arXiv subjects

Allison Youngblood

Publications and source records attributed to Allison Youngblood.

At least 37 records · Page 2Linked to original sources

arXiv2024

The Mega-MUSCLES Treasury Survey: X-ray to infrared Spectral Energy Distributions of a representative sample of M dwarfs

We present 5-1x10^7 Angstrom spectral energy distributions (SEDs) for twelve M dwarf stars covering spectral types M0-M8. Our SEDs are provided for community use as a sequel to the Measurements of the Ultraviolet Spectral Characteristics of Low-mass Exoplanetary Systems (MUSCLES) survey. The twelve stars include eight known exoplanet hosts and four stars chosen to fill out key parameter space in spectral type and rotation period. The SEDs are constructed from Hubble Space Telescope ultraviolet spectroscopy and XMM Newton, Chandra and/or Swift X-ray observations and completed with various model data, including Lyman alpha reconstructions, PHOENIX optical models, APEC coronal models and Differential Emission Measure models in the currently-unobservable Extreme Ultraviolet. We provide a complete overview of the Mega-MUSCLES program, including a description of the observations, models, and SED construction. The SEDs are available as MAST High-Level Science Products and we describe the various data products here. We also present ensemble measurements from our sample that are of particular relevance to exoplanet science, including the high-energy fluxes in the habitable zone and the FUV/NUV ratio. Combined with MUSCLES, Mega-MUSCLES provides SEDs covering a wide range of M\,dwarf spectral types and ages such that suitable proxies for any M dwarf planet host of interest may be found in our sample. However, we find that ultraviolet and X-ray fluxes can vary even between stars with similar parameters, such that observations of each exoplanet host star will remain the gold standard for interpreting exoplanet atmosphere observations.

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arXiv2024

The Featherweight Giant: Unraveling the Atmosphere of a 17 Myr Planet with JWST

The characterization of young planets (< 300 Myr) is pivotal for understanding planet formation and evolution. We present the 3-5$μ$m transmission spectrum of the 17 Myr, Jupiter-size ($R$ $\sim$10$R_{\oplus}$) planet, HIP 67522 b, observed with JWST/NIRSpec/G395H. To check for spot contamination, we obtain a simultaneous $g$-band transit with SOAR. The spectrum exhibits absorption features 30-50% deeper than the overall depth, far larger than expected from an equivalent mature planet, and suggests that HIP 67522 b's mass is $<$20 $M_{\oplus}$ irrespective of cloud cover and stellar contamination. A Bayesian retrieval analysis returns a mass constraint of $13.8\pm1.0M_{\oplus}$. This challenges the previous classification of HIP 67522 b as a hot Jupiter and instead, positions it as a precursor to the more common sub-Neptunes. With a density of $<$0.10g/cm$^{3}$, HIP 67522 b is one of the lowest density planets known. We find strong absorption from H$_{2}$O and CO$_{2}$ ($\ge7σ$), a modest detection of CO (3.5$σ$), and weak detections of H$_2$S and SO$_2$ ($\simeq2σ$). Comparisons with radiative-convective equilibrium models suggest supersolar atmospheric metallicities and solar-to-subsolar C/O ratios, with photochemistry further constraining the inferred atmospheric metallicity to 3$\times$10 Solar due to the amplitude of the SO$_2$ feature. These results point to the formation of HIP 67522 b beyond the water snowline, where its envelope was polluted by icy pebbles and planetesimals. The planet is likely experiencing substantial mass loss (0.01-0.03 M$_{\oplus}$ Myr$^{-1}$), sufficient for envelope destruction within a Gyr. This highlights the dramatic evolution occurring within the first 100 Myr of its existence.

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arXiv2024

The Smallsat Technology Accelerated Maturation Platform-1 (STAMP-1): A Proposal to Advance Ultraviolet Science, Workforce, and Technology for the Habitable Worlds Observatory

