An official website of the United States government
Abstract Young exoplanets are attractive targets for atmospheric characterization to explore the early phase of planetary evolution and the surrounding environment. Recent observations of the 10 Myr young Neptune-sized exoplanet K2-33b revealed that the planet’s transit depth drastically decreases from the optical to near-infrared wavelengths. Thao et al. suggested that a thick planetary haze and/or stellar spots may be the cause; however, even the best-fit model only barely explains the data. Here, we propose that the peculiar transmission spectrum may indicate that K2-33b possesses a circumplanetary dust ring; an analog of Jupiter’s dust ring. We demonstrate that the ring could produce a steep slope in the transmission spectrum even if its optical depth is as low as ∼10 −2 . We then apply a novel joint atmosphere-ring retrieval to K2-33b and find that the ring scenario could well explain the observed spectrum for various possible ring compositions. Importantly, the dust ring also exhibits prominent ring particle absorption features of ring particles around ∼10 μ m, whose shape and strength depend on the composition of the ring. Thus, future observations by JWST-MIRI would be able to test not only the ring hypothesis but also, if it indeed exists, to constrain the composition of the ring—providing a unique opportunity to explore the origins of the dust ring around its parent planet, soon after the planetary system’s formation.
more »
« less
Hazy with a Chance of Star Spots: Constraining the Atmosphere of Young Planet K2-33b
Abstract Although all-sky surveys have led to the discovery of dozens of young planets, little is known about their atmospheres. Here, we present multiwavelength transit data for the super-Neptune sized exoplanet, K2-33b—the youngest (∼10 Myr) transiting exoplanet to date. We combined photometric observations of K2-33 covering a total of 33 transits spanning >2 yr, taken from K2, MEarth, the Hubble Space Telescope (HST), and Spitzer. The transit photometry spanned from the optical to the near-infrared (0.6–4.5μm), enabling us to construct a transmission spectrum of the planet. We find that the optical transit depths are nearly a factor of 2 deeper than those from the near-infrared. This difference holds across multiple data sets taken over years, ruling out issues of data analysis and unconstrained systematics. Surface inhomogeneities on the young star can reproduce some of the difference, but required spot coverage fractions (>60%) are ruled out by the observed stellar spectrum (<20%). We find a better fit to the transmission spectrum using photochemical hazes, which were predicted to be strong in young, moderate-temperature, and large-radius planets like K2-33b. A tholin haze with CO as the dominant gaseous carbon carrier in the atmosphere can reasonably reproduce the data with small or no stellar surface inhomogeneities, consistent with the stellar spectrum. The HST data quality is insufficient for the detection of any molecular features. More observations would be required to fully characterize the hazes and spot properties and confirm the presence of CO suggested by current data.
more »
« less
- Award ID(s):
- 2143763
- PAR ID:
- 10386658
- Publisher / Repository:
- DOI PREFIX: 10.3847
- Date Published:
- Journal Name:
- The Astronomical Journal
- Volume:
- 165
- Issue:
- 1
- ISSN:
- 0004-6256
- Format(s):
- Medium: X Size: Article No. 23
- Size(s):
- Article No. 23
- Sponsoring Org:
- National Science Foundation
More Like this
-
-
Abstract Orbiting an M dwarf 12 pc away, the transiting exoplanet GJ 1132b is a prime target for transmission spectroscopy. With a mass of 1.7M⊕and radius of 1.1R⊕, GJ 1132b’s bulk density indicates that this planet is rocky. Yet with an equilibrium temperature of 580 K, GJ 1132b may still retain some semblance of an atmosphere. Understanding whether this atmosphere exists and its composition will be vital for understanding how the atmospheres of terrestrial planets orbiting M dwarfs evolve. We observe five transits of GJ 1132b with the Wide Field Camera 3 (WFC3) on the Hubble Space Telescope (HST). We find a featureless transmission spectrum from 1.1 to 1.7μm, ruling out cloud-free atmospheres with metallicities <300× solar with >4.8σconfidence. We combine our WFC3 results with transit depths from TESS and archival broadband and spectroscopic observations to find a featureless spectrum across 0.7 to 4.5μm. GJ 1132b therefore has a high mean molecular weight atmosphere, possesses a high-altitude aerosol layer, or has effectively no atmosphere. Higher-precision observations are required in order to differentiate between these possibilities. We explore the impact of hot and cold starspots on the observed transmission spectrum GJ 1132b, quantifying the amplitude of spot-induced transit depth features. Using a simple Poisson model, we estimate spot temperature contrasts, spot covering fractions, and spot sizes for GJ 1132. These limits, as well as the modeling framework, may be useful for future observations of GJ 1132b or other planets transiting similarly inactive M dwarfs.more » « less
