431 research outputs found
Occurrence and core-envelope structure of 1--4x Earth-size planets around Sun-like stars
Small planets, 1-4x the size of Earth, are extremely common around Sun-like
stars, and surprisingly so, as they are missing in our solar system. Recent
detections have yielded enough information about this class of exoplanets to
begin characterizing their occurrence rates, orbits, masses, densities, and
internal structures. The Kepler mission finds the smallest planets to be most
common, as 26% of Sun-like stars have small, 1-2 R_e planets with orbital
periods under 100 days, and 11% have 1-2 R_e planets that receive 1-4x the
incident stellar flux that warms our Earth. These Earth-size planets are
sprinkled uniformly with orbital distance (logarithmically) out to 0.4 AU, and
probably beyond. Mass measurements for 33 transiting planets of 1-4 R_e show
that the smallest of them, R < 1.5 R_e, have the density expected for rocky
planets. Their densities increase with increasing radius, likely caused by
gravitational compression. Including solar system planets yields a relation:
rho = 2.32 + 3.19 R/R_e [g/cc]. Larger planets, in the radius range 1.5-4.0
R_e, have densities that decline with increasing radius, revealing increasing
amounts of low-density material in an envelope surrounding a rocky core,
befitting the appellation "mini-Neptunes." Planets of ~1.5 R_e have the highest
densities, averaging near 10 g/cc. The gas giant planets occur preferentially
around stars that are rich in heavy elements, while rocky planets occur around
stars having a range of heavy element abundances. One explanation is that the
fast formation of rocky cores in protoplanetary disks enriched in heavy
elements permits the gravitational accumulation of gas before it vanishes,
forming giant planets. But models of the formation of 1-4 R_e planets remain
uncertain. Defining habitable zones remains difficult, without benefit of
either detections of life elsewhere or an understanding of life's biochemical
origins.Comment: 11 pages, 6 figures, accepted for publication Proc. Natl. Acad. Sc
A Neptune-Mass Planet Orbiting the Nearby M Dwarf GJ 436
We report precise Doppler measurements of GJ 436 (M2.5V) obtained at Keck
Observatory. The velocities reveal a planetary companion with orbital period of
2.644 d, eccentricity of 0.12 (consistent with zero) and velocity
semi-amplitude of \ms. The minimum mass (\msini) for the planet is
0.067 \mjup = 1.2 M = 21 M, making it the lowest mass
exoplanet yet found around a main sequence star and the first candidate in the
Neptune mass domain. GJ 436 (Mass = 0.41 \msune) is only the second M dwarf
found to harbor a planet, joining the two--planet system around GJ 876. The low
mass of the planet raises questions about its constitution, with possible
compositions of primarily H and He gas, ice/rock, or rock--dominated. The
implied semi--major axis is = 0.028 AU = 14 stellar radii, raising issues
of planet formation, migration, and tidal coupling with the star. GJ 436 is
Gyr old, based on both kinematic and chromospheric diagnostics. The star
exhibits no photometric variability on the 2.644-day Doppler period to a
limiting amplitude of 0.0004 mag, supporting the planetary interpretation of
the Doppler periodicity. Photometric transits of the planet across the star are
ruled out for gas giant compositions and are also unlikely for solid
compositions. As the third closest known planetary system, GJ 436 warrants
follow--up observations by high resolution optical and I
Future Exoplanet Research: Science Questions and How to Address Them
Started approximately in the late 1980s, exoplanetology has up to now
unveiled the main gross bulk characteristics of planets and planetary systems.
In the future it will benefit from more and more large telescopes and advanced
space missions. These instruments will dramatically improve their performance
in terms of photometric precision, detection speed, multipixel imaging,
high-resolution spectroscopy, allowing to go much deeper in the knowledge of
planets. Here we outline some science questions which should go beyond these
standard improvements and how to address them. Our prejudice is that one is
never too speculative: experience shows that the speculative predictions
initially not accepted by the community have been confirmed several years later
(like spectrophotometry of transits or circumbinary planets).Comment: Invited review, accepte
Fourier series for eclipses on exoplanet binaries
A double planet system or planet binary undergoes eclipses that modify the
reflective light curve. In the time domain, the eclipse events are fast and
weak. This would make their signal difficult to find and recognize in the phase
light curve, even for small inclinations when eclipses happen frequently.
However, due to the quasiperiodic nature of the phenomenon, the Fourier
transform of the direct reflection signal consists of a double sum of sharp
peaks. These peaks can be resolved for large close binaries and sufficiently
long observation times with a star coronagraph. Eclipses modulate the phase
curve, having an orbital period , with a contribution from the
relative motion in the binary plane of a period . This leads to a
spectral structure with basis frequencies and . We aim to
characterize these spectra. We studied the regime of short eclipses that occur
when the planet radii are small compared to the planet separation. We derived
formulas for the peak amplitudes applicable to homogeneous (Lambertian) planet
binaries in circular orbit with small inclination. The effects of an eclipse
and of double reflection appear as first- and second-order contributions (in
planet radius over separation) in the reflection signal respectively. Small
peaks appear as observable side bands in the spectrum. Identical structures
around are characteristic of short-duration eclipses. Deceasing side
bands could indicate double reflection between companions. Fourier analysis of
the light curve of non-transiting planets can be used to find planets and their
moons. Difficulties in interpreting the structures arise for small planet
separation and when there are several moons in mean-motion resonance.Comment: 12 pages, 10 figure
Bilayer characteristics of a diether phosphonolipid analog of the major lung surfactant glycerophospholipid dipalmitoyl phosphatidylcholine.
