arXiv subjects

David Kipping

Publications and source records attributed to David Kipping.

At least 73 records · Page 4Linked to original sources

arXiv2020

Hundreds of new periodic signals detected in the first year of $\it{TESS}$ with the ${\tt weirddetector}$

We apply the ${\tt weirddetector}$, a nonparametric signal detection algorithm based on phase dispersion minimization, in a search for low duty-cycle periodic signals in the Transiting Exoplanet Survey Satellite (${\it TESS}$) photometry. Our approach, in contrast to commonly used model-based approaches specifically for flagging transits, eclipsing binaries, or other similarly periodic events, makes minimal assumptions about the shape of a periodic signal, with the goal of finding "weird" signals of unexpected or arbitrary shape. In total, 248,301 ${\it TESS}$ sources from the first-year Southern sky survey are run through the ${\tt weirddetector}$, of which we manually inspect the top 21,500 for periodicity. To minimize false-positives, we here only report on the upper decile in terms of signal score, a sample for which we obtain 97% recall of ${\it TESS}$ eclipsing binaries and 62% of the TOIs. In our sample, we find 377 previously unreported periodic signals, for which we make a first-pass assignment that 26 are ultra-short periods ($<0.3$ d), 313 are likely eclipsing binaries, 28 appear planet-like, and 10 are miscellaneous signals.

astro-ph.EP↗

arXiv2020

A Bayesian Approach to the Simulation Argument

The Simulation Argument posed by Bostrom (2003) suggests that we may be living inside a sophisticated computer simulation. If post-human civilizations eventually have both the capability and desire to generate such Bostrom-like simulations, then the number of simulated realities would greatly exceed the one base reality, ostensibly indicating a high probability that we do not live in said base reality. In this work, it is argued that since the hypothesis that such simulations are technically possible remains unproven, then statistical calculations need to consider not just the number of state spaces, but the intrinsic model uncertainty. This is achievable through a Bayesian treatment of the problem, which is presented here. Using Bayesian model averaging, it is shown that the probability that we are sims is in fact less than 50%, tending towards that value in the limit of an infinite number of simulations. This result is broadly indifferent as to whether one conditions upon the fact that humanity has not yet birthed such simulations, or ignore it. As argued elsewhere, it is found that if humanity does start producing such simulations, then this would radically shift the odds and make it very probable that we are in fact sims.

physics.pop-ph↗

arXiv2020

An Independent Analysis of the Six Recently Claimed Exomoon Candidates

It has been recently claimed that KOIs-268.01, 303.01, 1888.01, 1925.01, 2728.01 & 3320.01 are exomoon candidates, based on an analysis of their transit timing. Here, we perform an independent investigation, which is framed in terms of three questions: 1) Are there significant excess TTVs? 2) Is there a significant periodic TTV? 3) Is there evidence for a non-zero moon mass? We applied rigorous statistical methods to these questions alongside a re-analysis of the Kepler photometry and find that none of the KOIs satisfy these three tests. Specifically, KOIs-268.01 & 3220.01 pass none of the tests and KOIs-303.01, 1888.01 & 1925.01 pass a single test each. Only KOI-2728.01 satisfies two, but fails the cross-validation test for predictions. Further, detailed photodynamical modeling reveals that KOI-2728.01 favours a negative radius moon (as does KOI-268.01). We also note that we find a significant photoeccentric for KOI-1925.01 indicating an eccentric orbit of e>(0.62+/-0.06). For comparison, we applied the same tests to Kepler-1625b, which reveals that 1) and 3) are passed, but 2) cannot be checked with the cross-validation method used here, due to the limited number of available epochs. In conclusion, we find no compelling evidence for exomoons amongst the six KOIs. Despite this, we're able to derive exomoon mass upper limits versus semi-major axis, with KOI-3220.01 leading to particularly impressive constraints of Ms/Mp < 0.4% [2 sigma] at a similar relative semi-major to that of the Earth-Moon.

astro-ph.EP↗

arXiv2020

A planet within the debris disk around the pre-main-sequence star AU Microscopii

AU Microscopii (AU Mic) is the second closest pre main sequence star, at a distance of 9.79 parsecs and with an age of 22 million years. AU Mic possesses a relatively rare and spatially resolved3 edge-on debris disk extending from about 35 to 210 astronomical units from the star, and with clumps exhibiting non-Keplerian motion. Detection of newly formed planets around such a star is challenged by the presence of spots, plage, flares and other manifestations of magnetic activity on the star. Here we report observations of a planet transiting AU Mic. The transiting planet, AU Mic b, has an orbital period of 8.46 days, an orbital distance of 0.07 astronomical units, a radius of 0.4 Jupiter radii, and a mass of less than 0.18 Jupiter masses at 3 sigma confidence. Our observations of a planet co-existing with a debris disk offer the opportunity to test the predictions of current models of planet formation and evolution.

