384 research outputs found
Exobiology in Earth orbit: The results of science workshops held at NASA, Ames Research Center
The Workshops on Exobiology in Earth Orbit were held to explore concepts for orbital experiments of exobiological interest and make recommendations on which classes of experiments should be carried out. Various observational and experimental opportunities in Earth orbit are described including those associated with the Space Shuttle laboratories, spacecraft deployed from the Space Shuttle and expendable launch vehicles, the Space Station, and lunar bases. Specific science issues and technology needs are summarized. Finally, a list of recommended experiments in the areas of observational exobiology, cosmic dust collection, and in situ experiments is presented
A 1.1 to 1.9 GHz SETI Survey of the Kepler Field: I. A Search for Narrow-band Emission from Select Targets
We present a targeted search for narrow-band (< 5 Hz) drifting sinusoidal
radio emission from 86 stars in the Kepler field hosting confirmed or candidate
exoplanets. Radio emission less than 5 Hz in spectral extent is currently known
to only arise from artificial sources. The stars searched were chosen based on
the properties of their putative exoplanets, including stars hosting candidates
with 380 K > T_eq > 230 K, stars with 5 or more detected candidates or stars
with a super-Earth (R_p 50 day orbit. Baseband voltage data
across the entire band between 1.1 and 1.9 GHz were recorded at the Robert C.
Byrd Green Bank Telescope between Feb--Apr 2011 and subsequently searched
offline. No signals of extraterrestrial origin were found. We estimate that
fewer than ~1% of transiting exoplanet systems host technological civilizations
that are radio loud in narrow-band emission between 1-2 GHz at an equivalent
isotropically radiated power (EIRP) of ~1.5 x 10^21 erg s^-1, approximately
eight times the peak EIRP of the Arecibo Planetary Radar, and we limit the the
number of 1-2 GHz narrow-band-radio-loud Kardashev type II civilizations in the
Milky Way to be < 10^-6 M_solar^-1. Here we describe our observations, data
reduction procedures and results.Comment: Accepted to the Astrophysical Journa
Bayesian approach to SETI
The search for technosignatures from hypothetical galactic civilizations is
going through a new phase of intense activity. For the first time, a
significant fraction of the vast search space is expected to be sampled in the
foreseeable future, potentially bringing informative data about the abundance
of detectable extraterrestrial civilizations, or the lack thereof. Starting
from the current state of ignorance about the galactic population of
non-natural electromagnetic signals, we formulate a Bayesian statistical model
to infer the mean number of radio signals crossing Earth, assuming either
non-detection or the detection of signals in future surveys of the Galaxy.
Under fairly noninformative priors, we find that not detecting signals within
about kly from Earth, while suggesting the lack of galactic emitters or at
best the scarcity thereof, is nonetheless still consistent with a probability
exceeding \% that typically over signals could be crossing
Earth, with radiated power analogous to that of the Arecibo radar, but coming
from farther in the Milky Way. The existence in the Galaxy of potentially
detectable Arecibo-like emitters can be reasonably ruled out only if all-sky
surveys detect no such signals up to a radius of about kly, an endeavor
requiring detector sensitivities thousands times higher than those of current
telescopes. Conversely, finding even one Arecibo-like signal within
light years, a possibility within reach of current detectors, implies almost
certainly that typically more than signals of comparable radiated
power cross the Earth, yet to be discovered.Comment: Published in PNAS ahead of print October 1, 2018. Preprint has 13
pages, 7 figures + 7 pages of Supplementary Information with 5 figure
Scintillation-Induced Intermittency in SETI
We consider interstellar scintillations as a cause of intermittency in radio
signals from extraterrestrial intelligence (ETI). We demonstrate that
scintillations are very likely to allow initial detections of narrowband
signals from distant sources (> 100 pc), while making redetections improbable.
We consider three models in order to assess the non-repeating, narrowband
events found in recent SETI and to analyze large surveys in general: (I)
Radiometer noise; (II) A population of constant Galactic sources undergoing
interstellar scintillation,; and (III) Real, transient signals (or hardware
errors) of either terrestrial or ET origin.
We apply likelihood and Bayesian tests of the models to The Planetary
Society/Harvard META data. We find that Models II and III are both highly
preferred to Model I, but that Models II and III are about equally likely.
Ruling out Model II in favor of Model III requires many more reobservations
than were conducted in META *or* the reobservation threshold must be much lower
than was used in META. *We cannot rule out the possibility that META events are
real, intrinsically steady ETI signals.*
We recommend that future surveys use thresholds far below the typical
false-alarm threshold to lessen the effects of intermittency. The threshold
level is best defined in terms of the recording and computational technology
that is available at a cost commensurate with other survey costs.Comment: 59 pages, LaTeX using aaspp4 style file, 12 figures in 14 PostScript
figures, ApJ, in press, 1997 Oct.