NASA's Great Observatories Maturation Program (GOMAP) will advance the science definition, technology, and workforce needed for the Habitable Worlds Observatory (HWO) with the goal of a Phase A start by the end of the current decade. GOMAP offers long-term cost and schedule savings compared to the 'TRL 6 by Preliminary Design Review' paradigm historically adopted by large NASA missions. Many of the key technologies in the development queue for HWO require the combined activities of 1) facility and process development for validation of technologies at the scale required for HWO and 2) deployment in the 'real world' environment of mission Integration & Test prior to on-orbit operations. We present a concept for the Smallsat Technology Accelerated Maturation Platform (STAMP), an integrated facility, laboratory, and instrument prototype development program that could be supported through the GOMAP framework and applied to any of NASA's Future Great Observatories (FGOs). This brief describes the recommendation for the first entrant into this program, "STAMP-1", an ESPA Grande-class mission advancing key technologies to enable the ultraviolet capabilities of HWO. STAMP-1 would advance new broadband optical coatings, high-sensitivity ultraviolet detector systems, and multi-object target selection technology to TRL 6 with a flight demonstration. STAMP-1 advances HWO technology on an accelerated timescale, building on current ROSES SAT+APRA programs, reducing cost and schedule risk for HWO while conducting a compelling program of preparatory science and workforce development with direct benefits for HWO mission implementation in the 2030s.

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arXiv2024

The TEMPO Survey II: Science Cases Leveraged from a Proposed 30-Day Time Domain Survey of the Orion Nebula with the Nancy Grace Roman Space Telescope

The TEMPO (Transiting Exosatellites, Moons, and Planets in Orion) Survey is a proposed 30-day observational campaign using the Nancy Grace Roman Space Telescope. By providing deep, high-resolution, short-cadence infrared photometry of a dynamic star-forming region, TEMPO will investigate the demographics of exosatellites orbiting free-floating planets and brown dwarfs -- a largely unexplored discovery space. Here, we present the simulated detection yields of three populations: extrasolar moon analogs orbiting free-floating planets, exosatellites orbiting brown dwarfs, and exoplanets orbiting young stars. Additionally, we outline a comprehensive range of anticipated scientific outcomes accompanying such a survey. These science drivers include: obtaining observational constraints to test prevailing theories of moon, planet, and star formation; directly detecting widely separated exoplanets orbiting young stars; investigating the variability of young stars and brown dwarfs; constraining the low-mass end of the stellar initial mass function; constructing the distribution of dust in the Orion Nebula and mapping evolution in the near-infrared extinction law; mapping emission features that trace the shocked gas in the region; constructing a dynamical map of Orion members using proper motions; and searching for extragalactic sources and transients via deep extragalactic observations reaching a limiting magnitude of $m_{AB}=29.7$\,mag (F146 filter).

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arXiv2024

Host Stars and How Their Compositions Influence Exoplanets

It has become a common practice within the exoplanet field to say that "to know the star is to know the planet." The properties of the host star have a strong, direct influence on the interior and surface conditions of the orbiting planet and oftentimes measurements of planetary properties are made relative to the star's properties. Not only are observational measurements of the star necessary to determine even the most basic aspects of the planet (such as mass and radius), but the stellar environment influences how the planet evolves. Therefore, in this chapter, we begin by discussing the basics of stars, providing an overview of stellar formation, structure, photon and particle emissions, and evolution. Next, we go over the possible ways to determine the age of a star. We then outline how different kinds of stars are distributed within the Milky Way galaxy. Afterwards, we explain how to measure the composition of stars and the underlying math inherent to those observations, including caveats that are important when using the data for research applications. Finally, we explain the underlying physics and observations that enable stellar composition to be used as a proxy for planetary composition. In addition, given that this chapter focuses more on astronomy/astrophysics and uses a variety of important terms that may not be familiar to all readers, we have defined many terms either within the text or as a footnote for better interdisciplinary comprehension.

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arXiv2024

A Multiwavelength Survey of Nearby M dwarfs: Optical and Near-Ultraviolet Flares and Activity with Contemporaneous TESS, Kepler/K2, \textit{Swift}, and HST Observations

We present a comprehensive multiwavelength investigation into flares and activity in nearby M~dwarf stars. We leverage the most extensive contemporaneous dataset obtained through the Transiting Exoplanet Sky Survey (TESS), Kepler/K2, the Neil Gehrels Swift Observatory (\textit{Swift}), and the Hubble Space Telescope (HST), spanning the optical and near-ultraviolet (NUV) regimes. In total, we observed 213 NUV flares on 24 nearby M dwarfs, with $\sim$27\% of them having detected optical counterparts, and found that all optical flares had NUV counterparts. We explore NUV/optical energy fractionation in M dwarf flares. Our findings reveal a slight decrease in the ratio of optical to NUV energies with increasing NUV energies, a trend in agreement with prior investigations on G-K stars' flares at higher energies. Our analysis yields an average NUV fraction of flaring time for M0-M3 dwarfs of 2.1\%, while for M4-M6 dwarfs, it is 5\%. We present an empirical relationship between NUV and optical flare energies and compare to predictions from radiative-hydrodynamic and blackbody models. We conducted a comparison of the flare frequency distribution (FFDs) of NUV and optical flares, revealing the FFDs of both NUV and optical flares exhibit comparable slopes across all spectral subtypes. NUV flares on stars affect the atmospheric chemistry, the radiation environment, and the overall potential to sustain life on any exoplanets they host. We find that early and mid-M dwarfs (M0-M5) have the potential to generate NUV flares capable of initiating abiogenesis.

