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David Kipping

Publications and source records attributed to David Kipping.

At least 37 records · Page 2Linked to original sources
arXiv2025

Strong Evidence That Abiogenesis Is a Rapid Process on Earth Analogs

The early start to life naively suggests that abiogenesis is a rapid process on Earth-like planets. However, if evolution typically takes ~4Gyr to produce intelligent life-forms like us, then the limited lifespan of Earth's biosphere (~5-6Gyr) necessitates an early (and possibly highly atypical) start to our emergence - an example of the weak anthropic principle. Our previously proposed objective Bayesian analysis of Earth's chronology culminated in a formula for the minimum odds ratio between the fast and slow abiogenesis scenarios (relative to Earth's lifespan). Timing from microfossils (3.7Gya) yields 3:1 odds in favor of rapid abiogenesis, whereas evidence from carbon isotopes (4.1Gya) gives 9:1, both below the canonical threshold of "strong evidence" (10:1). However, the recent result of a 4.2Gya LUCA pushes the odds over the threshold for the first time (nominally 13:1). In fact, the odds ratio is >10:1 for all possible values of the biosphere's ultimate lifespan and speculative hypotheses of ancient civilizations. For the first time, we have formally strong evidence that favors the hypothesis that life rapidly emerges in Earth-like conditions (although such environments may themselves be rare).

astro-ph.EP↗
arXiv2025

Near-circular orbits for planets around M/K-type stars with Earth-like sizes and instellations

Recent advances have enabled the discovery of a population of potentially Earth-like planets, yet their orbital eccentricity, which governs their climate and provides clues about their origin and dynamical history, is still largely unconstrained. We identify a sample of 17 transiting exoplanets around late-type stars with similar radii and irradiation to that of Earth and use the "photoeccentric effect" - which exploits transit durations - to infer their eccentricity distribution via hierarchical Bayesian modelling. Our analysis establishes that these worlds further resemble Earth in that their eccentricities are nearly circular (mean eccentricity $=0.060_{-0.028}^{+0.040}$ and $\leq0.15$), with the exception of one outlier of moderate eccentricity. The results hint at a subset population of dynamically warmer Earths, but this requires a larger sample to statistically confirm. The planets in our sample are thus largely subject to minimal eccentricity-induced seasonal variability and are consistent with emerging via smooth disk migration rather than violent planet-planet scattering.

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arXiv2024

Do SETI Optimists Have a Fine-Tuning Problem?

In ecological systems, be it a petri dish or a galaxy, populations evolve from some initial value (say zero) up to a steady state equilibrium, when the mean number of births and deaths per unit time are equal. This equilibrium point is a function of the birth and death rates, as well as the carrying capacity of the ecological system itself. We show that the occupation fraction versus birth-to-death rate ratio is S-shaped, saturating at the carrying capacity for large birth-to-death rate ratios and tending to zero at the other end. We argue that our astronomical observations appear inconsistent with a cosmos saturated with ETIs, and thus SETI optimists are left presuming that the true population is somewhere along the transitional part of this S-curve. Since the birth and death rates are a-priori unbounded, we argue that this presents a fine-tuning problem. Further, we show that if the birth-to-death rate ratio is assumed to have a log-uniform prior distribution, then the probability distribution of the ecological filling fraction is bi-modal - peaking at zero and unity. Indeed, the resulting distribution is formally the classic Haldane prior, conceived to describe the prior expectation of a Bernoulli experiment, such as a technological intelligence developing (or not) on a given world. Our results formally connect the Drake Equation to the birth-death formalism, the treatment of ecological carrying capacity and their connection to the Haldane perspective.