-
Abstract The upcoming deployment of the James Webb Space Telescope will dramatically advance our ability to characterize exoplanet atmospheres, both in terms of precision and sensitivity to smaller and cooler planets. Disequilibrium chemical processes dominate these cooler atmospheres, requiring accurate photochemical modeling of such environments. The host star’s UV spectrum is a critical input to these models, but most exoplanet hosts lack UV observations. For cases in which the host UV spectrum is unavailable, a reconstructed or proxy spectrum will need to be used in its place. In this study, we use the MUSCLES catalog and UV line scaling relations to understand how well reconstructed host star spectra reproduce photochemically modeled atmospheres using real UV observations. We focus on two cases: a modern Earth-like atmosphere and an Archean Earth-like atmosphere that forms copious hydrocarbon hazes. We find that modern Earth-like environments are well-reproduced with UV reconstructions, whereas hazy (Archean Earth) atmospheres suffer from changes at the observable level. Specifically, both the stellar UV emission lines and the UV continuum significantly influence the chemical state and haze production in our modeled Archean atmospheres, resulting in observable differences in their transmission spectra. Our modeling results indicate that UV observations of individual exoplanet host stars are needed to accurately characterize and predict the transmission spectra of hazy terrestrial atmospheres. In the absence of UV data, reconstructed spectra that account for both UV emission lines and continuum are the next best option, albeit at the cost of modeling accuracy.more » « less
-
Abstract Transmission spectroscopy offers an invaluable opportunity to characterize the atmospheres of exoplanets. We present new ground-based optical transmission spectra of the hot Jupiter HD 189733b, derived from nine transits observed over a six year time span (2016–2021) using near-simultaneous broadband observations. We achieve an average (best) precision of 435 (280) ppm by implementing an optical diffuser on the prime focus spectrograph from the 2.3 m Wyoming Infrared Observatory telescope. The data provide new measurements of the apparent planetary radius with respect to the stellar radius, the spectral index of atmospheric opacity, and the time variability of the two quantities. Our results indicate an enhanced spectral slope in the optical regime ≈2.4 times steeper than would be expected from canonical Rayleigh scattering and that is consistent with earlier measurements of a super-Rayleigh slope (SRS). While the effect of stellar activity on the transmission spectrum complicates the measurement of the spectral slope, our multiepoch data set over six years can measure and average over stellar variations, yielding a mean spectral index of −9.9 ± 4.4. The 1200 K equilibrium temperature of HD 189733b places it in a sweet spot for the formation of SRSs and is consistent with vigorously mixing hazes in the atmosphere. Additionally, we find variations in the depth of the lightcurve during two of the transits, explainable as an increase in occulted star spots during June 2021. Although the star is active, the mean level of stellar activity does not seem to vary dramatically over our six years of observations, leading us to conclude that the variability in stellar activity is modest at most.more » « less
-
Abstract The characterization of young planets (<300 Myr) is pivotal for understanding planet formation and evolution. We present the 3–5μm transmission spectrum of the 17 Myr, Jupiter-size (R∼10R⊕) planet, HIP 67522b, observed with JWST NIRSpec/G395H. To check for spot contamination, we obtain a simultaneousg-band transit with the Southern Astrophysical Research Telescope. 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 67522b’s mass is <20M⊕irrespective of cloud cover and stellar contamination. A Bayesian retrieval analysis returns a mass constraint of 13.8 ± 1.0M⊕. This challenges the previous classification of HIP 67522b as a hot Jupiter and instead, positions it as a precursor to the more common sub-Neptunes. With a density of <0.10 g cm−3, HIP 67522 b is one of the lowest-density planets known. We find strong absorption from H2O and CO2(≥7σ), a modest detection of CO (3.5σ), and weak detections of H2S and SO2(≃2σ). 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 × 10 solar due to the amplitude of the SO2feature. These results point to the formation of HIP 67522b beyond the water snowline, where its envelope was polluted by icy pebbles and planetesimals. The planet is likely experiencing substantial mass loss (0.01–0.03M⊕Myr−1), sufficient for envelope destruction within a gigayear. This highlights the dramatic evolution occurring within the first 100 Myr of its existence.more » « less