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arXiv2024

Correlating Intrinsic Stellar Parameters with Mg II Self-Reversal Depths

The Mg II h&k emission lines (2803, 2796 A) are a useful tool for understanding stellar chromospheres and transition regions due to their intrinsic brightness, relatively low interstellar medium (ISM) absorption interference, and abundance of archival spectra available. Similar to other optically thick chromospheric emission lines such as H I Lya, Mg II emissions commonly present with a self-reversed line core, the depth and shape of which vary from star to star. We explore the relationship between self-reversal and the stellar atmosphere by investigating the extent that fundamental stellar parameters affect self-reversal. We present a search for correlations between photospheric parameters such as effective temperature, surface gravity, and metallicity with the Mg II k self-reversal depth for a group of 135 FGKM main sequence stars with high-resolution near-ultraviolet spectra from the Hubble Space Telescope. We modeled the observed MgII k line profiles to correct for ISM attenuation and recover the depth of emission line's self-reversal in relation to the intensity of the line. We used the PHOENIX atmosphere code to homogeneously determine the stellar parameters by computing a suite of stellar atmosphere models that include a chromosphere and transition region, and using archival photometry to guide the models of each star. We quantify the sensitivity of the visible and near-infrared photometry to chromospheric and photospheric parameters. We find weak trends between Mg II k self-reversal depth and age, rotation period, Mg II luminosity, temperature, and mass. All stars in our sample older than ~2 Gyr or rotating slower than ~10 days exhibit self-reversal.

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arXiv2023

Atmospheric Escape From Three Terrestrial Planets in the L 98-59 System

A critically important process affecting the climate evolution and potential habitability of an exoplanet is atmospheric escape, in which high-energy radiation from a star drives the escape of hydrogen atoms and other light elements from a planet's atmosphere. L 98-59 is a benchmark system for studying such atmospheric processes, with three transiting terrestrial-size planets receiving Venus-like instellations (4-25 S$_\oplus$) from their M3 host star. We use the VPLanet model to simulate the evolution of the L 98-59 system and the atmospheric escape of its inner three small planets, given different assumed initial water quantities. We find that, regardless of their initial water content, all three planets accumulate significant quantities of oxygen due to efficient water photolysis and hydrogen loss. All three planets also receive enough XUV flux to drive rapid water loss, which considerably affects their developing climates and atmospheres. Even in scenarios of low initial water content, our results suggest that the James Webb Space Telescope (JWST) will be sensitive to observations of retained oxygen on the L 98-59 planets in its future scheduled observations, with planets b and c being the most likely targets to possess an extended atmosphere. Our results constrain the atmospheric evolution of these small rocky planets, and they provide context for current and future observations of the L 98-59 system to generalize our understanding of multi-terrestrial planet systems.

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arXiv2023

Degenerate interpretations of O$_3$ spectral features in exoplanet atmosphere observations due to stellar UV uncertainties: a 3D case study with TRAPPIST-1e

TRAPPIST-1e is a potentially habitable terrestrial exoplanet orbiting an ultra-cool M Dwarf star and is a key target for observations with the James Webb Space Telescope (JWST). One-dimensional photochemical modelling of terrestrial planetary atmospheres has shown the importance of the incoming stellar UV flux in modulating the concentration of chemical species, such as O$_3$ and H$_2$O. In addition, three-dimensional (3D) modelling has demonstrated anisotropy in chemical abundances due to transport in tidally locked exoplanet simulations. We use the Whole Atmosphere Community Climate Model Version 6 (WACCM6), a 3D Earth System Model, to investigate how uncertainties in the incident UV flux, combined with transport, affect observational predictions for TRAPPIST-1e (assuming an initial Earth-like atmospheric composition). We use two semi-empirical stellar spectra for TRAPPIST-1 from the literature. The UV flux ratio between them can be as large as a factor of 5000 in some wavelength bins. Consequently, the photochemically-produced total O$_3$ columns differ by a factor of 26. Spectral features of O$_3$ in both transmission and emission spectra vary between these simulations (e.g. differences of 19 km in transmission spectra effective altitude for O$_3$ at 0.6 $μ$m). This leads to potential ambiguities when interpreting observations, including overlap with scenarios that assume alternative O$_2$ concentrations. Hence, to achieve robust interpretations of terrestrial exoplanetary spectra, characterisation of the UV spectra of their host stars is critical. In the absence of such stellar measurements, atmospheric context can still be gained from other spectral features (e.g. H$_2$O), or by comparing direct imaging and transmission spectra in conjunction.

