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The famous "Wow" signal detected in 1977 remains arguably the most compelling SETI signal ever found. The original Big Ear data requires that the signal turned on/off over the span of ~3 minutes (time difference between the dual antennae), yet persisted for 72 seconds (duration of a single beam sweep). Combined with the substantial and negative follow-up efforts, these observations limit the allowed range of signal repeat schedules, to the extent that one might question the credibility of the signal itself. Previous work has largely excluded the hypothesis of a strictly periodic repeating source, for periods shorter than 40 hours. However, a non-periodic, stochastic repeater remains largely unexplored. Here, we employ a likelihood emulator using the Big Ear observing logs to infer the probable signal properties under this hypothesis. We find that the maximum a-posteriori solution has a likelihood of 32.3%, highly compatible with the Big Ear data, with a broad 2 $\sigma$ credible interval of signal duration 72 secs < T < 77 mins and mean repeat rate 0.043 1/days < $\lambda$ < 59.8 1/days. We extend our analysis to include 192 hours of subsequent observations from META, Hobart and ATA, which drops the peak likelihood to 1.78%, and thus in tension with the available data at the 2.4 $\sigma$ level. Accordingly, the Wow signal cannot be excluded as a stochastic repeater with available data, and we estimate that 62 days of accumulated additional observations would be necessary to surpass 3 $\sigma$ confidence., Comment: Re-submitted to MNRAS after first review

Accurate quantification of the signal-to-noise ratio (SNR) of a given observational phenomenon is central to associated calculations of sensitivity, yield, completeness and occurrence rate. Within the field of exoplanets, the SNR of a transit has been widely assumed to be the formula that one would obtain by assuming a boxcar light curve, yielding an SNR of the form $(\delta/\sigma_0) \sqrt{D}$. In this work, a general framework is outlined for calculating the SNR of any analytic function and it is applied to the specific case of a trapezoidal transit as a demonstration. By refining the approximation from boxcar to trapezoid, an improved SNR equation is obtained that takes the form $(\delta/\sigma_0) \sqrt{(T_{14}+2T_{23})/3}$. A solution is also derived for the case of a trapezoid convolved with a top-hat, corresponding to observations with finite integration time, where it is proved that SNR is a monotonically decreasing function of integration time. As a rule of thumb, integration times exceeding $T_{14}/3$ lead to a 10% loss in SNR. This work establishes that the boxcar transit is approximate and it is argued that efforts to calculate accurate completeness maps or occurrence rate statistics should either use the refined expression, or even better numerically solve for the SNR of a more physically complete transit model., Comment: Accepted to MNRAS

Co-orbital systems contain two or more bodies sharing the same orbit around a planet or star. The best-known flavors of co-orbital systems are tadpoles (in which two bodies' angular separations oscillate about the L4/L5 Lagrange points $60^\circ$ apart) and horseshoes (with two bodies periodically exchanging orbital energy to trace out a horseshoe shape in a co-rotating frame). Here, we use N-body simulations to explore the parameter space of many-planet horseshoe systems. We show that up to 24 equal-mass, Earth-mass planets can share the same orbit at 1 au, following a complex pattern in which neighboring planets undergo horseshoe oscillations. We explore the dynamics of horseshoe constellations, and show that they can remain stable for billions of years and even persist through their stars' post-main sequence evolution. With sufficient observations, they can be identified through their large-amplitude, correlated transit timing variations. Given their longevity and exotic orbital architectures, horseshoe constellations may represent potential SETI beacons., 10 pages, 10 figures. Published in MNRAS. YouTube playlist with animations of horseshoe constellation systems here: https://www.youtube.com/playlist?list=PLelMZVM3ka3F335LGLxkxrD1ieiLJYQ5N . Blog post here: https://planetplanet.net/2023/04/20/constellations-of-co-orbital-planets/

Communication is an essential asset enabling humankind to forge an advanced civilization. Using approximately 31,000 languages from the Stone Age to our present digital information society, humans have connected and collaborated to accomplish remarkable feats. As the newly dawned Space Age progresses, we are attempting to communicate with intelligent species beyond our world, on distant planets and in Earth’s far future. Absent mutually understood signs, symbols, and semiotic conventions, this study, the “Message in a Bottle”, uses scientific methods to assess and design a means of communication encapsulating the story of humanity, conveying our thoughts, emotions, ingenuity, and aspirations. The message will be structured to provide a universal yet contextual understanding of modern human society, evolution of life on Earth, and challenges for the future. In assembling this space and time capsule, we aim to energize and unite current generations to celebrate and preserve humanity.

Systems of two gravitationally bound exoplanets orbiting a common barycenter outside their physical radii ("binary planets") may result from tidal capture during planet-planet scattering. These objects are expected to form in tight orbits of just a few times their summed radii due to dynamical tides. As a result of their close proximity, their transits overlap heavily, leading to the deceptive illusion of a single planet of larger effective size, an effect compounded in the presence of noisy data and/or long integration times. We show that these illusory single-component transits, dubbed "chimera transits", exhibit large-amplitude Transit Duration Variation (TDV) effects on the order of hours, as well as smaller Transit Timing Variations (TTVs). We compute an analytic approximation for the transit duration upper bound, assuming binary planets with low impact parameter and orbits coplanar with the stellarcentric orbit. We verify the accuracy of our expressions against dynamical simulations of binary Jupiters using the luna algorithm, and provide a Python code for numerical calculations of the TDV signal in binary planet systems (github.com/joheenc/binary-planet-transits). Additionally, chimera transits from binary planets exhibit TTVs of detectable amplitude and high frequency, falling within the recently identified exomoon corridor. Due to their anomalous shapes, depths, and durations, such objects may be flagged as false positives, but could be clearly surveyed for in existing archives., Accepted to MNRAS (https://doi.org/10.1093/mnras/stac3604). Code available at github.com/joheenc/binary-planet-transits

