 0.3 Gyr.Comment: Accepted by ApJ; 16 pages, 18 figure
A Super-Earth Transiting a Naked-Eye Star
We have detected transits of the innermost planet "e" orbiting 55 Cnc
(V=6.0), based on two weeks of nearly continuous photometric monitoring with
the MOST space telescope. The transits occur with the period (0.74 d) and phase
that had been predicted by Dawson & Fabrycky, and with the expected duration
and depth for the crossing of a Sun-like star by a hot super-Earth. Assuming
the star's mass and radius to be 0.963_{-0.029}^{+0.051} M_sun and 0.943 +/-
0.010 R_sun, the planet's mass, radius, and mean density are 8.63 +/- 0.35
Mearth, 2.00 +/- 0.14 Rearth, and 5.9_{-1.1}^{+1.5} g/cm^3. The mean density is
comparable to that of Earth, despite the greater mass and consequently greater
compression of the interior of 55 Cnc e. This suggests a rock-iron composition
supplemented by a significant mass of water, gas, or other light elements.
Outside of transits, we detected a sinusoidal signal resembling the expected
signal due to the changing illuminated phase of the planet, but with a full
range (168 +/- 70 ppm) too large to be reflected light or thermal emission.
This signal has no straightforward interpretation and should be checked with
further observations. The host star of 55 Cnc e is brighter than that of any
other known transiting planet, which will facilitate future investigations.Comment: ApJ Letters, in press (v3) Minor changes/clarifications; (v2) Revised
after fixing an error in the data reduction pipeline; for details see
http://web.mit.edu/~jwinn/www/55cnc_response.tx
Asteroseismic Investigation of Known Planet Hosts in the Kepler Field
In addition to its great potential for characterizing extra-solar planetary
systems the Kepler mission is providing unique data on stellar oscillations. A
key aspect of Kepler asteroseismology is the application to solar-like
oscillations of main-sequence stars. As an example we here consider an initial
analysis of data for three stars in the Kepler field for which planetary
transits were known from ground-based observations. For one of these, HAT-P-7,
we obtain a detailed frequency spectrum and hence strong constraints on the
stellar properties. The remaining two stars show definite evidence for
solar-like oscillations, yielding a preliminary estimate of their mean
densities.Comment: Astrophysical Journal Letters, in the pres
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