Mutual detectability: a targeted SETI strategy that avoids the SETI Paradox

As our ability to undertake more powerful Searches for Extraterrestrial Intelligence (SETI) grows, so does interest in the more controversial endeavour of Messaging Extraterrestrial Intelligence (METI). METI proponents point to the SETI Paradox - if all civilisations refrain from METI then SETI is futile. I introduce Mutual Detectability as a game-theoretic strategy aimed at increasing the success potential of targeted SETI. Mutual detectability is embodied by four laws: mutuality, symmetry, opportunity and superiority. These laws establish how SETI participants can engage each other using game theory principles applied to mutual evidence of mutual existence. The law of superiority establishes an "onus to transmit" on the party whom both SETI participants can judge to have better quality evidence, or common denominator information (CDI), thus avoiding the SETI Paradox. I argue that transiting exoplanets within the Earth Transit Zone form a target subset that satisfies mutual detectability requirements. I identify the intrinsic time-integrated transit signal strength as suitable CDI. Civilisations on habitable-zone planets of radius $R_{\rm p}/R_{\oplus} \lesssim (L_*/L_{\odot})^{-1/7}$ have superior CDI on us, so have game-theory incentive (onus) to transmit. Whilst this implies that the onus to transmit falls on us for habitable planets around $L_* > L_{\odot}$ stars, considerations of relative stellar frequency, main-sequence lifetime and planet occurrence mean such systems are likely a small minority. Surveys of the Earth Transit Zone for Earth-analogue transits around sub-solar luminosity hosts, followed up by targeted SETI monitoring of them, represent an efficient strategy compliant with mutual detectability. A choice to remain silent, by not engaging in METI towards such systems, does not in this case fuel concerns of a SETI Paradox.Comment: Author Accepted Manuscript. Published in A

Reference image selection for difference imaging analysis

Difference image analysis (DIA) is an effective technique for obtaining photometry in crowded fields, relative to a chosen reference image. As yet, however, optimal reference image selection is an unsolved problem. We examine how this selection depends on the combination of seeing, background and detector pixel size. Our tests use a combination of simulated data and quality indicators from DIA of well-sampled optical data and under-sampled near-infrared data from the OGLE and VVV surveys, respectively. We search for a figure-of-merit (FoM) which could be used to select reference images for each survey. While we do not find a universally applicable FoM, survey-specific measures indicate that the effect of spatial under-sampling may require a change in strategy from the standard DIA approach, even though seeing remains the primary criterion. We find that background is not an important criterion for reference selection, at least for the dynamic range in the images we test. For our analysis of VVV data in particular, we find that spatial under-sampling is best handled by reversing the standard DIA procedure and convolving target images to a better-sampled (poor seeing) reference image.Comment: 14 pages, 8 figures, 4 tables, accepted for publication in MNRA

The detectability of habitable exomoons with Kepler

In this paper, the detectability of habitable exomoons orbiting around giant planets in M-dwarf systems using Transit Timing Variations (TTVs) and Transit Timing Durations (TDVs) with Kepler-class photometry is investigated. Light curves of systems with various configurations were simulated around M-dwarf hosts of mass 0.5 Msun and radius 0.55 Rsun. Jupiter-like giant planets which offer the best potential for hosting habitable exomoons were considered with rocky super-Earth-mass moons. The detectability is measured by using the phase-correlation between TTV and TDV signals. Since the TDV signal is typically weaker than the TTV signal, confirmation of an exomoon detection will depend on being able to detect a TDV signal. We find that exomoons around planets orbiting within the habitable zone of an M-dwarf host star can produce both detectable TTV and TDV signatures with Kepler-class photometry. While aliasing between the planet period and moon period may hinder exomoon detection, we also find some strong correlation signatures in our simulation (eg. correlation: >0.7) which would provide convincing exomoon signatures. With the addition of red noise stellar variability, correlations generally weaken. However simulated examples with planet masses less than around 25 Mearth, moons of mass 8-10 Mearth and specific values of planet and moon periods still yield detectable correlation in 25-50% of cases. Our simulation indicates that Kepler provides one of the best available opportunities for exomoon detection.Comment: 14 pages, 8 figures, 2 tables, Accepted by MNRA

