Information in the Reflected-light Spectra of Widely Separated Giant Exoplanets

Hu, Renyu

doi:10.3847/1538-4357/ab58c7

Cited by 32 publications

(37 citation statements)

References 86 publications

(151 reference statements)

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“…The degeneracy is between the atmospheric scenario of a higher cloud and greater mixing ratio of CH 4 versus the scenario of a lower or absent cloud and smaller mixing ratio of CH 4 . This type of degeneracy has been discussed in Hu (2014Hu ( , 2019 and it is shown here with realistic noise estimates and rigorous retrieval.…”

Section: Discussionmentioning

confidence: 55%

“…We simulated the atmospheric abundance and cloud structure of this exoplanet in our previous work (Damiano & Hu 2020). Here we use EXOREL (Hu 2019;Damiano & Hu 2020) to synthesize the reflected light spectra at the orbital phases of π/3 and π/2. These phases are picked because the phase of π/3 is a good compromise between the angular separation between the planet and the star and the brightness in the reflected light (and thus the go-to phase of a single observation).…”

Section: Simulated Planet Spectramentioning

confidence: 99%

“…Reflected light spectra from these cold gaseous planets contain rich information on the chemical composition and cloud formation in their atmospheres (e.g., Sudarsky et al 2000Sudarsky et al , 2003Burrows et al 2004;Cahoy et al 2010;Lupu et al 2016;MacDonald et al 2018;Hu 2019;Damiano & Hu 2020;Carrión-González et al 2020). When the wavelength coverage is limited and with moderate wavelength resolution and signalto-noise ratio (S/N), there can be substantial degeneracy between the atmospheric abundance and the cloud pressure (e.g., Lupu et al 2016;Nayak et al 2017;Hu 2019;Carrión-González et al 2020). In short, a deep cloud and a small abundance of an absorber (e.g., CH 4 ) may result in a similar spectrum as a shallower cloud and a higher abundance of absorber, leading to the degeneracy.…”

Section: Introductionmentioning

confidence: 99%

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Multi-orbital-phase and Multiband Characterization of Exoplanetary Atmospheres with Reflected Light Spectra

Damiano

Hildebrandt

2020

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Direct imaging of widely separated exoplanets from space will obtain their reflected light spectra and measure atmospheric properties. Previous calculations have shown that a change in the orbital phase would cause a spectral signal, but whether this signal may be used to characterize the atmosphere has not been shown. We simulate starshade-enabled observations of the planet 47 UMa b, using the present most realistic simulator Starshade Imaging Simulation Toolkit for Exoplanet Reconnaissance to estimate the uncertainties due to residual starlight, solar glint, and exozodiacal light. We then use the Bayesian retrieval algorithm EXOREL R to determine the constraints on the atmospheric properties from observations using a Roman-or Habitable Exoplanet Observatory (HabEx)-like telescope, comparing the strategies to observe at multiple orbital phases or in multiple wavelength bands. With a ∼20% bandwidth in 600-800 nm on a Roman-like telescope, the retrieval finds a degenerate scenario with a lower gas abundance and a deeper or absent cloud than the truth. Repeating the observation at a different orbital phase or at a second 20% wavelength band in 800-1000 nm, with the same integration time and thus degraded signal-to-noise ratio (S/N), would effectively eliminate this degenerate solution. Single observation with a HabEx-like telescope would yield high-precision constraints on the gas abundances and cloud properties, without the degenerate scenario. These results are also generally applicable to high-contrast spectroscopy with a coronagraph with a similar wavelength coverage and S/N, and can help design the wavelength bandwidth and the observation plan of exoplanet direct-imaging experiments in the future.

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Section: Discussionmentioning

confidence: 55%

Section: Simulated Planet Spectramentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

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Multi-orbital-phase and Multiband Characterization of Exoplanetary Atmospheres with Reflected Light Spectra

Damiano

Hildebrandt

2020

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“…We adopt a requirement of S∕N ¼ 20 per spectral element as this is a conservative estimate of the precision needed to measure atmospheric abundances from the reflected-light spectra and potentially distinguish the types of planets. [44][45][46][47][48][49] The expected integration time to detect planets in the broadband is substantially less than what is shown in this section. Also all results shown in this section assume unbiased calibration of the background to the photon-noise limit.…”