astro-ph.EP↗

arXiv2020

An Objective Bayesian Analysis of Life's Early Start and Our Late Arrival

Life emerged on the Earth within the first quintile of its habitable window, but a technological civilization did not blossom until its last. Efforts to infer the rate of abiogenesis, based on its early emergence, are frustrated by the selection effect that if the evolution of intelligence is a slow process, then life's early start may simply be a prerequisite to our existence, rather than useful evidence for optimism. In this work, we interpret the chronology of these two events in a Bayesian framework, extending upon previous work by considering that the evolutionary timescale is itself an unknown that needs to be jointly inferred, rather than fiducially set. We further adopt an objective Bayesian approach, such that our results would be agreed upon even by those using wildly different priors for the rates of abiogenesis and evolution - common points of contention for this problem. It is then shown that the earliest microfossil evidence for life indicates that the rate of abiogenesis is at least 2.8 times more likely to be a typically rapid process, rather than a slow one. This modest limiting Bayes factor rises to 8.7 if we accept the more disputed evidence of C13 depleted zircon deposits (Bell et al. 2015). For intelligence evolution, it is found that a rare-intelligence scenario is slightly favored at 3:2 betting odds. Thus, if we re-ran Earth's clock, one should statistically favor life to frequently re-emerge, but intelligence may not be as inevitable.

astro-ph.EP↗

arXiv2020

8 in 10 Stars in the Milky Way Bulge Experience Stellar Encounters Within 1000 AU in a Gigayear

The Galactic bulge is a tumultuous dense region of space, packed with stars separated by far smaller distances than those in the Solar neighborhood. A quantification of the frequency and proximity of close stellar encounters in this environment dictates the exchange of material, disruption of planetary orbits, and threat of sterilizing energetic events. We present estimated encounter rates for stars in the Milky Way bulge found using a combination of numerical and analytical methods. By integrating the orbits of bulge stars with varying orbital energy and angular momentum to find their positions over time, we were able to estimate how many close stellar encounters the stars should experience as a function of orbit shape. We determined that ~80% of bulge stars have encounters within 1000 AU and that half of bulge stars will have >35 such encounters, both over a gigayear. Our work has interesting implications for the long-term survivability of planets in the Galactic bulge.

astro-ph.GA↗

arXiv2020

Detection of the phase curve and occultation of WASP-100b with TESS

We report the detection of the full orbital phase curve and occultation of the hot-Jupiter WASP-100b using TESS photometry. The phase curve is isolated by suppressing low frequency stellar and instrumental modes using both a non-parametric harmonic notch filter (phasma) and semi-sector long polynomials. This yields a phase curve signal of (73 +/- 9) ppm amplitude, preferred over a null-model by deltaBIC = 25, indicating very strong evidence for an observed effect. We recover the occultation event with a suite of five temporally localized tools, including Gaussian processes and cosine filtering. This allows us to infer an occultation depth of (100 +/- 14) ppm, with an additional +/- 16 ppm systematic error from the differences between methods. We regress a model including atmospheric reflection, emission, ellipsoidal variations and Doppler beaming to the combined phase curve and occultation data. This allows us to infer that WASP-100b has a geometric albedo of A_g = 0.16 (+0.04, -0.03) in the TESS bandpass, with a maximum dayside brightness temperature of (2710 +/- 100) K and a warm nightside temperature of (2380 [+170, -200]) K. Additionally, we find evidence that WASP-100b has a high thermal redistribution efficiency, manifesting as a substantial eastward hotspot offset of (71 [+2, -4]) degrees. These results present the first measurement of a thermal phase shift among the phase curves observed by TESS so far, and challenge the predicted efficiency of heat transport in the atmospheres of ultra-hot Jupiters.

astro-ph.EP↗

arXiv2020

Kepler's Dark Worlds: a Low Albedo for an Ensemble of Neptunian and Terran Exoplanets