Pre-discovery transits of the exoplanets WASP-18 b and WASP-33 b from Hipparcos

We recover transits of WASP-18 b and WASP-33 b from Hipparcos (1989-1993) photometry. Marginal detections of HAT-P-56 b and HAT-P-2 b may be also present in the data. New ephemerides are fitted to WASP-18 b and WASP-33 b. A tentative (~1.3 sigma) orbital decay is measured for WASP-18 b, but the implied tidal quality factor (Q' ~ 5 x 10^5) is small and survival time (<10^6 years) is too short to be likely. No orbital decay is measured for WASP-33 b, and a limit of Q' > 2 x 10^5 is placed. For both planets, the uncertainties in published ephemerides appear underestimated: the uncertainty in the period derivative of WASP-18 b would be greatly reduced if its current ephemeris could be better determined.Comment: 4 pages, 3 figures, Accepted MNRAS Letter

Microlensing Halo Models with Abundant Brown Dwarfs

All previous attempts to understand the microlensing results towards the Large Magellanic Cloud (LMC) have assumed homogeneous present day mass functions (PDMFs) for the lensing populations. Here, we present an investigation into the microlensing characteristics of haloes with spatially varying PDMFs and anisotropic velocity dispersion tensors. One attractive possibility -- suggested by baryonic dark cluster formation in pregalactic and protogalactic cooling flows -- is that the inner halo is dominated by stellar mass objects, whereas low mass brown dwarfs become more prevalent on moving outwards. The contribution to the microlensing rate must be dominated by dark remnants (of about 0.5 solar masses) to recover the observed timescales of the microlensing experiments. But, even though stellar remnants control the rate, they do not dominate the mass of the baryonic halo, and so the well-known enrichment and mass budget problems are much less severe. Using a simple ansatz for the spatial variation of the PDMF, models are constructed in which the contribution of brown dwarfs to the mass of the baryonic halo is 55 % and to the total halo is 30 %. An unusual property of the models is that they predict that the average timescale of events towards M31 is shorter than the average timescale towards the LMC. This is because the longer line of sight towards M31 probes more of the far halo where brown dwarfs are the most common constituent.Comment: 17 pages, 1 figure, in press at The Astrophysical Journal (Letters

Breaking up with the continuous exoplanet mass-radius relation

We use a carefully selected subsample of 1053 confirmed exoplanets from the NASA Exoplanet Archive to construct empirical power-law exoplanet mass-radius-temperature ($M$-$R$-$T$) relations. Using orthogonal distance regression to account for errors in both mass and radius, we allow the data to decide: 1) the number of distinct planetary regimes; 2) whether the boundaries of these regimes are best described by broken power laws joined at mass break points, or by discontinuous power laws motivated by changes in equations of state and temperature. We find strong support from the data for three distinct planetary $M$-$R$ regimes and for those regimes to be discontinuous. Our most successful model involves an $M$-$R$-$T$ relation in which ice/rock (rocky) and ice-giant (neptunian) planets are segregated by a pure-ice equation of state, whilst neptunes and gas giant (jovian) planets are segregated by a mass break at $M_{\rm br} = 115\pm19~M_{\oplus}$. The rocky planet regime is shown to follow $M \propto R^{0.34\pm0.01}$, whilst neptunes have $M\propto R^{0.55\pm0.02}$. Planets in both regimes are seen to extend to similar maximum masses. In the jovian regime, we find that $M \propto R^{0.00\pm0.01}T^{0.35\pm 0.02}$, where $T$ is the planet equilibrium temperature. This implies that, for jovian planets detected so far, equilibrium temperature alone provides a robust estimator of mass.Comment: 11 pages, 11 figures. For submission to The Open Journal of Astrophysic