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arXiv2024

squishyplanet: Modeling Transits of Non-spherical Exoplanets in JAX

While astronomers often assume that exoplanets are perfect spheres when analyzing observations, the subset of these distant worlds that are subject to strong tidal forces and/or rapid rotations are expected to be distinctly ellipsoidal or even triaxial. Since a planet's response to these forces is determined in part by its interior structure, measurements of an exoplanet's deviations from spherical symmetry can lead to powerful insights into its composition and surrounding environment. These shape deformations will imprint themselves on a planet's phase curve and transit lightcurve and cause small (1s-100s of parts per million) deviations from their spherical-planet counterparts. Until recently, these deviations were undetectable in typical real-world datasets due to limitations in photometric precision. Now, however, current and soon-to-come-online facilities such as JWST will routinely deliver observations that warrant the consideration of more complex models. To this end we present squishyplanet, a JAX-based Python package that implements an extension of the polynomial limb-darkened transit model presented in Agol et al. 2020 to non-spherical (triaxial) planets, as well as routines for modeling reflection and emission phase curves.

astro-ph.IM↗
arXiv2024

The SNR of idealised radial velocity signals

One of the most basic quantities relevant to planning observations and assessing detection bias is the signal-to-noise ratio (SNR). Remarkably, the SNR of an idealised radial velocity (RV) signal has not been previously derived beyond scaling behaviours and ignoring orbital eccentricity. In this work, we derive the RV SNR for three relevant cases to observers. First, we consider a single mass orbiting a star, revealing the expected result that $\mathrm{SNR}\propto K \sqrt{T}$, where $T$ is the observing window, but an additional dependency on eccentricity and argument of periastron. We show that the RV method is biased towards companions with their semi-major axes aligned to the observer, which is physically intuitive, but also less obviously that the marginalised bias to eccentricity is negligible until one reaches very high eccentricities. Second, we derive the SNR necessary to discriminate eccentric companions from 2:1 resonance circular orbits, although our result is only valid for eccentricities $e\lesssim0.3$. We find that the discriminatory SNR is $(9/8) e^2 (1-e^2)^{-1/2}$ times that of the eccentric planet solution's SNR, and is thus typically an order-of-magnitude less. Finally, we have obtained a semi-empirical expression for the SNR of the idealised Rossiter-McLaughlin effect, revealing the bias with respect to spin-orbit alignment angle. Our formula is valid to within 10% accuracy in 95.45% of the training samples used (for $b\leq0.8$), but larger deviations occur when comparing to different RM models.

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arXiv2024

A Reply to: Large Exomoons unlikely around Kepler-1625 b and Kepler-1708 b

Recently, Heller & Hippke argued that the exomoon candidates Kepler-1625 b-i and Kepler-1708 b-i were allegedly 'refuted'. In this Matters Arising, we address these claims. For Kepler-1625 b, we show that their Hubble light curve is identical to that previously published by the same lead author, in which the moon-like dip was recovered. Indeed, our fits of their data again recover the moon-like dip with improved residuals than that obtained by Heller & Hippke. Their fits therefore appear to have somehow missed this deeper likelihood maximum, as well producing apparently unconverged posteriors. Consequently, their best-fitting moon is the same radius as the planet, Kepler-1625 b; a radically different signal from that which was originally claimed. The authors then inject this solution into the Kepler data and remark, as a point of concern, how retrievals obtain much higher significances than originally reported. However, this issue stems from the injection of a fundamentally different signal. We demonstrate that their Hubble light curve exhibits ~20% higher noise and discards 11% of the useful data, which compromises its ability to recover the subtle signal of Kepler-1625 b-i. For Kepler-1708 b-i it was claimed that the exomoon model's Bayes factor is highly sensitive to detrending choices, yielding reduced evidence with a biweight filter versus the original claim. We use their own i) detrended light curve and ii) biweight filter code to investigate these claims. For both, we recover the original moon signal, to even higher confidence than before. The discrepancy is explained by comparing to their quoted fit metrics, where we again demonstrate that the Heller & Hippke regression definitively missed the deeper likelihood maximum corresponding to Kepler-1708 b-i. We conclude that both candidates remain viable but certainly demand further observations.

