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The JWST Early-release Science Program for Direct Observations of Exoplanetary Systems II: A 1 to 20 μm Spectrum of the Planetary-mass Companion VHS 1256–1257 b
We present the highest fidelity spectrum to date of a planetary-mass object. VHS 1256 b is a <20 M Jup widely separated (∼8″, a = 150 au), young, planetary-mass companion that shares photometric colors and spectroscopic features with the directly imaged exoplanets HR 8799c, d, and e. As an L-to-T transition object, VHS 1256 b exists along the region of the color–magnitude diagram where substellar atmospheres transition from cloudy to clear. We observed VHS 1256 b with JWST's NIRSpec IFU and MIRI MRS modes for coverage from 1 to 20 μm at resolutions of ∼1000–3700. Water, methane, carbon monoxide, carbon dioxide, sodium, and potassium are observed in several portions of the JWST spectrum based on comparisons from template brown dwarf spectra, molecular opacities, and atmospheric models. The spectral shape of VHS 1256 b is influenced by disequilibrium chemistry and clouds. We directly detect silicate clouds, the first such detection reported for a planetary-mass companion.
Journal Article
Exoplanets : hidden worlds and the quest for extraterrestrial life
\"Astronomers have recently discovered thousands of exotic planets that orbit stars throughout our Milky Way galaxy. With his characteristic wit and style, Donald Goldsmith shows how these observations have already broadened our planetary horizons, and tells us what may come next, including the ultimate discovery: life beyond our home planet.\"--Publisher's description.
Book
Direct Imaging Constraints on Binary Planets and Exomoons around Epsilon Indi A b
Epsilon Indi A b (hereafter Eps Ind A b) is a directly imaged ∼6 MJup exoplanet orbiting a nearby (3.6 pc) K dwarf at ∼30 au. We analyze archival JWST/MIRI 15 μm coronagraphic imaging of this planet to search for directly imaged satellites orbiting Eps Ind A b. Within the planet’s Hill sphere (radius RH ≈ 2.3 au or 1.3 λ/D), we compare single and double point-spread-function (PSF) models using Bayesian evidence. We find that a double-PSF (binary planet) fit is preferred. This apparent preference can most plausibly be explained by systematics, although follow-up observations would be required to fully rule out a binary planet interpretation. We construct a contrast curve of the exoplanet after removing this feature, demonstrating sensitivity to companions as faint as 0.03× the F1550C flux of Eps Ind A b (equivalent to T = 130 K, 1.3 MJup) at large separations (> 2 au). We also demonstrate sensitivity to brighter companions 0.2× the F1550C flux of Eps Ind A b (equivalent to T = 180 K, 2.5 MJup) down to separations of 0.52 au (1.3 pixels; 0.29 λ/D; 144 mas). This study demonstrates that JWST/MIRI can directly detect exomoons or binary planets inside the Hill sphere of directly imaged exoplanets orbiting neighboring stars.
Journal Article
Detecting Exomoons from Radial Velocity Measurements of Self-luminous Planets: Application to Observations of HR 7672 B and Future Prospects
The detection of satellites around extrasolar planets, so called exomoons, remains a largely unexplored territory. In this work, we study the potential of detecting these elusive objects from radial velocity monitoring of self-luminous, directly imaged planets. This technique is now possible thanks to the development of dedicated instruments combining the power of high-resolution spectroscopy and high-contrast imaging. First, we demonstrate a sensitivity to satellites with a mass ratio of 1%–4% at separations similar to the Galilean moons from observations of a brown-dwarf companion (HR 7672 B; K mag = 13; 0.″7 separation) with the Keck Planet Imager and Characterizer (R ∼ 35,000 in the K band) at the W. M. Keck Observatory. Current instrumentation is therefore already sensitive to large unresolved satellites that could be forming from gravitational instability akin to binary star formation. Using end-to-end simulations, we then estimate that future instruments such as the Multi-Object Diffraction-limited High-resolution Infrared Spectrograph, planned for the Thirty Meter Telescope, should be sensitive to satellites with mass ratios of ∼10−4. Such small moons would likely form in a circumplanetary disk similar to the Jovian satellites in the solar system. Looking for the Rossiter–McLaughlin effect could also be an interesting pathway to detecting the smallest moons on short orbital periods. Future exomoon discoveries will allow precise mass measurements of the substellar companions that they orbit and provide key insight into the formation of exoplanets. They would also help constrain the population of habitable Earth-sized moons orbiting gas giants in the habitable zone of their stars.
Journal Article
Occurrence Rates of Exosatellites Orbiting 3–30 M Jup Hosts from 44 Spitzer Light Curves
We conduct a comprehensive search for transiting exomoons and exosatellites within 44 archival Spitzer light curves of 32 substellar worlds with estimated masses ranging between 3 and 30 M Jup. This sample’s median host mass is 16 M Jup, inclusive of 14 planetary-mass objects, among which one is a wide-orbit exoplanet. We search the light curves for exosatellite signatures and implement a transit injection-recovery test, illustrating our survey’s capability to detect >0.7 R ⊕ exosatellites. Our findings reveal no substantial (>5σ) evidence for individual transit events. However, an unusual fraction of light curves favor the transit model at the 2–3σ significance level, with fitted transit depths consistent with terrestrial-sized (0.7–1.6 R ⊕) bodies. Comparatively, fewer than 2.2% of randomly generated normal distributions from an equivalent sample size exhibit a similar prevalence of outliers. Should one or two of these outliers represent a real exosatellite transit, it would imply an occurrence rate of η=0.61−0.34+0.49 short-period terrestrial exosatellites per system, consistent with the known occurrences rates for both solar system moons and mid-M dwarf exoplanets. We explore alternative astrophysical interpretations for these outliers, underscoring that transits are not the only plausible explanation. For orbital periods <0.8 days, the typical duration of the light curves, we constrain the occurrence rate of sub-Neptunes to η < 0.35 (95% confidence) and, if none of the detected outlier signals are real, the occurrence rate of terrestrial (∼Earth-sized) exosatellites to η < 0.51 (95% confidence). Forthcoming JWST observations of substellar light curves will enable the detection of sub-Io-sized exosatellites, allowing for much stronger constraints on this exosatellite population.
Journal Article