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Stephen Kane

Publications and source records attributed to Stephen Kane.

At least 19 recordsLinked to original sources
arXiv2025

Identifying rocky planets and water worlds among sub-Neptune-sized exoplanets with the Habitable Worlds Observatory

Astronomers are debating whether the plentiful "sub-Neptune" exoplanets -- worlds a bit larger than Earth but smaller than Neptune -- are predominantly rocky planets, water-rich "ocean worlds," or gas-enshrouded mini-Neptunes. This question is crucial because such sub-Neptune-sized planets are among the most common in our galaxy, yet we have no analog in our own solar system, making them a key to understanding planet formation and diversity. It also directly impacts the search for habitable worlds: larger-than-Earth planets with solid surfaces or oceans could support life, whereas gas-rich mini-Neptunes likely cannot. However, distinguishing these types using only a planet's mass and radius is very challenging, because different compositions can produce similar densities, leaving a world's nature ambiguous with current data. The proposed Habitable Worlds Observatory (HWO), a future NASA flagship telescope, offers a solution. HWO could directly image and spectroscopically analyze starlight reflected from 50~100 sub-Neptunes around nearby stars, aiming to reveal their atmospheric compositions and potential surfaces. Using visible and near-infrared spectroscopy along with sensitive polarimetry, HWO would detect atmospheric gases (such as water vapor, methane, and carbon dioxide) and search for telltale surface signatures, including rock absorption features and the characteristic reflectivity patterns of oceans. By analyzing these signals, we could determine whether sub-Neptunes are large rocky planets or water worlds rather than gas-dominated mini-Neptunes. Crucially, expanding the search beyond Earth-sized planets to include these abundant sub-Neptunes may uncover entirely new classes of potentially habitable worlds, directly advancing HWO's mission to identify and characterize planets that could support life.

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arXiv2023

The development of HISPEC for Keck and MODHIS for TMT: science cases and predicted sensitivities

HISPEC is a new, high-resolution near-infrared spectrograph being designed for the W.M. Keck II telescope. By offering single-shot, R=100,000 between 0.98 - 2.5 um, HISPEC will enable spectroscopy of transiting and non-transiting exoplanets in close orbits, direct high-contrast detection and spectroscopy of spatially separated substellar companions, and exoplanet dynamical mass and orbit measurements using precision radial velocity monitoring calibrated with a suite of state-of-the-art absolute and relative wavelength references. MODHIS is the counterpart to HISPEC for the Thirty Meter Telescope and is being developed in parallel with similar scientific goals. In this proceeding, we provide a brief overview of the current design of both instruments, and the requirements for the two spectrographs as guided by the scientific goals for each. We then outline the current science case for HISPEC and MODHIS, with focuses on the science enabled for exoplanet discovery and characterization. We also provide updated sensitivity curves for both instruments, in terms of both signal-to-noise ratio and predicted radial velocity precision.

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arXiv2022

Spinning up a Daze: TESS Uncovers a Hot Jupiter orbiting the Rapid-Rotator TOI-778

NASA's Transiting Exoplanet Survey Satellite (TESS) mission, has been uncovering a growing number of exoplanets orbiting nearby, bright stars. Most exoplanets that have been discovered by TESS orbit narrow-line, slow-rotating stars, facilitating the confirmation and mass determination of these worlds. We present the discovery of a hot Jupiter orbiting a rapidly rotating ($v\sin{(i)}= 35.1\pm1.0$km/s) early F3V-dwarf, HD115447 (TOI-778). The transit signal taken from Sectors 10 and 37 of TESS's initial detection of the exoplanet is combined with follow-up ground-based photometry and velocity measurements taken from Minerva-Australis, TRES, CORALIE and CHIRON to confirm and characterise TOI-778b. A joint analysis of the light curves and the radial velocity measurements yield a mass, radius, and orbital period for TOI-778b of $2.76^{+0.24}_{-0.23}$Mjup, $1.370\pm0.043$Rjup and $\sim4.63$ days, respectively. The planet orbits a bright ($V = 9.1$mag) F3-dwarf with $M=1.40\pm0.05$Msun, $R=1.70\pm0.05$Rsun, and $\log g=4.05\pm0.17$. We observed a spectroscopic transit of TOI-778b, which allowed us to derive a sky-projected spin-orbit angle of $18^{\circ}\pm11^{\circ}$, consistent with an aligned planetary system. This discovery demonstrates the capability of smaller aperture telescopes such as Minerva-Australis to detect the radial velocity signals produced by planets orbiting broad-line, rapidly rotating stars.