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arXiv2024

Giant Outer Transiting Exoplanet Mass (GOT 'EM) Survey. IV. Long-term Doppler Spectroscopy for 11 Stars Thought to Host Cool Giant Exoplanets

Discovering and characterizing exoplanets at the outer edge of the transit method's sensitivity has proven challenging owing to geometric biases and the practical difficulties associated with acquiring long observational baselines. Nonetheless, a sample of giant exoplanets on orbits longer than 100 days has been identified by transit hunting missions. We present long-term Doppler spectroscopy for 11 such systems with observation baselines spanning a few years to a decade. We model these radial velocity observations jointly with transit photometry to provide initial characterizations of these objects and the systems in which they exist. Specifically, we make new precise mass measurements for four long-period giant exoplanets (Kepler-111 c, Kepler-553 c, Kepler-849 b, and PH-2 b), we place new upper limits on mass for four others (Kepler-421 b, KOI-1431.01, Kepler-1513 b, and Kepler-952 b), and we show that several "confirmed" planets are in fact not planetary at all. We present these findings to complement similar efforts focused on closer-in short-period giant planets, and with the hope of inspiring future dedicated studies of cool giant exoplanets.

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arXiv2023

The giant nature of WD 1856 b implies that transiting rocky planets are rare around white dwarfs

White dwarfs (WDs) have roughly Earth-sized radii - a fact long recognized to facilitate the potential discovery of sub-Earth sized planets via transits, as well atmospheric characterization including biosignatures. Despite this, the first (and still only) transiting planet discovered in 2020 was a roughly Jupiter-sized world, found using TESS photometry. Given the relative paucity of giant planets compared to terrestrials indicated by both exoplanet demographics and theoretical simulations (a "bottom-heavy" radius distribution), this is perhaps somewhat surprising. Here, we quantify the surprisingness of this fact accounting for geometric bias and detection bias assuming 1) a bottom-heavy Kepler derived radius distribution, and 2) a top-heavy radial velocity inspired radius distribution. Both are concerning, with the latter implying rocky planets are highly unusual and the former implying WD 1856 b would have to be highly surprising event at the <0.5% level. Using an HBM, we infer the implied power-law radius distribution conditioned upon WD 1856 b and arrive at a top-heavy distribution, such that 0.1-2 REarth planets are an order-of-magnitude less common than 2-20 REarth planets in the period range of 0.1-10 days. The implied hypothesis is that transiting WD rocky planets are rare. We discuss ways to reconcile this with other evidence for minor bodies around WDs, and ultimately argue that it should be easily testable.

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arXiv2023

Deconstructing Alien Hunting

The search for extraterrestrial (alien) life is one of the greatest scientific quests yet raises fundamental questions about just what we should be looking for and how. We approach alien hunting from the perspective of an experimenter engaging in binary classification with some true and confounding positive probability (TPP and CPP). We derive the Bayes factor in such a framework between two competing hypotheses, which we use to classify experiments as either impotent, imperfect or ideal. Similarly, the experimenter can be classified as dogmatic, biased or agnostic. We show how the unbounded explanatory and evasion capability of aliens poses fundamental problems to experiments directly seeking aliens. Instead, we advocate framing the experiments as looking for that outside of known processes, which means the hypotheses we test do not directly concern aliens per se. To connect back to aliens requires a second level of model selection, for which we derive the final odds ratio in a Bayesian framework. This reveals that it is fundamentally impossible to ever establish alien life at some threshold odds ratio, $\mathcal{O}_{\mathrm{crit}}$, unless we deem the prior probability that some as-yet-undiscovered natural process could explain the event is less than $(1+\mathcal{O}_{\mathrm{crit}})^{-1}$. This elucidates how alien hunters need to carefully consider the challenging problem of how probable unknown unknowns are, such as new physics or chemistry, and how it is arguably most fruitful to focus on experiments for which our domain knowledge is thought to be asymptotically complete.