An exomoon will produce transit timing variations (TTVs) upon the parent planet and their undersampled nature causes half of such TTVs to manifest within a frequency range of 2 to 4 cycles, irrespective of exomoon demographics. Here, we search through published Kepler TTV data for such signals, applying a battery of significance and robustness checks, plus independent light curve analyses for candidate signals. Using the original transit times, we identify 11 (ostensibly) single-planets with a robust, significant and fast ($P_{TTV}20 sigma) TTV signal with a periodicity, amplitude and shape consistent with that caused by an exomoon. Our analysis finds that this planet is $8.2_{-0.5}^{+0.7}$ $R_{\oplus}$ orbiting at $0.53_{-0.03}^{+0.04}$ AU around a late G-type dwarf. After forecasting the planetary mass, we expect it to be capable of maintaining at least a 0.3 $M_{\oplus}$ exomoon for 5 Gyr, and the TTV signal corresponds to a moon mass as low as 0.75 Lunar masses. We thus encourage follow-up observations and dynamical analysis of this unique signal, but caution skepticism until such data can be obtained., Accepted to MNRAS. Code and posteriors available at https://github.com/CoolWorlds/fastTTVs

How might an advanced alien civilization manipulate the orbits within a planetary system to create a durable signpost that communicates its existence? While it is still debated whether such a purposeful advertisement would be prudent and wise, we propose that mean-motion resonances between neighboring planets -- with orbital periods that form integer ratios -- could in principle be used to encode simple sequences that one would not expect to form in nature. In this Letter we build four multi-resonant planetary systems and test their long-term orbital stability. The four systems each contain 6 or 7 planets and consist of: (i) consecutive integers from 1 to 6; (ii) prime numbers from 2 to 11; (iii) the Fibonacci sequence from 1 to 13; and (iv) the Lazy Caterer sequence from 1 to 16. We built each system using N-body simulations with artificial migration forces. We evaluated the stability of each system over the full 10 Gyr integration of the Sun's main sequence phase. We then tested the stability of these systems for an additional 10 Gyr, during and after post-main sequence evolution of the central stars (assumed to be Sun-like) to their final, white dwarf phase. The only system that was destabilized was the consecutive integer sequence (system i). The other three sequences therefore represent potential SETI beacons., Comment: Updated acknowledgements. 6 pages, 2 figures, 1 table, accepted for publication in MRNAS

The growing catalog of circumbinary planets strengthens the notion that planets form in a diverse range of conditions across the cosmos. Transiting circumbinary planets yield especially important insights and many examples are now known, in broadly coplanar obits with respect to their binary. Studies of circumbinary disks suggest misaligned transiting examples could also plausibly exist, but their existence would exacerbate the already challenging feat of automatic detection. In this work, we synthesize populations of such planets and consider the number of transits per epoch they produce, forming integer sequences. For isotropic distributions, such sequences will appear foreign to conventional expectation, rarely (~1%) producing the signature double-transits we have come to expect for circumbinaries, instead producing sparse sequences dominated by zero-transit epochs (~80%). Despite their strangeness, we demonstrate that these sequences will be non-random and that the two preceding epochs predict the next to high accuracy. Additionally, we show that even when clustering the transits into grouped epochs, they often appear unphysical if erroneously assuming a single star, due to the missing epochs. Crucially, missing epochs mean highly isotropic populations can trick the observer into assigning the wrong period in up to a quarter of cases, adding further confusion. Finally, we show that the transit sequences encode the inclination distribution and demonstrate a simple inference method that successfully matches the injected truth. Our work highlights how the simple act of flagging transits can be used to provide an initial, vetting-level analysis of misaligned transiting circumbinary planets., 12 pages, 6 figures, 4 tables. Accepted for publication in Monthly Notices of the Royal Astronomical Society, 3 May 2022

Stellar limb darkening affects a wide range of astronomical measurements and is frequently modelled with a parametric model using polynomials in the cosine of the angle between the line of sight and the emergent intensity. Two-parameter laws are particularly popular for cases where one wishes to fit freely for the limb darkening coefficients (i.e. an uninformative prior) due to the compact prior volume and the fact that more complex models rarely obtain unique solutions with present data. In such cases, we show that the two limb darkening coefficients are constrained by three physical boundary conditions, describing a triangular region in the two-dimensional parameter space. We show that uniformly distributed samples may be drawn from this region with optimal efficiency by a technique developed by computer graphical programming: triangular sampling. Alternatively, one can make draws using a uniform, bivariate Dirichlet distribution. We provide simple expressions for these parametrizations for both techniques applied to the case of quadratic, square-root and logarithmic limb darkening laws. For example, in the case of the popular quadratic law, we advocate fitting for q_1 = (u_1+u_2)^2 and q_2 = 0.5u_1(u_1+u_2)^{-1} with uniform priors in the interval [0,1] to implement triangular sampling easily. Employing these parametrizations allows one to derive model parameters which fully account for our ignorance about the intensity profile, yet never explore unphysical solutions, yielding robust and realistic uncertainty estimates. Furthermore, in the case of triangular sampling with the quadratic law, our parametrization leads to significantly reduced mutual correlations and provides an alternative geometric explanation as to why naively fitting the quadratic limb darkening coefficients precipitates strong correlations in the first place., Comment: 11 pages, 5 figures, 2 tables, accepted in MNRAS