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arXiv2020

Deep Atmosphere of Venus Probe as a Mission Priority for the Upcoming Decade

This is a white paper submitted to the Planetary Science and Astrobiology Decadal Survey. The deep atmosphere of Venus is largely unexplored and yet may harbor clues to the evolutionary pathways for a major silicate planet with implications across the solar system and beyond. In situ data is needed to resolve significant open questions related to the evolution and present-state of Venus, including questions of Venus' possibly early habitability and current volcanic outgassing. Deep atmosphere "probe-based" in situ missions carrying analytical suites of instruments are now implementable in the upcoming decade (before 2030), and will both reveal answers to fundamental questions on Venus and help connect Venus to exoplanet analogs to be observed in the JWST era of astrophysics.

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arXiv2019

High-resolution Infrared Spectrograph for Exoplanet Characterization with the Keck and Thirty Meter Telescopes

HISPEC (High-resolution Infrared Spectrograph for Exoplanet Characterization) is a proposed diffraction-limited spectrograph for the W.M. Keck Observatory, and a pathfinder for the MODHIS facility project (Multi-Object Diffraction-limited High-resolution Infrared Spectrograph) on the Thirty Meter Telescope. HISPEC/MODHIS builds on diffraction-limited spectrograph designs which rely on adaptively corrected single-mode fiber feeds. Seeing-limited high-resolution spectrographs, by virtue of the conservation of beam etendue, grow in volume following a D^3 power law (D is the telescope diameter), and are subject to daunting challenges associated with their large size. Diffraction-limited spectrographs fed by single mode fibers are decoupled from the telescope input, and are orders of magnitude more compact and have intrinsically stable line spread functions. Their efficiency is directly proportional to the performance of the adaptive optics (AO) system. AO technologies have matured rapidly over the past two decades and are baselined for future extremely large telescopes. HISPEC/MODHIS will take R>100,000 spectra of a few objects in a 10" field-of-view sampled at the diffraction limit (~10-50 mas), simultaneously from 0.95 to 2.4 microns (y-K). The scientific scope ranges from exoplanet infrared precision radial velocities, spectroscopy of transiting, close-in, and directly imaged exoplanets (atmospheric composition and dynamics, RM effect, spin measurements, Doppler imaging), brown dwarf characterization, stellar physics/chemistry, proto-planetary disk kinematics/composition, Solar system, extragalactic science, and cosmology. HISPEC/MODHIS features a compact, cost-effective design optimized to fully exploit the existing Keck-AO and future TMT-NFIRAOS infrastructures and boost the scientific reach of Keck Observatory and TMT soon after first light.

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arXiv2019

The Need for Laboratory Measurements and Ab Initio Studies to Aid Understanding of Exoplanetary Atmospheres

We are now on a clear trajectory for improvements in exoplanet observations that will revolutionize our ability to characterize their atmospheric structure, composition, and circulation, from gas giants to rocky planets. However, exoplanet atmospheric models capable of interpreting the upcoming observations are often limited by insufficiencies in the laboratory and theoretical data that serve as critical inputs to atmospheric physical and chemical tools. Here we provide an up-to-date and condensed description of areas where laboratory and/or ab initio investigations could fill critical gaps in our ability to model exoplanet atmospheric opacities, clouds, and chemistry, building off a larger 2016 white paper, and endorsed by the NAS Exoplanet Science Strategy report. Now is the ideal time for progress in these areas, but this progress requires better access to, understanding of, and training in the production of spectroscopic data as well as a better insight into chemical reaction kinetics both thermal and radiation-induced at a broad range of temperatures. Given that most published efforts have emphasized relatively Earth-like conditions, we can expect significant and enlightening discoveries as emphasis moves to the exotic atmospheres of exoplanets.