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arXiv2023

Message in a Bottle -- An Update to the Golden Record

In this first part of our series, we delve into the foundational aspects of the "Message in a Bottle" (henceforth referred to as MIAB). This study stands as a continuation of the legacy set by the Voyager Golden Records launched aboard Voyager 1 and 2 in 1977, which aimed to communicate with intelligent species beyond our world. These Records continue to serve not only as a snapshot of Earth and humanity but also carry forth our desire for establishing contact with advanced alien civilizations. Given the absence of mutually understood signs, symbols, and semiotic conventions, MIAB, like its predecessor, seeks to use scientific methods to design an innovative means of communication encapsulating the story of humanity. Our aim is to convey our collective knowledge, feelings, innovations, and aspirations in a manner that offers a universal, yet contextual understanding of human society, the evolution of life on Earth, and our hopes and concerns for the future. Through this time and space traveling capsule, we also strive to inspire and unify current and future generations to celebrate and safeguard our shared human experience.

cs.CY↗
arXiv2023

Not So Fast Kepler-1513: A Perturbing Planetary Interloper in the Exomoon Corridor

Transit Timing Variations (TTVs) can be induced by a range of physical phenomena, including planet-planet interactions, planet-moon interactions, and stellar activity. Recent work has shown that roughly half of moons would induce fast TTVs with a short period in the range of two-to-four orbits of its host planet around the star. An investigation of the Kepler TTV data in this period range identified one primary target of interest, Kepler-1513 b. Kepler-1513 b is a $8.05^{+0.58}_{-0.40}$ $R_\oplus$ planet orbiting a late G-type dwarf at $0.53^{+0.04}_{-0.03}$ AU. Using Kepler photometry, this initial analysis showed that Kepler-1513 b's TTVs were consistent with a moon. Here, we report photometric observations of two additional transits nearly a decade after the last Kepler transit using both ground-based observations and space-based photometry with TESS. These new transit observations introduce a previously undetected long period TTV, in addition to the original short period TTV signal. Using the complete transit dataset, we investigate whether a non-transiting planet, a moon, or stellar activity could induce the observed TTVs. We find that only a non-transiting perturbing planet can reproduce the observed TTVs. We additionally perform transit origami on the Kepler photometry, which independently applies pressure against a moon hypothesis. Specifically, we find that Kepler-1513 b's TTVs are consistent with an exterior non-transiting $\sim$Saturn mass planet, Kepler-1513 c, on a wide orbit, $\sim$5$\%$ outside a 5:1 period ratio with Kepler-1513 b. This example introduces a previously unidentified cause for planetary interlopers in the exomoon corridor, namely an insufficient baseline of observations.

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arXiv2023

RoSETZ: Roman Survey of the Earth Transit Zone -- a SETI-optimized survey for habitable-zone exoplanets

In this White Paper for Nancy Grace Roman Space Telescope (Roman) science, we propose the Roman Survey of the Earth Transit Zone (RoSETZ), a transit search for rocky planets within the habitable zones (HZs) of stars located within the Earth Transit Zone (ETZ). The ETZ holds special interest in the search for extra-terrestrial intelligence (SETI) - observers on planets within the ETZ can see Earth as a transiting planet. RoSETZ would augment the Roman Galactic Bulge Time Domain Survey (GBTDS) as an additional field located $\sim 5$~degrees away from other GBTDS fields. Our simulations show that RoSETZ alone can find from 120 to 630 Earth-sized HZ planets around K- and M-type hosts, with the range reflecting different survey design assumptions. These yields are 5-20 times the number currently known. Such a sample will transform our knowledge of ``Eta-Earth'' ($η_{\oplus}$) -- the occurrence of Earth-sized HZ planets -- and would be the first catalogue of exoplanets selected in a manner optimized according to the Mutual Detectability targetted-SETI strategy. If it can be accommodated alongside the existing GBTDS design, we favour a RoSETZ-Max design that is observed for the duration of the GBTDS. If not, we show that a slimmed-down RoSETZ-Lite design, occupying two GBTDS seasons, would not significantly impact overall GBTDS exoplanet yields, even if time allocated to it had to come from time allocations to other fields. We argue that the angular separation of RoSETZ from other GBTDS fields permits self-calibration of systematic uncertainties that would otherwise hamper exoplanet demographic modelling of both microlensing and transit datasets. Other science possible with RoSETZ data include studies of small solar system bodies and high resolution 3D extinction mapping.