Photometric phase curves provide an important window onto exoplanetary atmospheres and potentially even their surfaces. With similar amplitudes to occultations but far longer baselines, they have a higher sensitivity to planetary photons at the expense of a more challenging data reduction in terms of long-term stability. In this work, we introduce a novel non-parametric algorithm dubbed "phasma" to produce clean, robust exoplanet phase curves and apply it to 115 Neptunian and 50 Terran exoplanets observed by Kepler. We stack the signals to further improve signal-to-noise, and measure an average Neptunian albedo of Ag < 0.23 to 95% confidence, indicating a lack of bright clouds consistent with theoretical models. Our Terran sample provides the first constraint on the ensemble albedo of exoplanets which are most likely solid, constraining Ag < 0.42 to 95% confidence. In agreement with our constraint on the greenhouse effect, our work implies that Kepler's solid planets are unlikely to resemble cloudy Venusian analogs, but rather dark Mercurian rocks.

astro-ph.EP↗

arXiv2020

Loose Ends for the Exomoon Candidate Host Kepler-1625b

The claim of an exomoon candidate in the Kepler-1625b system has generated substantial discussion regarding possible alternative explanations for the purported signal. In this work we examine in detail these possibilities. First, the effect of more flexible trend models is explored and we show that sufficiently flexible models are capable of attenuating the signal, although this is an expected byproduct of invoking such models. We also explore trend models using X and Y centroid positions and show that there is no data-driven impetus to adopt such models over temporal ones. We quantify the probability that the 500 ppm moon-like dip could be caused by a Neptune-sized transiting planet to be < 0.75%. We show that neither autocorrelation, Gaussian processes nor a Lomb-Scargle periodogram are able to recover a stellar rotation period, demonstrating that K1625 is a quiet star with periodic behavior < 200 ppm. Through injection and recovery tests, we find that the star does not exhibit a tendency to introduce false-positive dip-like features above that of pure Gaussian noise. Finally, we address a recent re-analysis by Kreidberg et al (2019) and show that the difference in conclusions is not from differing systematics models but rather the reduction itself. We show that their reduction exhibits i) slightly higher intra-orbit and post-fit residual scatter, ii) $\simeq$ 900 ppm larger flux offset at the visit change, iii) $\simeq$ 2 times larger Y-centroid variations, and iv) $\simeq$ 3.5 times stronger flux-centroid correlation coefficient than the original analysis. These points could be explained by larger systematics in their reduction, potentially impacting their conclusions.

astro-ph.EP↗

arXiv2019

The "Terrascope": On the Possibility of Using the Earth as an Atmospheric Lens

Distant starlight passing through the Earth's atmosphere is refracted by an angle of just over one degree near the surface. This focuses light onto a focal line starting at an inner (and chromatic) boundary out to infinity - offering an opportunity for pronounced lensing. It is shown here that the focal line commences at ~85% of the Earth-Moon separation, and thus placing an orbiting detector between here and one Hill radius could exploit this refractive lens. Analytic estimates are derived for a source directly behind the Earth (i.e. on-axis) showing that starlight is lensed into a thin circular ring of thickness $W H_Δ/R$, yielding an amplification of $8 H_Δ/W$, where $H_Δ$ is the Earth's refractive scale height, $R$ is its geopotential radius and $W$ is the detector diameter. These estimates are verified through numerical ray-tracing experiments from optical to 30 micron light with standard atmospheric models. The numerical experiments are extended to include extinction from both a clear atmosphere and one with clouds. It is found that a detector at one Hill radius is least affected by extinction since lensed rays travel no deeper than 13.7 km, within the stratosphere and above most clouds. Including extinction, a 1 metre Hill radius 'terrascope' is calculated to produce an amplification of ~45,000 for a lensing timescale of ~20 hours. In practice, the amplification is likely halved in order to avoid daylight scattering i.e. 22,500 ($Δ$mag=10.9) for $W=$1 metre, or equivalent to a 150 metre optical/infrared telescope.

astro-ph.IM↗

arXiv2019

Not Gone with the Wind: Planet Occurrence is Independent of Stellar Galactocentric Velocity

