arXiv subjects

Ted M. Johnson

Publications and source records attributed to Ted M. Johnson.

5 recordsLinked to original sources

arXiv2026

Evidence for LP 890-9d via Transit Timing Variations

LP 890-9, also known as SPECULOOS-2 and TOI-4306, is a nearby late-M dwarf hosting two confirmed transiting rocky exoplanets. We analyze 20 JWST/NIRSpec PRISM transits of LP 890-9b and LP 890-9c obtained as part of GO program 7073 and detect statistically significant transit timing variations (TTVs), with peak-to-peak amplitudes of ~17 s and ~35 s, respectively. Using analytic linear TTV theory, we find that the known two-planet configuration cannot reproduce the measured TTV amplitudes or super-period, whereas three-planet models provide substantially better fits. The best-fit configuration places the candidate third planet, LP 890-9d, between planets b and c, with an orbital period of ~4.4 days; however, the current data do not uniquely determine its orbital architecture, and periods spanning 4.0-6.9 days remain plausible. TESS is insensitive to transits of LP 890-9d and we find no evidence for the candidate in JWST observations, although the phase coverage (ranging from ~50% to ~80%) depends strongly on the candidate orbital period. Additional high-precision transit observations of LP 890-9b and LP 890-9c are needed to refine their TTV solutions and further constrain the orbital properties of the third planet.

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arXiv2026

A New Strategy for Using Spectroscopic Phase Curves to Characterize Non-Transiting Planets

We introduce a new time-series analysis strategy for combined-light exoplanet spectroscopic phase curves called the Variable Planetary Infrared Excess (VPIE) method. VPIE can be used to extract information about the planetary flux contribution without the need for the planet to transit or the use of a stellar spectral model. VPIE utilizes a linear combination of a small set of individual spectra to produce an empirical model of the stellar contribution at each time step, thereby normalizing each spectrum and leaving only an imprint of the planet's flux in the residual data. We demonstrate the effectiveness of VPIE through simulated James Webb Space Telescope (JWST) observations of three known exoplanet orbiting late-type M stars: the warm giant TOI-519 b, the warm sub-Neptune GJ 876 d, and the temperate rocky planet Proxima Centauri b. Our results indicate that though VPIE is less sensitive to very efficient heat redistribution, it can successfully distinguish between various atmospheric circulation regimes (low, moderate, or high heat redistribution) and constrain planetary radii for non-unity day-night temperature ratios. While current performance for cooler targets may be limited by JWST spectroscopic capabilities at longer wavelengths, future VPIE improvements or new instrumentation could enable characterization of potentially habitable planets. VPIE offers a promising new framework for pulling back the veil on the population of non-transiting planets around nearby M-stars that are otherwise inaccessible to current techniques.

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arXiv2026

Accretion Rate Changes Detected in a Polluted White Dwarf

This letter reports statistically significant changes in the equivalent widths of MgII and CaII lines in the dusty and polluted white dwarf WD 0106-328, based on six epochs of spectroscopy using the VLT and Keck spanning 25 yr. Furthermore, the ratio of these two equivalent widths may also vary, with a 7% probability of being constant. Between 2000 and 2025, both Mg and Ca have experienced decreases in accretion rates, of approximately 20 and 60%, respectively, but with individual variation during the interim. These metal abundance decreases are the first empirical corroboration of diffusion theory in white dwarfs, which predict sinking timescales on the order of days for this star. However, the persistent atmospheric metals require a more gradual, circumstellar process, where one possibility is viscous spreading in an ionized disk of metals, consistent with $\alpha\approx0.1$ within that formalism. The combination of optical and ultraviolet spectroscopy with the Hubble Space Telescope detects all the major rock-forming elements (O, Mg, Si, Fe), and demonstrates that Fe dominates the accreted material by mass, and that it is delivered mostly as pure metal from within a differentiated parent body. This inference is consistent with the possibility that chemically-segregated accretion may result from a combination of planetary assembly, fragmentation, disk evolution, and be observed on relatively short timescales.

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arXiv2025

The fraction of polar aligned circumbinary disks

Circumbinary gas disks that are misaligned to the binary orbital plane evolve toward either a coplanar or a polar-aligned configuration with respect to the binary host. The preferred alignment depends on the dynamics of the disk: whether it undergoes librating or circulating nodal precession, with librating disks evolving to polar inclinations and circulating disks evolving to coplanar. We quantify the fraction of binary star systems whose disks are expected to have polar orbits $f_\text{polar}$, extending previous work to include disks with non-zero mass. Our results suggest that, for low mass disks, the polar fraction is highly sensitive to the distribution of binary eccentricity with a higher fraction expected for higher binary eccentricities, $f_{\rm polar}\sim e_{\rm b}$. However, for massive discs, the fraction is independent of the binary eccentricity and $f_{\rm polar}\approx 0.37$. The value of $f_\text{polar}$ is always reduced in a population with a greater preference for low initial mutual inclination. We also explore the consequences of the finite lifetime and non-zero radial extent of a real disk, both of which affect a disk's ability to complete its evolution to a stationary configuration. Our findings can be used to make predictions given populations with well-understood distributions of binary eccentricity, initial mutual inclination, and disk angular momentum.

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arXiv2025

The climates and thermal emission spectra of prime nearby temperate rocky exoplanet targets

Over the course of the past decade, advances in the radial velocity and transit techniques have enabled the detection of rocky exoplanets in the habitable zones of nearby stars. Future observations with novel methods are required to characterize this sample of planets, especially those that are non-transiting. One proposed method is the Planetary Infrared Excess (PIE) technique, which would enable the characterization of non-transiting planets by measuring the excess infrared flux from the planet relative to the star's spectral energy distribution. In this work, we predict the efficacy of future observations using the PIE technique by potential future observatories such as the MIRECLE mission concept. To do so, we conduct a broad suite of 21 General Circulation Model (GCM) simulations with ExoCAM of seven nearby habitable zone targets for three choices of atmospheric composition with varying partial pressure of CO$_2$. We then construct thermal phase curves and emission spectra by post-processing our ExoCAM GCM simulations with the Planetary Spectrum Generator (PSG). We find that all cases have distinguishable carbon dioxide and water features assuming a 90$^\circ$ orbital inclination. Notably, we predict that CO$_2$ is potentially detectable at 15 $\mu\mathrm{m}$ with MIRECLE for at least four nearby known non-transiting rocky planet candidate targets in the habitable zone: Proxima Cenaturi b, GJ 1061 d, GJ 1002 b, and Teegarden's Star c. Our ExoCAM GCMs and PSG post-processing demonstrate the potential to observationally characterize nearby non-transiting rocky planets and better constrain the potential for habitability in our Solar neighborhood.

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