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arXiv2023

The Variable Detection of Atmospheric Escape around the young, Hot Neptune AU Mic b

Photoevaporation is a potential explanation for several features within exoplanet demographics. Atmospheric escape observed in young Neptune-sized exoplanets can provide insight into and characterize which mechanisms drive this evolution and at what times they dominate. AU Mic b is one such exoplanet, slightly larger than Neptune (4.19 Earth radii). It closely orbits a 23 Myr pre-Main Sequence M dwarf with a period of 8.46 days. We obtained two visits of AU Mic b at Lyman-alpha with HST/STIS. One flare within the first HST visit is characterized and removed from our search for a planetary transit. We present a non-detection in our first visit followed by the detection of escaping neutral hydrogen ahead of the planet in our second visit. The outflow absorbed about 30% of the star's Lyman-alpha blue-wing 2.5 hours before the planet's white-light transit. We estimate the highest velocity escaping material has a column density of 10^13.96 cm^-2 and is moving 61.26 km/s away from the host star. AU Mic b's large high energy irradiation could photoionize its escaping neutral hydrogen in 44 minutes, rendering it temporarily unobservable. Our time-variable Lyman-alpha transit ahead of AU Mic b could also be explained by an intermediate stellar wind strength from AU Mic that shapes the escaping material into a leading tail. Future Lyman-alpha observations of this system will confirm and characterize the unique variable nature of its Lyman-alpha transit, which combined with modeling will tune the importance of stellar wind and photoionization.

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arXiv2023

Enabling Stellar Flare Science in the Roman Galactic Bulge Survey: Cadence, Filters, and the Read-Out Strategy Matter

As was discovered with other wide field, precise imagers, the stable photometry necessary for the microlensing surveys is well-suited to general stellar astrophysics, including stellar flares, which are important for understanding stellar magnetic activity and even the space weather environments of exoplanets. Large stellar flare surveys have never been performed before in the Roman spectral range, and Roman may reveal new information about flare emission mechanisms (blackbody, recombination continuum, chromospheric emission lines) and how flare rates change with stellar age and metallicity. For instance, the Galactic Bulge stars will be much older than the typical studied flare stars, and Roman's wide field and exquisite imaging may provide sufficient statistics to probe the flare behavior and properties of such an old stellar population. However, the information yield will likely depend on sky location, cadence, read-out strategy, and filter choices. Stellar flare timescales range from seconds to hours, so Roman may only be able to resolve the longest, most energetic events. However, because a single exposure in the Galactic Bulge Time Domain Survey will consist of several non-destructive reads, short duration events can be modeled from the flux variations within a single exposure. Here we provide a proof of concept, showing that flare morphologies can be significantly better constrained if sub-exposure data are analyzed. As a result, we advocate that such data be made publicly available for Roman flare studies, with minimal on-board processing.

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arXiv2023

The MUSCLES Extension for Atmospheric and Transmission Spectroscopy: UV and X-ray Host-star Observations for JWST ERS & GTO Targets