Habitability of the Goldilocks Planet Gliese 581g: Results from Geodynamic Models
Aims: In 2010, detailed observations have been published that seem to
indicate another super-Earth planet in the system of Gliese 581 located in the
midst of the stellar climatological habitable zone. The mass of the planet,
known as Gl 581g, has been estimated to be between 3.1 and 4.3 Earth masses. In
this study, we investigate the habitability of Gl 581g based on a previously
used concept that explores its long-term possibility of photosynthetic biomass
production, which has already been used to gauge the principal possibility of
life regarding the super-Earths Gl 581c and Gl 581d. Methods: A thermal
evolution model for super-Earths is used to calculate the sources and sinks of
atmospheric carbon dioxide. The habitable zone is determined by the limits of
photosynthetic biological productivity on the planetary surface. Models with
different ratios of land / ocean coverage are pursued. Results: The maximum
time span for habitable conditions is attained for water worlds at a position
of about 0.14+/-0.015 AU, which deviates by just a few percent (depending on
the adopted stellar luminosity) from the actual position of Gl 581g, an
estimate that does however not reflect systematic uncertainties inherent in our
model. Therefore, in the framework of our model an almost perfect Goldilock
position is realized. The existence of habitability is found to critically
depend on the relative planetary continental area, lending a considerable
advantage to the possibility of life if Gl 581g's ocean coverage is relatively
high. Conclusions: Our results are a further step toward identifying the
possibility of life beyond the Solar System, especially concerning super-Earth
planets, which appear to be more abundant than previously surmised.Comment: 5 pages, 3 figures, 1 table; in pres
The habitability of super-Earths in Gliese 581
Aims: The planetary system around the M star Gliese 581 consists of a hot
Neptune (Gl 581b) and two super-Earths (Gl 581c and Gl 581d). The habitability
of this system with respect to the super-Earths is investigated following a
concept that studies the long-term possibility of photosynthetic biomass
production on a dynamically active planet. Methods: A thermal evolution model
for a super-Earth is used to calculate the sources and sinks of atmospheric
carbon dioxide. The habitable zone is determined by the limits of biological
productivity on the planetary surface. Models with different ratios of land /
ocean coverage are investigated. Results: The super-Earth Gl 581c is clearly
outside the habitable zone, since it is too close to the star. In contrast, Gl
581d is a tidally locked habitable super-Earth near the outer edge of the
habitable zone. Despite the adverse conditions on this planet, at least some
primitive forms of life may be able to exist on its surface.Therefore, Gl 581d
is an interesting target for the planned TPF/Darwin missions to search for
biomarkers in planetary atmospheres.Comment: 6 pages, 4 figures, 2 table
Discovery of the Transiting Planet Kepler-5B
We present 44 days of high duty cycle, ultra precise photometry of the 13th magnitude star Kepler-5 (KIC 8191672, T(eff) = 6300 K, log g = 4.1), which exhibits periodic transits with a depth of 0.7%. Detailed modeling of the transit is consistent with a planetary companion with an orbital period of 3.548460 +/- 0.000032 days and a radius of 1.431(-0.052)(+0.041) R(J). Follow-up radial velocity measurements with the Keck HIRES spectrograph on nine separate nights demonstrate that the planet is more than twice as massive as Jupiter with a mass of 2.114(-0.059)(+0.056) M(J) and a mean density of 0.894 +/- 0.079 g cm(-3).NASA's Science Mission DirectorateAstronom
The Benefits and Harms of Transmitting Into Space
Deliberate and unintentional radio transmissions from Earth propagate into
space. These transmissions could be detected by extraterrestrial watchers over
interstellar distances. Here, we analyze the harms and benefits of deliberate
and unintentional transmissions relevant to Earth and humanity. Comparing the
magnitude of deliberate radio broadcasts intended for messaging to
extraterrestrial intelligence (METI) with the background radio spectrum of
Earth, we find that METI attempts to date have much lower detectability than
emissions from current radio communication technologies on Earth. METI
broadcasts are usually transient and several orders of magnitude less powerful
than other terrestrial sources such as astronomical and military radars, which
provide the strongest detectable signals. The benefits of radio communication
on Earth likely outweigh the potential harms of detection by extraterrestrial
watchers; however, the uncertainty regarding the outcome of contact with
extraterrestrial beings creates difficulty in assessing whether or not to
engage in long-term and large-scale METI.Comment: Published in Space Polic
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