Section: Results and Analysesmentioning

confidence: 68%

Overview and reassessment of noise budget of starshade exoplanet imaging

Lisman

Shaklan

et al. 2021

J. Astron. Telesc. Instrum. Syst.

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High-contrast imaging enabled by a starshade in formation flight with a space telescope can provide a near-term pathway to search for and characterize temperate and small planets of nearby stars. NASA's Starshade Technology Development Activity to TRL5 (S5) is rapidly maturing the required technologies to the point at which starshades could be integrated into potential future missions. We reappraise the noise budget of starshade-enabled exoplanet imaging to incorporate the experimentally demonstrated optical performance of the starshade and its optical edge. Our analyses of stray light sources-including the leakage through micrometeoroid damage and the reflection of bright celestial bodies-indicate that sunlight scattered by the optical edge (i.e., the solar glint) is by far the dominant stray light. With telescope and observation parameters that approximately correspond to Starshade Rendezvous with Roman and Habitable Exoplanet Observatory (HabEx), we find that the dominating noise source is exozodiacal light for characterizing a temperate and Earth-sized planet around Sun-like and earlier stars and the solar glint for later-type stars. Further, reducing the brightness of solar glint by a factor of 10 with a coating would prevent it from becoming the dominant noise for both Roman and HabEx. With an instrument contrast of 10 −10 , the residual starlight is not a dominant noise, and increasing the contrast level by a factor 10 does not lead to any appreciable change in the expected science performance. If unbiased calibration of the background to the photon-noise limit can be achieved, Starshade Rendezvous with Roman could provide nearly photon-limited spectroscopy of temperate and Earth-sized planets of F, G, and K stars <4 parsecs away, and HabEx could extend this capability to many more stars <8 parsecs. Larger rocky planets around stars <8 parsecs would be within the reach of Roman. To achieve these capabilities, the exozodiacal light may need to be calibrated to a precision better than 2% and the solar glint to better than 5%. Our analysis shows that the expected temporal variability of the solar glint is unlikely to hinder the calibration, and the main challenge for background calibration likely comes from the unsmooth spatial distribution of exozodiacal dust in some stars. Taken together, these results validate the optical noise budget and technology milestones adopted by S5 against key science objectives and inform the priorities of future technology developments and science and industry community partnerships. © The Authors. Published by SPIE under a Creative Commons Attribution 4.0 Unported License. Distribution or reproduction of this work in whole or in part requires full attribution of the original publication, including its DOI.

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“…Although the above space missions are still years away, there is already significant interest to understand their prospective scientific output (see e.g. Cahoy et al 2010;Greco & Burrows 2015;Lupu et al 2016;Robinson et al 2016;Nayak et al 2017;Batalha et al 2018;Feng et al 2018;Guimond & Cowan 2018;Damiano & Hu 2019;Hu 2019;Lacy et al 2019). Such investigations will have an impact on the design of future telescopes and their observing strategies.…”

Section: Introductionmentioning

confidence: 99%

Directly imaged exoplanets in reflected starlight. The importance of knowing the planet radius

Carrión-González¹,

Muñoz²,

Cabrera³

et al. 2024

Preprint

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Context. The direct imaging of exoplanets in reflected starlight will represent a major advance in the study of cold and temperate exoplanet atmospheres. Understanding how basic planet and atmospheric properties may affect the measured spectra is key to their interpretation. Aims. We have investigated the information content in reflected-starlight spectra of exoplanets. We specify our analysis to Barnard's Star b candidate super-Earth, for which we assume a radius 0.6 times that of Neptune, an atmosphere dominated by H 2-He, and a CH 4 volume mixing ratio of 5•10 −3. The main conclusions of our study are however planet-independent. Methods. We set up a model of the exoplanet described by seven parameters including its radius, atmospheric methane abundance and basic properties of a cloud layer. We generate synthetic spectra at zero phase (full disk illumination) from 500 to 900 nm and spectral resolution R∼125-225. We simulate a measured spectrum with a simplified, wavelength-independent noise model at Signal-to-Noise ratio S/N=10. With an MCMC-based retrieval methodology, we analyse which planet/atmosphere parameters can be inferred from the measured spectrum and the theoretical correlations amongst them. We consider limiting cases in which the planet radius is either known or completely unknown, and intermediate cases in which the planet radius is partly constrained. Results. If the planet radius is known, we can generally discriminate between cloud-free and cloudy atmospheres, and constrain the methane abundance to within two orders of magnitude. If the planet radius is unknown, new correlations between model parameters occur and the accuracy of the retrievals decreases. Without a radius determination, it is challenging to discern whether the planet has clouds, and the estimates on methane abundance degrade. However, we find the planet radius is constrained to within a factor of two for all the cases explored. Having a priori information on the planet radius, even if approximate, helps improve the retrievals. Conclusions. Reflected-starlight measurements will open a new avenue for characterising long-period exoplanets, a population that remains poorly studied. For this task to be complete, direct-imaging observations should be accompanied by other techniques. We urge exoplanet detection efforts to extend the population of long-period planets with mass and radius determinations.

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Information in the Reflected-light Spectra of Widely Separated Giant Exoplanets

Cited by 32 publications

References 86 publications

Multi-orbital-phase and Multiband Characterization of Exoplanetary Atmospheres with Reflected Light Spectra

Multi-orbital-phase and Multiband Characterization of Exoplanetary Atmospheres with Reflected Light Spectra

Overview and reassessment of noise budget of starshade exoplanet imaging

Directly imaged exoplanets in reflected starlight. The importance of knowing the planet radius

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