X-ray through infrared spectral energy distributions (SEDs) are essential for understanding a star's effect on exoplanet atmospheric composition and evolution. We present a catalog of panchromatic SEDs, hosted on the Barbara A. Mikulski Archive for Space Telescopes (MAST), for 11 exoplanet hosting stars which have guaranteed JWST observation time as part of the ERS or GTO programs but have no previous UV characterization. The stars in this survey range from spectral type F4-M6 (0.14-1.57 M$_\odot$), rotation periods of ~4-132 days, and ages of approximately 0.5-11.4 Gyr. The SEDs are composite spectra using data from the Chandra X-ray Observatory and XMM-Newton, the Hubble Space Telescope, BT-Settl stellar atmosphere models, and scaled spectra of proxy stars of similar spectral type and activity. From our observations, we have measured a set of UV and X-ray fluxes as indicators of stellar activity level. We compare the chromospheric and coronal activity indicators of our exoplanet-hosting stars to the broader population of field stars and find that a majority of our targets have activity levels lower than the average population of cool stars in the solar neighborhood. This suggests that using SEDs of stars selected from exoplanet surveys to compute generic exoplanet atmosphere models may underestimate the typical host star's UV flux by an order of magnitude or more, and consequently, that the observed population of exoplanetary atmospheres receive lower high-energy flux levels than the typical planet in the solar neighborhood.

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arXiv2023

Three long period transiting giant planets from TESS

We report the discovery and orbital characterization of three new transiting warm giant planets. These systems were initially identified as presenting single transit events in the light curves generated from the full frame images of the Transiting Exoplanet Survey Satellite (TESS). Follow-up radial velocity measurements and additional light curves were used to determine the orbital periods and confirm the planetary nature of the candidates. The planets orbit slightly metal-rich late F- and early G-type stars. We find that TOI 4406b has a mass of $M_P$= 0.30 $\pm$ 0.04 $M_J$ , a radius of $R_P$= 1.00 $\pm$ 0.02 $R_J$ , and a low eccentricity orbit (e=0.15 $\pm$ 0.05) with a period of P= 30.08364 $\pm$ 0.00005 d . TOI 2338b has a mass of $M_P$= 5.98 $\pm$ 0.20 $M_J$ , a radius of $R_P$= 1.00 $\pm$ 0.01 $R_J$ , and a highly eccentric orbit (e= 0.676 $\pm$ 0.002 ) with a period of P= 22.65398 $\pm$ 0.00002 d . Finally, TOI 2589b has a mass of $M_P$= 3.50 $\pm$ 0.10 $M_J$ , a radius of $R_P$= 1.08 $\pm$ 0.03 $R_J$ , and an eccentric orbit (e = 0.522 $\pm$ 0.006 ) with a period of P= 61.6277 $\pm$ 0.0002 d . TOI 4406b and TOI 2338b are enriched in metals compared to their host stars, while the structure of TOI 2589b is consistent with having similar metal enrichment to its host star.

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arXiv2023

Observations of GRB 230307A by TESS

We present the TESS light curve of GRB 230307A. We find two distinct components: a bright, prompt optical component at the time of the Fermi observation that peaked at TESS magnitude 14.49 (averaged over 200 seconds), followed by a gradual rise and fall over 0.5 days, likely associated with the afterglow, that peaked at 17.65 mag. The prompt component is observed in a single 200s Full Frame Image and was undetectable in the next TESS image ($T_{\rm mag} > 17.79$). Assuming that the onset of the optical transient was coincident with the gamma-ray emission, the prompt emission lasted less than 73.6 seconds, which implies the true peak was actually brighter than $T_{\rm mag} =$ 13.40. We also fit parametric models to the afterglow to characterize its shape. The TESS light curve can be retrieved at https://tess.mit.edu/public/tesstransients/light_curves/lc_grb230307A_cleaned.txt.

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arXiv2023

A High-Eccentricity Warm Jupiter Orbiting TOI-4127

We report the discovery of TOI-4127 b, a transiting, Jupiter-sized exoplanet on a long-period ($P = 56.39879^{+0.00010}_{-0.00010}$ d), high-eccentricity orbit around a late F-type dwarf star. This warm Jupiter was first detected and identified as a promising candidate from a search for single-transit signals in TESS Sector 20 data, and later characterized as a planet following two subsequent transits (TESS Sectors 26 and 53) and follow-up ground-based RV observations with the NEID and SOPHIE spectrographs. We jointly fit the transit and RV data to constrain the physical ($R_p = 1.096^{+0.039}_{-0.032} R_J$, $M_p = 2.30^{+0.11}_{-0.11} M_J$) and orbital parameters of the exoplanet. Given its high orbital eccentricity ($e=0.7471^{+0.0078}_{-0.0086}$), TOI-4127 b is a compelling candidate for studies of warm Jupiter populations and of hot Jupiter formation pathways. We show that the present periastron separation of TOI-4127 b is too large for high-eccentricity tidal migration to circularize its orbit, and that TOI-4127 b is unlikely to be a hot Jupiter progenitor unless it is undergoing angular momentum exchange with an undetected outer companion. Although we find no evidence for an external companion, the available observational data are insufficient to rule out the presence of a perturber that can excite eccentricity oscillations and facilitate tidal migration.

