The search for water-rich Earth-sized exoplanets around low-mass stars is rapidly gaining attention because they represent the best opportunity to characterize habitable planets in the near future. Understanding the atmospheres of these planets and determining the optimal strategy for characterizing them through transmission spectroscopy with our upcoming instrumentation is essential in order to constrain their environments. For this study, we present simulated transmission spectra of tidally locked Earth-sized ocean-covered planets around late-M to mid-K stellar spectral types, utilizing GCM modeling results previously published by Kopparapu et al. (2017) as inputs for our radiative transfer calculations performed using NASA's Planetary Spectrum Generator (psg.gsfc.nasa.gov; Villanueva et al. (2018)). We identify trends in the depth of H 2 O spectral features as a function of planet surface temperature and rotation rate. These trends allow us to calculate the exposure times necessary to detect water vapor in the atmospheres of aquaplanets through transmission spectroscopy with the upcoming James Webb Space Telescope (JWST) as well as several future flagship space telescope concepts under consideration (LUVOIR and OST) for a target list constructed from the TESS Input Catalog (TIC). Our calculations reveal that transmission spectra for water-rich Earth-sized planets around low-mass stars will be dominated by clouds, with spectral features < 20 ppm, and only a small subset of TIC stars would allow for the characterization of an ocean planet in the Habitable Zone. We thus present a careful prioritization of targets that are most amenable to follow-up characterizations with next-generation instrumentation, in order to assist the community in efficiently utilizing precious telescope time.
We present the discovery and validation of a three-planet system orbiting the nearby (31.1 pc) M2 dwarf star TOI-700 (TIC 150428135). TOI-700 lies in the TESS continuous viewing zone in the Southern Ecliptic Hemisphere; observations spanning 11 sectors reveal three planets with radii ranging from 1 R ⊕ to 2.6 R ⊕ and orbital periods ranging from 9.98 to 37.43 days. Ground-based follow-up combined with diagnostic vetting and validation tests enable us to rule out common astrophysical false-positive scenarios and validate the system of planets. The outermost planet, TOI-700 d, has a radius of 1.19 ± 0.11 R ⊕ and resides in the conservative habitable zone of its host star, where it receives a flux from its star that is approximately 86% of the Earth's insolation. In contrast to some other low-mass stars that host Earth-sized planets in their habitable zones, TOI-700 exhibits low levels of stellar activity, presenting a valuable opportunity to study potentially-rocky planets over a wide range of conditions affecting atmospheric escape. While atmospheric characterization of TOI-700 d with the James Webb Space Telescope (JWST) will be challenging, the larger sub-Neptune, TOI-700 c (R = 2.63 R ⊕), will be an excellent target for JWST and beyond. TESS is scheduled to return to the Southern Hemisphere and observe TOI-700 for an additional 11 sectors in its extended mission, which should provide further constraints on the known planet parameters and searches for additional planets and transit timing variations in the system.
Seven rocky planets orbit the nearby dwarf star TRAPPIST-1, providing a unique opportunity to search for atmospheres on small planets outside the Solar System1. Thanks to the recent launch of the James Webb Space Telescope (JWST), possible atmospheric constituents such as carbon dioxide (CO2) are now detectable2,3. Recent JWST observations of the innermost planet TRAPPIST-1 b showed that it is most probably a bare rock without any CO2 in its atmosphere4. Here we report the detection of thermal emission from the dayside of TRAPPIST-1 c with the Mid-Infrared Instrument (MIRI) on JWST at 15 µm. We measure a planet-to-star flux ratio of fp/f⁎ = 421 ± 94 parts per million (ppm), which corresponds to an inferred dayside brightness temperature of 380 ± 31 K. This high dayside temperature disfavours a thick, CO2-rich atmosphere on the planet. The data rule out cloud-free O2/CO2 mixtures with surface pressures ranging from 10 bar (with 10 ppm CO2) to 0.1 bar (pure CO2). A Venus-analogue atmosphere with sulfuric acid clouds is also disfavoured at 2.6σ confidence. Thinner atmospheres or bare-rock surfaces are consistent with our measured planet-to-star flux ratio. The absence of a thick, CO2-rich atmosphere on TRAPPIST-1 c suggests a relatively volatile-poor formation history, with less than $${9.5}_{-2.3}^{+7.5}$$ 9.5 − 2.3 + 7.5 Earth oceans of water. If all planets in the system formed in the same way, this would indicate a limited reservoir of volatiles for the potentially habitable planets in the system.