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arXiv2023

Lazarus Stars: Numerical investigations of stellar evolution with star-lifting as a life extension strategy

The aging and gradual brightening of the Sun will challenge Earth's habitability in the next few billion years. If life exists elsewhere in the Universe, the aging of its host star similarly poses an existential threat. One solution, which we dub a Lazarus star, is for an advanced civilization to remove (or star-lift) mass from their host star at a rate that offsets the increase in luminosity, keeping the flux on the habitable planet(s) constant and extending the lifetime of their star. While this idea has existed since 1985 when it was first proposed by Criswell, numerical investigations of star-lifting have been lacking. Here, we use the stellar evolution code MESA to find mass vs. age and $\dot{M}$ vs. age relations which would hold the flux on surrounding planets constant. We explore initial mass ranging from $0.2{\rm M}_{\odot}$ to $1.2{\rm M}_{\odot}$. For most stars with a mass initially below about $ 0.4 {\rm M}_{\odot}$, we find that star-lifting increases their main-sequence lifetimes up to $500$ Gyr until they approach the hydrogen burning limit and star-lifting is no longer possible. For more massive stars, star-lifting increase main-sequence lifetimes by 1 Gyr to 100 Gyr, though they still enter the red-giant phase. For example, a Sun-like star has a main-sequence lifetime which can be increased by up to 3 Gyr. This requires a mass-loss rate of about $0.05 {\rm M}_{\mathrm{Ceres}}$ per year. We compare star-lifting to other survival strategies and briefly discuss methods for detecting these engineered stars.

astro-ph.SR↗
arXiv2023

The SNR of a Transit

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 $(δ/σ_0) \sqrt{D}$. 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 $(δ/σ_0) \sqrt{(T_{14}+2T_{23})/3}$. 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.

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arXiv2023

Survival and dynamics of rings of co-orbital planets under perturbations

In co-orbital planetary systems, two or more planets share the same orbit around their star. Here we test the dynamical stability of co-orbital rings of planets perturbed by outside forces. We test two setups: i) 'stationary' rings of planets that, when unperturbed, remain equally-spaced along their orbit; and ii) horseshoe constellation systems, in which planets are continually undergoing horseshoe librations with their immediate neighbors. We show that a single rogue planet crossing the planets' orbit more massive than a few lunar masses (0.01-0.04 Earth masses) systematically disrupts a co-orbital ring of 6, 9, 18, or 42 Earth-mass planets located at 1 au. Stationary rings are more resistant to perturbations than horseshoe constellations, yet when perturbed they can transform into stable horseshoe constellation systems. Given sufficient time, any co-orbital ring system will be perturbed into either becoming a horseshoe constellation or complete destabilization.

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arXiv2023

Constellations of co-orbital planets: horseshoe dynamics, long-term stability, transit timing variations, and potential as SETI beacons

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.

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arXiv2022

Transit Duration and Timing Variations from Binary Planets

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.

astro-ph.EP↗
arXiv2022

A search for transit timing variations within the exomoon corridor using Kepler data

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 ($P_{TTV}<4$ cycles) TTV signal. However, of these, only 5 are recovered in an independent analysis of the original photometry, underscoring the importance of such checks. The surviving signals are subjected to an additional trifecta of statistical tests to ensure signal significance, predictive capability and consistency with an exomoon. KOI-3678.01, previously validated as Kepler-1513b, is the only case that passes every test, exhibiting a highly significant (>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.

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