We demonstrate that planet occurrence does not depend on stellar galactocentric velocity in the Solar neighborhood. Using Gaia DR2 astrometry and radial velocity data, we calculate 3D galactocentric velocities for 197,090 Kepler field stars, 1647 of which are confirmed planet hosts. When we compare the galactocentric velocities of planet hosts to those of the entire field star sample, we observe a statistically significant (KS p-value = $10^{-70}$) distinction, with planet hosts being apparently slower than field stars by $\sim$40 km/s. We explore some potential explanations for this difference and conclude that it is not a consequence of the planet-metallicity relation or distinctions in the samples' thin/thick disk membership, but rather an artefact of Kepler's selection function. Non Kepler-host stars that have nearly identical distances, temperatures, surface gravities, and Kepler magnitudes to the confirmed planet hosts also have nearly identical velocity distributions. Using one of these identical non-host samples, we consider that the probability of a star with velocity $v_{\mathrm{tot}}$ hosting a planet can be described by an exponential function proportional to $e^{(-v_{\mathrm{tot}}/v_0)}$. Using a Markov Chain Monte Carlo sampler, we determine that $v_0 > $976 km/s to 99% confidence, which implies that planets in the Solar neighborhood are just as likely to form around fast stars as they are around slow stars. Our work highlights the subtle ways in which selection biases can create strong correlations without physical underpinnings.

astro-ph.EP↗

arXiv2019

Technosignatures in Transit

Kepler, K2, TESS, and similar time-domain photometric projects, while designed with exoplanet detection in mind, are also well-suited projects for searches for large artificial structures orbiting other stars in the Galaxy. An effort to examine these data sets with an eye towards non-spherical or otherwise anomalous transit events, and a robust follow-up program to understand the stars and occulters that generate them, would enable the first robust upper limits on such megastructures in terms of their sizes, occurrence rates, and orbital properties. Such work also has the ancillary benefit of improving our understanding of stellar photometric variability and orbital and physical parameter estimation of exoplanets from photometric time series, and may lead to the identification of new, unexpected classes of stellar variables and exoplanets. Ultimately, searching for the most unusual and anomalous signatures benefits not only the search for technologies, but also the entire astronomical community by uncovering new mysteries to advance our understanding of the Universe.

astro-ph.IM↗

arXiv2019

The multiplicity distribution of Kepler's exoplanets

The true multiplicity distribution of transiting planet systems is obscured by strong observational biases, leading low-multiplicity systems to be overrepresented in the observed sample. Using the Kepler FGK planet hosts, we employ approximate Bayesian computation to infer the multiplicity distribution by comparing simulated catalogs to the observed one. After comparing a total of ten different multiplicity distributions, half of which were two-population models, to the observed data, we find that a single-population model following a Zipfian distribution is able to explain the Kepler data as well as any of the dichotomous models we test. Our work provides another example of a way to explain the observed Kepler multiplicities without invoking a dichotomous planet population. Using our preferred Zipfian model, we estimate that an additional $2393_{-717}^{+904}$ planets likely reside in the 1537 FGK Kepler systems studied in this work, which would increase the planet count by a factor of $2.22_{-0.36}^{+0.46}$. Of these hidden worlds, $663_{-151}^{+158}$ are expected to reside in ostensibly single-transiting-planet systems, meaning that an additional planet(s) is expected for approximately 1-in-2 such Kepler systems.

astro-ph.EP↗

arXiv2019

Proxima Centauri b is not a transiting exoplanet

We report Spitzer Space Telescope observations during predicted transits of the exoplanet Proxima Centauri b. As the nearest terrestrial habitable-zone planet we will ever discover, any potential transit of Proxima b would place strong constraints on its radius, bulk density, and atmosphere. Subsequent transmission spectroscopy and secondary-eclipse measurements could then probe the atmospheric chemistry, physical processes, and orbit, including a search for biosignatures. However, our photometric results rule out planetary transits at the 200~ppm level at 4.5$~μm$, yielding a 3$σ$ upper radius limit of 0.4~$R_\rm{\oplus}$ (Earth radii). Previous claims of possible transits from optical ground- and space-based photometry were likely correlated noise in the data from Proxima Centauri's frequent flaring. Follow-up observations should focus on planetary radio emission, phase curves, and direct imaging. Our study indicates dramatically reduced stellar activity at near-to-mid infrared wavelengths, compared to the optical. Proxima b is an ideal target for space-based infrared telescopes, if their instruments can be configured to handle Proxima's brightness.