We present Spitzer 4.5µm observations of the transit of TOI-700 d, a habitable zone Earth-sized planet in a multiplanet system transiting a nearby M-dwarf star (TIC 150428135, 2MASS J06282325-6534456). TOI-700 d has a radius of 1.220 +0.073 −0.063 R ⊕ and orbits within its host star's conservative habitable zone with a period of 37.42 days (T eq ∼ 269 K). TOI-700 also hosts two small inner planets (R b =1.044 +0.065 −0.063 R ⊕ & R c =2.64 +0.16 −0.14 R ⊕) with periods of 9.98 and 16.05 days, respectively. Our Spitzer observations confirm the TESS detection of TOI-700 d and remove any remaining doubt that it is a genuine planet. We analyze the Spitzer light curve combined
We present self-consistent three-dimensional climate simulations of possible habitable states for the newly discovered habitable-zone Earth-sized planet TOI-700 d. We explore a variety of atmospheric compositions, pressures, and rotation states for both ocean-covered and completely desiccated planets in order to assess the planet’s potential for habitability. For all 20 of our simulated cases, we use our climate model outputs to synthesize transmission spectra, combined-light spectra, and integrated broadband phase curves. These climatologically informed observables will help the community assess the technological capabilities necessary for future characterization of this planet—as well as similar transiting planets discovered in the future—and will provide a guide for distinguishing possible climate states if one day we do obtain sensitive spectral observations of a habitable planet around an M star. We find that TOI-700 d is a strong candidate for a habitable world and can potentially maintain temperate surface conditions under a wide variety of atmospheric compositions. Unfortunately, the spectral feature depths from the resulting transmission spectra and the peak flux and variations from our synthesized phase curves for TOI-700 d do not exceed 10 ppm. This will likely prohibit the James Webb Space Telescope from characterizing its atmosphere; however, this motivates the community to invest in future instrumentation that perhaps can one day reveal the true nature of TOI-700 d and to continue to search for similar planets around less distant stars.
The interior structure of an exoplanet is hidden from direct view yet likely plays a crucial role in influencing the habitability of Earth analogs. Inferences of the interior structure are impeded by a fundamental degeneracy that exists between any model comprising of more than two layers and observations constraining just two bulk parameters: mass and radius. In this work, we show that although the inverse problem is indeed degenerate, there exists two boundary conditions that enables one to infer the minimum and maximum core radius fraction, CRF min & CRF max . These hold true even for planets with light volatile envelopes, but require the planet to be fully differentiated and that layers denser than iron are forbidden. With both bounds in hand, a marginal CRF can also be inferred by sampling inbetween. After validating on the Earth, we apply our method to Kepler-36b and measure CRF min = (0.50 ± 0.07), CRF max = (0.78±0.02) and CRF marg = (0.64±0.11), broadly consistent with the Earth's true CRF value of 0.55. We apply our method to a suite of hypothetical measurements of synthetic planets to serve as a sensitivity analysis. We find that CRF min & CRF max have recovered uncertainties proportional to the relative error on the planetary density, but CRF marg saturates to between 0.03 to 0.16 once (∆ρ/ρ) drops below 1-2%. This implies that mass and radius alone cannot provide any better constraints on internal composition once bulk density constraints hit around a percent, providing a clear target for observers.
We report the discovery of TOI-700 e, a 0.95 R ⊕ 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 ⊕), 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 s 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.
The TRAPPIST-1 system provides an exquisite laboratory for understanding exoplanetary atmospheres and interiors. Their mutual gravitational interactions leads to transit timing variations, from which Grimm et al. (2018) recently measured the planetary masses with precisions ranging from 5% to 12%. Combined with < 5% radius measurements on each planet, TRAPPIST-1 provides a unique opportunity to examine the range of permissible planetary interiors. Grimm et al. (2018) used their new masses and radii and compared them to those expected for planets comprised of pure silicate (no iron or volatiles). This revealed that planets b, d, f, g and h likely contain volatile layers to explain their properties, but c and e are compatible with being rocky. This is an example of a boundary condition comparison, first described in Kipping et al. (2013) in the context of planetary interiors, where the authors show how a minimum envelope height can be derived by comparison to pure water models. We briefly note that planets b through h all have a minimum envelope height compatible with zero when applying the method of Kipping et al. (2013) to the Grimm et al. (2018) masses and radii, to a confidence of ≥ 99.999%.Grimm et al. (2018)'s inference that planets c and e are consistent with a rock-iron composition is useful, but it is possible to go further and actually quantify the minimum and maximum size of an iron core using boundary condition arguments. Such an approach is laid out in our recent paper Suissa et al. (2018). In that work, we considered that the maximum core size is found by solving when the mass and radius of the planet equals that of an iron core surrounded by a light hydrogen/helium envelope. However, recent atmospheric studies by de Wit et al. (2016 exclude the possibility of such envelopes for planets b through f. Accordingly, we updated our model, hardCORE, such that the maximum core size corresponds to the next lightest layer plausibly found around the core, a water layer (where as in our original paper we use the Zeng & Sasselov (2013) interior model).Using the Grimm et al. (2018) posteriors, we are then able to derive a minimum and maximum core size for each planet (using the original maximum formulation for planets g and h). We find that the minimum core size is consistent with zero for all of the planets except e. In particular, for planet c, unlike the result of Grimm et al. (2018), we find that the radius of the planet if pure silicate would be 1.09 +0.04 −0.04 R ⊕ , which is consistent with the observed radius of 1.09 +0.03 −0.03 R ⊕ . The difference is likely a product of the different equations-of-state used in each model, that of Zeng & Sasselov (2013) used in our work, versus that of Connolly (2009) used by Grimm et al. (2018). We find that the probability of an iron core is modest at 57% and thus ambiguity remains regarding c's interior. For planet e, however, 99.3% of the posterior samples are consistent with a silicate-iron model indicating strong evidence for an iron core. Substituting the silicate ...
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.