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arXiv2019

Key Technologies for the Wide Field Infrared Survey Telescope Coronagraph Instrument

The Wide Field Infrared Survey Telescope (WFIRST) Coronagraph Instrument (CGI) is a high-contrast imager and integral field spectrograph that will enable the study of exoplanets and circumstellar disks at visible wavelengths. Ground-based high-contrast instrumentation has fundamentally limited performance at small working angles, even under optimistic assumptions for 30m-class telescopes. There is a strong scientific driver for better performance, particularly at visible wavelengths. Future flagship mission concepts aim to image Earth analogues with visible light flux ratios of more than 10^10. CGI is a critical intermediate step toward that goal, with a predicted 10^8-9 flux ratio capability in the visible. CGI achieves this through improvements over current ground and space systems in several areas: (i) Hardware: space-qualified (TRL9) deformable mirrors, detectors, and coronagraphs, (ii) Algorithms: wavefront sensing and control; post-processing of integral field spectrograph, polarimetric, and extended object data, and (iii) Validation of telescope and instrument models at high accuracy and precision. This white paper, submitted to the 2018 NAS Exoplanet Science Strategy call, describes the status of key CGI technologies and presents ways in which performance is likely to evolve as the CGI design matures.

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arXiv2018

Revisiting the HIP41378 system with K2 and Spitzer

We present new observations of the multi-planet system HIP~41378, a bright star ($V$ = 8.9, $K_s$ = 7.7) with five known transiting planets. Observations in Campaign 5 of the K2 mission showed multiple transits of two Neptune-sized bodies and single transits of three larger planets ($R_P= 0.33R_J, 0.47R_J, 0.88 R_J$). K2 recently observed the system again in Campaign 18. We observe one new transit each of two of the larger planets, HIP41378~d and~f, giving maximal possible orbital periods of 1114 and 1084 days, respectively. Other possible periods include integer divisions of these maximum values down to a lower limit of about 50 days. We use all available photometry to determine the eccentricity distributions of HIP41378~d \& f, finding that periods $\lesssim$300 days require non-zero eccentricity. We also perform a stability analysis on the orbits of planets d and f to assess the likelihood of their different possible periods, finding that short periods (P $<$ 300 days) are disfavored. In addition, we observe transits of the inner two planets b and c with Spitzer/IRAC, which we combine with the new K2 observations of these planets to search for transit timing variations (TTVs). We find a linear ephemeris for planet b, but see a significant TTV signal for planet c that could be induced by planet d, e, or f. The ability to recover the two smaller planets with Spitzer shows that the several planets in this fascinating system will continue to be detectable with Spitzer, CHEOPS, TESS, and other observatories. This will allow us to precisely determine the periods of d and f, characterize the TTVs of planet~c, recover the transits of planet~e, and further enhance our view of this remarkable dynamical laboratory.

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arXiv2018

Disentangling Blended K2 Photometry: Determining the Planetary Host Star

The presence of companion stars, whether bound or unbound, make correct identification of the planetary hosting star difficult when a planet has been detected through a photometrically blended transiting event. We present an approach that uses a combination of light curve analysis and stellar modeling to disentangle 8 K2 photometrically blended binary systems that have either a confirmed or suspected planet to identify the probable host star. The key to our approach is the use of the mean stellar density, calculated using the transit geometry, as a discriminator. The approach is strongly dependent on the difference in magnitude between the stars and the observed transit depth, which is corrected by the flux ratio between the two stars. While our approach does not lead to a definitive result for all systems, we were able to determine for the 8 systems in this case study: two systems where the primary was highly likely to be the planet hosting star ($>$ 90\% likelihood); three systems where the primary was likely to be the hosting star ($>$ 55\% likelihood); one system where the secondary was likely to be the planet hosting star ($>$ 55\% likelihood); and two systems where we were uncertain which star was the planet hosting star ($\approx$ 50\% likelihood to be either the primary or the secondary). Two systems, denoted by their EPIC number, EPIC 201546283 and EPIC 201862715, had confirmed planets, K2-27b and K2-94b respectively, with radii of $\mathrm{4.0 \pm 0.1}$ and $\mathrm{16.4 \pm 2.9}$ $\mathrm{R_\oplus}$ respectively.