astro-ph.EP↗

arXiv2019

A resonant pair of warm giant planets revealed by TESS

We present the discovery of a pair of transiting giant planets, TOI-216b and c, using four sectors of TESS photometry. TOI-216 is a $0.87 M_{\odot}$ dwarf orbited by two transiters with radii of $8.2 R_{\oplus}$ and $11.3 R_{\oplus}$, and periods of $17.01$d and $34.57$d, respectively. Anti-correlated TTVs are clearly evident indicating that the transiters orbit the same star and interact via a near 2:1 mean motion resonance. By fitting the TTVs with a dynamical model, we infer masses of $26_{-11}^{+24} M_{\oplus}$ and $190_{-80}^{+220} M_{\oplus}$, establishing that the objects are planetary in nature and have likely sub-Kronian and Kronian densities. TOI-216 lies close to the southern ecliptic pole and thus will be observed by TESS throughout the first year, providing an opportunity for continuous dynamical monitoring and considerable refinement of the dynamical masses presented here. TOI-216 closely resembles Kepler-9 in architecture, and we hypothesize that in such systems these Saturn-analogs failed to fully open a gap and thus migrated far deeper into the system before becoming trapped into resonance, which would imply that future detections of new analogs may also have sub-Jupiter masses.

astro-ph.EP↗

arXiv2019

The Weird Detector: Flagging periodic, coherent signals of arbitrary shape in time series photometry

By design, model-based approaches for flagging transiting exoplanets in light curves, such as boxed least squares, excel at detecting planets with low S/N at the expense of finding signals that are not well described by the assumed model, such as self-lensing binaries, disintegrating or evaporating planets, or planets with large rings. So far, such signals have typically been found through visual searches by professional or citizen scientists, or by inspection of the photometric power-spectra. We present a nonparametric detection algorithm, for short duty-cycle periodic signals in photometric time series based on phase dispersion minimization. We apply our code to 161,786 Kepler sources and detect 18 new periodic signals consistent with heartbeat binaries/planets, 4 new singly-transiting systems, and 2 new doubly-transiting systems. We show that our code is able to recover the majority of known Kepler objects of interest (KOIs) to high confidence, as well as more unusual events such as Boyajian's star and a comet passing through the Kepler field. Nonparametric signal-flagging techniques, such as the one presented here, will become increasingly valuable with the coming data from TESS and future transit surveys as the volume of data available to us exceeds that which can be feasibly examined manually.

astro-ph.EP↗

arXiv2019

The Halo Drive: Fuel-Free Relativistic Propulsion of Large Masses via Recycled Boomerang Photons

Gravitational slingshots around a neutron star in a compact binary have been proposed as a means of accelerating large masses to potentially relativistic speeds. Such a slingshot is attractive since fuel is not expended for the acceleration, however it does entail a spacecraft diving into close proximity of the binary, which could be hazardous. It is proposed here that such a slingshot can be performed remotely using a beam of light which follows a boomerang null geodesic. Using a moving black hole as a gravitational mirror, kinetic energy from the black hole is transferred to the beam of light as a blueshift and upon return the recycled photons not only accelerate, but also add energy to, the spacecraft. It is shown here that this gained energy can be later expended to reach a terminal velocity of approximately 133% the velocity of the black hole. A civilization could exploit black holes as galactic way points but would be difficult to detect remotely, except for an elevated binary merger rate and excess binary eccentricity.

gr-qc↗

arXiv2018

Shadow Imaging of Transiting Objects

We consider the problem of inferring the shape of a transiting object's silhouette from its light curve alone, without assuming a physical model for the object. We model the object as a grid of pixels which transits the star; each pixel has an opacity, ranging from transparent to opaque, which we infer from the light curve. We explore three interesting degeneracies inherent to this problem, in which markedly different transiting shapes can produce identical light curves: (i) the "flip" degeneracy, by which two pixels transiting at the same impact parameter on opposite sides of the star's horizontal midplane generate the same light curve; (ii) the "arc" degeneracy, by which opacity can be redistributed along the semicircular arc of pixels which undergoes ingress or egress at the same time without consequence to the light curve, and (iii) the "stretch" degeneracy, by which a wide shape moving fast can produce the same light curve as a narrow shape moving more slowly. By understanding these degeneracies and adopting some additional assumptions, we are able to numerically recover informative shadow images of transiting objects, and we explore a number of different algorithmic approaches to this problem. We apply our methods to real data, including the TRAPPIST-1c,e,f triple transit and two dips of Boyajian's Star. We provide Python code to calculate the transit light curve of any grid and, conversely, infer the image grid which generates any light curve in a software package accompanying this paper, EightBitTransit.

astro-ph.EP↗