Thermal and lyotropic phase behavior was studied by X-ray diffraction and differential scanning calorimetry for a diether phosphonolipid analog (DEPN-8) of the major lung surfactant glycerophospholipid dipalmitoyl phosphatidylcholine (DPPC). DEPN-8 differs in an ether, rather than an ester, bond at the acyl chain-backbone linkage and a headgroup phosphonate (isosteric methylene substitution) versus phosphate constituent. Analysis of lamellar diffraction maxima demonstrated that at high relative humidity (98%) and temperatures below the liquid crystal phase transition (approximately 45 degrees C), DEPN-8 formed interdigitated bilayers with a characteristic periodicity of 41.9-46.5 A. At low humidity the gel phase DEPN-8 bilayers were characteristic of a normal L beta phase with a periodicity equivalent to DPPC (57-59 A). Above the liquid crystal thermal phase transition, bilayer spacing for both DEPN-8 and DPPC was 51-52 A, characteristic of the L alpha phase. Complete assessments of both lamellar and in-plane X-ray scattering used to construct electron density profiles and structure-factor plots for DEPN-8 defined more fully the interdigitated bilayer state at high humidity and low temperature. Compared to DPPC, it is energetically favorable for DEPN-8 to form interdigitated bilayers under conditions of excess water and low temperature. The flexible character of the ether bonds in DEPN-8 allows increased hydrophobic interactions between acyl chains, without generating a steric penalty from the increased packing density of the molecules. Additionally, the ether bond and the phosphonate moiety may allow for more energetically favorable interactions between the choline portion of the headgroup and water. The DEPN-8 ether linkage may also contribute to the improved adsorption and film respreading found previously for this phosphonolipid compared to DPPC
Exoplanet Biosignatures: Observational Prospects
Exoplanet hunting efforts have revealed the prevalence of exotic worlds with
diverse properties, including Earth-sized bodies, which has fueled our endeavor
to search for life beyond the Solar System. Accumulating experiences in
astrophysical, chemical, and climatological characterization of uninhabitable
planets are paving the way to characterization of potentially habitable
planets. In this paper, we review our possibilities and limitations in
characterizing temperate terrestrial planets with future observational
capabilities through 2030s and beyond, as a basis of a broad range of
discussions on how to advance "astrobiology" with exoplanets. We discuss the
observability of not only the proposed biosignature candidates themselves, but
also of more general planetary properties that provide circumstantial evidence,
since the evaluation of any biosignature candidate relies on their context.
Characterization of temperate Earth-size planets in the coming years will focus
on those around nearby late-type stars. JWST and later 30 meter-class
ground-based telescopes will empower their chemical investigations.
Spectroscopic studies of potentially habitable planets around solar-type stars
will likely require a designated spacecraft mission for direct imaging,
leveraging technologies that are already being developed and tested as part of
the WFIRST mission. Successful initial characterization of a few nearby targets
will be an important touchstone toward a more detailed scrutiny and a larger
survey that are envisioned beyond 2030. The broad outlook this paper presents
may help develop new observational techniques to detect relevant features as
well as frameworks to diagnose planets based on the observables.Comment: part of a series of 5 review manuscripts of the NExSS Exoplanet
Biosgnatures Worksho
An Introduction into the Physics of Self-folding Thin Structures
Preprint.
The article was published in: Friedman, Michael/Schäffner, Wolfgang (eds.) (2016): On Folding. Towards a New Field of Interdisciplinary Research. Bielefeld: transcript, pp. 175–210
The GAPS Programme with HARPS-N at TNG. XII. Characterization of the planetary system around HD 108874
In order to understand the observed physical and orbital diversity of extrasolar planetary systems, a full investigation of these objects and of their host stars is necessary. Within this field, one of the main purposes of the GAPS observing project with HARPS-N at TNG is to provide a more detailed characterization of already known systems. In this framework we monitored the star, hosting two giant planets, HD 108874, with HARPS-N for three years in order to refine the orbits, to improve the dynamical study and to search for additional low-mass planets in close orbits. We subtracted the radial velocity (RV) signal due to the known outer planets, finding a clear modulation of 40.2 d period. We analysed the correlation between RV residuals and the activity indicators and modelled the magnetic activity with a dedicated code. Our analysis suggests that the 40.2 d periodicity is a signature of the rotation period of the star. A refined orbital solution is provided, revealing that the system is close to a mean motion resonance of about 9:2, in a stable configuration over 1 Gyr. Stable orbits for low-mass planets are limited to regions very close to the star or far from it. Our data exclude super-Earths with Msini ≳ 5M⊕ within 0.4 AU and objects with Msini ≳ 2M⊕ with orbital periods of a few days. Finally we put constraints on the habitable zone of the system, assuming the presence of an exomoon orbiting the inner giant planet. Based on observations made with the Italian Telescopio Nazionale Galileo (TNG) operated on the island of La Palma by the Fundación Galileo Galilei of the INAF at the Spanish Observatorio del Roque de los Muchachos of the IAC in the frame of the programme Global Architecture of Planetary Systems (GAPS).Table A.1 is also available at the CDS via anonymous ftp to http://cdsarc.u-strasbg.fr (http://130.79.128.5) or via http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/599/A90</A
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