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arXiv2023

Photochemically-produced SO$_2$ in the atmosphere of WASP-39b

Photochemistry is a fundamental process of planetary atmospheres that regulates the atmospheric composition and stability. However, no unambiguous photochemical products have been detected in exoplanet atmospheres to date. Recent observations from the JWST Transiting Exoplanet Early Release Science Program found a spectral absorption feature at 4.05 $μ$m arising from SO$_2$ in the atmosphere of WASP-39b. WASP-39b is a 1.27-Jupiter-radii, Saturn-mass (0.28 M$_J$) gas giant exoplanet orbiting a Sun-like star with an equilibrium temperature of $\sim$1100 K. The most plausible way of generating SO$_2$ in such an atmosphere is through photochemical processes. Here we show that the SO$_2$ distribution computed by a suite of photochemical models robustly explains the 4.05 $μ$m spectral feature identified by JWST transmission observations with NIRSpec PRISM (2.7$σ$) and G395H (4.5$σ$). SO$_2$ is produced by successive oxidation of sulphur radicals freed when hydrogen sulphide (H$_2$S) is destroyed. The sensitivity of the SO$_2$ feature to the enrichment of the atmosphere by heavy elements (metallicity) suggests that it can be used as a tracer of atmospheric properties, with WASP-39b exhibiting an inferred metallicity of $\sim$10$\times$ solar. We further point out that SO$_2$ also shows observable features at ultraviolet and thermal infrared wavelengths not available from the existing observations.

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arXiv2023

Disentangling Stellar and Airglow Emission Lines from HST-COS Spectra

H I Ly$α$ (1215.67 Å) and the O I triplet (1302.17, 1304.86, and 1306.03 Å) are bright far-ultraviolet (FUV) emission lines that trace the stellar chromosphere. Observations of stellar Ly$α$ and O I using the Hubble Space Telescope's (HST) most sensitive FUV spectrograph, the Cosmic Origins Spectrograph (COS), are contaminated with geocoronal emission, or airglow. This study demonstrates that airglow emission profiles as observed by COS are sufficiently stable to create airglow templates which can be reliably subtracted from the data, recovering the underlying stellar flux. We developed a graphical user interface to implement the airglow subtraction on a sample of 171 main sequence F, G, K, and M-type dwarfs from the COS data archive. Correlations between recovered stellar emission and measures of stellar activity were investigated. Several power law relationships are presented for predicting the stellar Ly$α$ and O I emission. The apparent brightness of the stellar emission relative to the airglow is a critical factor in the success or failure of an airglow subtraction. We developed a predictor for the success of an airglow subtraction using the signal-to-noise ratio (SNR) of the nearby chromospheric emission line Si III (1206.51 Å). The minimum attenuated Ly$α$ flux which was successfully recovered is 1.39$\times$10$^{-14}$ erg cm$^{-2}$ s$^{-1}$, and we recommend this as a minimum flux for COS Ly$α$ recoveries.

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arXiv2023

A Second Earth-Sized Planet in the Habitable Zone of the M Dwarf, TOI-700

We report the discovery of TOI-700 e, a 0.95 R$_\oplus$ planet residing in the Optimistic Habitable Zone (HZ) of its host star. This discovery was enabled by multiple years of monitoring from NASA's Transiting Exoplanet Survey Satellite (TESS) mission. The host star, TOI-700 (TIC 150428135), is a nearby (31.1 pc), inactive, M2.5 dwarf ($V_{mag} = 13.15$). TOI-700 is already known to host three planets, including the small, HZ planet, TOI-700 d. The new planet has an orbital period of 27.8 days and, based on its radius (0.95 R$_\oplus$), it is likely rocky. TOI-700 was observed for 21 sectors over Years 1 and 3 of the TESS mission, including 10 sectors at 20-second cadence in Year 3. Using this full set of TESS data and additional follow-up observations, we identify, validate, and characterize TOI-700 e. This discovery adds another world to the short list of small, HZ planets transiting nearby and bright host stars. Such systems, where the stars are bright enough that follow-up observations are possible to constrain planet masses and atmospheres using current and future facilities, are incredibly valuable. The presence of multiple small, HZ planets makes this system even more enticing for follow-up observations.

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