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arXiv2018

Venus as an Analog for Hot Earths

Here, we evaluate our nearest planetary neighbor, Venus, as an exemplar of the runaway greenhouse state that bounds the inner edge of the habitable zone. Despite its current hellish surface environment, Venus may once have been habitable with oceans of surface liquid water. Over time, it lost its potentially clement environment as the sun brightened. Today, it represents the end-state of habitable planet evolution, and it therefore provides valuable lessons on habitability as a planetary process. Beyond the solar system, exo-Venus analogs are likely common types of planets, and we likely have already discovered many of Venus' sisters orbiting other stars. Furthermore, our near-future exoplanet detection and characterization methods are biased towards observing them. Therefore, it is instructive to consider what Venus can teach us about exo-Venus analogs. By observing exo-Venus planets of differing ages in in different astrophysical contexts, these distant hot terrestrial worlds may likewise allow us to witness processes that occurred on Venus in the past.

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arXiv2018

The WFIRST Exoplanet Microlensing Survey

The Wide Field Infrared Survey Telescope (WFIRST) was the top ranked large space mission in the 2010 New Worlds, New Horizons decadal survey, and it was formed by merging the science programs of 3 different mission concepts, including the Microlensing Planet Finder (MPF) concept (Bennett \etal\ 2010). The WFIRST science program (Spergel \etal\ 2015) consists of a general observer program, a wavefront controlled technology program, and two targeted science programs: a program to study dark energy, and a statistical census of exoplanets with a microlensing survey, which uses nearly one quarter of WFIRST's observing time in the current design reference mission. The New Worlds, New Horizons (decadal survey) midterm assessment summarizes the science case for the WFIRST exoplanet microlensing survey with this statement: "WFIRST's microlensing census of planets beyond 1 AU will perfectly complement Kepler's census of compact systems, and WFIRST will also be able to detect free-floating planets unbound from their parent stars\rlap."

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arXiv2018

The Origins Space Telescope: Towards An Understanding of Temperate Planetary Atmospheres

The Origins Space Telescope (OST) is one of four mission concepts currently being studied by NASA in preparation for the Astrophysics 2020 Decadal Survey. With active cooling (~4 K), OST will be sensitive in mid- to far-IR wavelengths, using imaging and spectroscopy to probe the furthest reaches of our galaxies, trace the path of water through star and planet formation, and place thermochemical constraints on the atmospheres of exoplanets ranging in size from Jupiter to Earth. This contribution to the Exoplanet Science Strategy committee discusses the significant advancements that the OST Mid-Infrared Imager, Spectrometer, and Coronagraph (MISC) instrument can make in studying cool planetary atmospheres. We particularly focus on the atmospheres of transiting rocky planets in the habitable zones of mid-to-late M stars. We discuss how OST thermal infrared observations can significantly enhance our understanding of the temperature structure and molecular abundances of biologically interesting gases on these worlds, including O3, CH4, H2O, and CO2.

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arXiv2018

Geoscience and the Search for Life Beyond the Solar System

How can scientists conclude with high confidence that an exoplanet hosts life? As telescopes come on line over the next 20 years that can directly observe photons from terrestrial exoplanets, this question will dictate the activities of many scientists across many fields. The expected data will be sparse and with low signal-to-noise, which will make disentangling biosignatures from abiotic features challenging. Our Earth is not just unique in that it hosts life, it is also the only terrestrial planet with direct observations of its interior through seismic waves, and compositional evolution through field and laboratory measurements. This extensive research reveals a planet born from collisions between worlds, followed by a complicated biogeochemical evolution. Exoplanet interiors, on the other hand, can only be constrained by the following observations: 1) photometric and spectroscopic analysis of the planet's atmosphere, 2) spectroscopic and photometric analysis of the host star, and 3) companion planet properties. From these (future) data, astrobiologists must generate plausible compositional and evolutionary models that constrain a potentially habitable exoplanet's internal properties and history, provide environmental context, and rule out geochemical explanations for any putative biosignatures. The goal of this white paper is to frame the role of geophysical and geochemical processes relevant to the search for life beyond the Solar System and to identify critical, but understudied, areas of future research.

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arXiv2018

A procedure for observing rocky exoplanets to maximize the likelihood that atmospheric oxygen will be a biosignature

Here we advocate an observational strategy to help prioritize exoplanet observations. It starts with more easily obtained observational data, and ranks exoplanets for more difficult follow-up observations based on the likelihood of avoiding planets for which oxygen is a false positives or even an inconclusive signature of life. We find that for oxygen to be a robust biosignature, both land and surface water must be present. Landless exoplanets have much slower biogeochemical cycles, so while oxygenic photosynthesizing life could exist on such planets, it could not produce oxygen at a rate competitive with abiotic rates such as photolysis. These habitable planets, whose life would not be detectable, should be avoided.

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arXiv2017

Exploring Other Worlds: Science Questions for Future Direct Imaging Missions (EXOPAG SAG15 Report)

The NASA Exoplanet Exploration Program's SAG15 group has solicited, collected, and organized community input on high-level science questions that could be addressed with future direct imaging exoplanet missions and the type and quality of data answering these questions will require. Input was solicited through a variety of forums and the report draft was shared with the exoplanet community continuously during the period of the report development (Nov 2015 -- May 2017). The report benefitted from the input of over 50 exoplanet scientists and from multiple open-forum discussions at exoplanet and astrobiology meetings. The SAG15 team has identified three group of high-level questions, those that focus on the properties of planetary systems (Questions A1--A2), those that focus on the properties of individual planets (Questions B1--B4), and questions that relate to planetary processes (Questions C1--C4). The questions in categories A, B, and C require different target samples and often different observational approaches. For each questions we summarize the current body of knowledge, the available and future observational approaches that can directly or indirectly contribute to answering the question, and provide examples and general considerations for the target sample required.

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arXiv2017

Kepler-1649b: An Exo-Venus in the Solar Neighborhood

The Kepler mission has revealed that Earth-sized planets are common, and dozens have been discovered to orbit in or near their host star's habitable zone. A major focus in astronomy is to determine which of these exoplanets are likely to have Earth-like properties that are amenable to follow-up with both ground- and future space-based surveys, with an ultimate goal of probing their atmospheres to look for signs of life. Venus-like atmospheres will be of particular interest in these surveys. While Earth and Venus evolved to have similar sizes and densities, it remains unclear what factors led to the dramatic divergence of their atmospheres. Studying analogs to both Earth and Venus can thus shed light on the limits of habitability and the potential for life on known exoplanets. Here we present the discovery and confirmation of Kepler-1649b, an Earth-sized planet orbiting a nearby M5V star that receives incident flux at a level similar to that of Venus. We present our methods for characterizing the star, using a combination of PSF photometry, ground-based spectroscopy and imaging, to confirm the planetary nature of Kepler-1649b. Planets like Kepler-1649b will be prime candidates for atmospheric and habitability studies in the next generation of space missions.

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arXiv2016

A map of the large day-night temperature gradient of a super-Earth exoplanet

Over the past decade, observations of giant exoplanets (Jupiter-size) have provided key insights into their atmospheres, but the properties of lower-mass exoplanets (sub-Neptune) remain largely unconstrained because of the challenges of observing small planets. Numerous efforts to observe the spectra of super-Earths (exoplanets with masses of one to ten times that of Earth) have so far revealed only featureless spectra. Here we report a longitudinal thermal brightness map of the nearby transiting super-Earth 55 Cancri e revealing highly asymmetric dayside thermal emission and a strong day-night temperature contrast. Dedicated space-based monitoring of the planet in the infrared revealed a modulation of the thermal flux as 55 Cancri e revolves around its star in a tidally locked configuration. These observations reveal a hot spot that is located 41 +- 12 degrees east of the substellar point (the point at which incident light from the star is perpendicular to the surface of the planet). From the orbital phase curve, we also constrain the nightside brightness temperature of the planet to 1380 +- 400 kelvin and the temperature of the warmest hemisphere (centred on the hot spot) to be about 1300 kelvin hotter (2700 +- 270 kelvin) at a wavelength of 4.5 microns, which indicates inefficient heat redistribution from the dayside to the nightside. Our observations are consistent with either an optically thick atmosphere with heat recirculation confined to the planetary dayside, or a planet devoid of atmosphere with low-viscosity magma flows at the surface.

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