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The Thermal Emission Spectrum of the Rocky Exoplanet LTT 1445A b Observed with JWST MIRI/LRS


Core Concepts
Observations of the rocky exoplanet LTT 1445A b using JWST's MIRI/LRS instrument reveal a dayside brightness temperature consistent with a dark, rocky surface, suggesting the planet lacks a thick CO2 atmosphere.
Abstract
  • Bibliographic Information: Wachiraphan, P., et al. (2024). The Thermal Emission Spectrum of the Nearby Rocky Exoplanet LTT 1445A b from JWST MIRI/LRS. arXiv preprint arXiv:2410.10987v1.
  • Research Objective: This study aims to characterize the atmosphere of the rocky exoplanet LTT 1445A b using thermal emission spectroscopy obtained with the James Webb Space Telescope (JWST).
  • Methodology: The researchers observed three secondary eclipses of LTT 1445A b using JWST's Mid-Infrared Instrument (MIRI) Low Resolution Spectrometer (LRS). They analyzed the data using two independent data reduction pipelines, Eureka! and SPARTA, to ensure robustness. The team then fit the light curves to constrain the planet's orbital parameters and derive its dayside brightness temperature. They compared the observed emission spectrum to theoretical models, including those with different atmospheric compositions and surface pressures.
  • Key Findings: The observations detected a broadband secondary eclipse depth of 41 ± 9 ppm, consistent with a circular orbit. The planet's dayside brightness temperature was constrained to 525 ± 15 K, yielding a temperature ratio relative to the maximum average dayside temperature expected from instant thermal reradiation by a rocky surface (R = Tday,obs/Tmax) of 0.952 ± 0.057. This finding suggests that LTT 1445A b likely has a dark, rocky surface.
  • Main Conclusions: The study concludes that LTT 1445A b lacks a thick CO2 atmosphere, possibly due to atmospheric erosion driven by stellar activity. The data disfavor thick atmospheres with surface pressures of 100 bar or higher. However, the presence of a moderately thin atmosphere, similar to those on Mars, Titan, or Earth, remains uncertain.
  • Significance: This research contributes to our understanding of atmospheric retention on rocky exoplanets orbiting M dwarfs. The findings suggest that even planets receiving relatively low stellar insolation, like LTT 1445A b, may lose their primordial atmospheres over time.
  • Limitations and Future Research: The study acknowledges that the presence of a thin atmosphere cannot be definitively ruled out. Future observations with higher signal-to-noise ratios and broader wavelength coverage could help to further constrain the atmospheric properties of LTT 1445A b.
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Stats
LTT 1445A b has a radius of 1.34 +0.11/-0.06 R⊕ and a mass of 2.73 +0.25/-0.23 M⊕. The planet receives an instellation of 5.7+1.3/-1.1 S⊕, comparable to Mercury. The observed broadband secondary eclipse depth is 41 ± 9 ppm. The planet's dayside brightness temperature is constrained to 525 ± 15 K. The temperature ratio R = Tday,obs/Tmax is 0.952 ± 0.057.
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Deeper Inquiries

How do the atmospheric properties of LTT 1445A b compare to those of other rocky exoplanets orbiting M dwarfs with similar instellation levels?

LTT 1445A b, with an instellation similar to Mercury, presents as a hot, likely dry planet with a tentatively tenuous atmosphere at best. This stands in contrast to some other well-studied rocky exoplanets around M dwarfs with comparable instellation levels: TRAPPIST-1 planets (b, c, & d): These planets receive similar or even lower instellation than LTT 1445A b. Early observations were inconclusive, but JWST data suggests they too may lack thick atmospheres. However, TRAPPIST-1 c shows a slightly lower eclipse depth than a bare-rock model, hinting at a possible thin atmosphere or non-zero albedo. This difference, though subtle, highlights the diversity possible even at similar instellation levels. GJ 1132 b: This planet, with a slightly higher equilibrium temperature, has been studied in detail with JWST. Observations indicate a lack of a thick atmosphere and favor a bare-rock composition, aligning with the findings for LTT 1445A b. LHS 3844 b: This planet, with an instellation about twice that of LTT 1445A b, also shows no signs of a substantial atmosphere based on Spitzer observations. The emerging picture suggests that while high instellation from M dwarfs might generally hinder substantial atmospheric retention, there's a spectrum of possibilities. Factors like planet mass, density, magnetic field strength, and the specific history of stellar activity (and thus the strength and duration of XUV-driven atmospheric escape) likely play crucial roles in shaping the final atmospheric properties.

Could tidal heating play a role in the atmospheric evolution of LTT 1445A b, given its proximity to its host star?

While LTT 1445A b orbits close to its host star, making tidal heating a relevant consideration, current evidence suggests it's unlikely to be a dominant factor in its atmospheric evolution. Here's why: Constraints on eccentricity: The study indicates that LTT 1445A b likely has a low eccentricity, supported by radial velocity data and eclipse timing measurements. Tidal heating scales with eccentricity; a near-circular orbit implies minimal tidal stresses and thus reduced internal heating. Timescale argument: Even if tidal heating were initially significant, LTT 1445A b's estimated age suggests ample time for the initial heat to dissipate. Unless there's an ongoing mechanism sustaining high eccentricity (like a perturbing third body), tidal heating would have likely subsided, making it a less influential factor in the planet's current state. However, it's worth noting that the precise internal structure and composition of LTT 1445A b remain unknown. Subtle tidal effects, while not the primary driver, could still contribute to outgassing or influence long-term atmospheric escape processes. Further observations and modeling efforts are needed to fully disentangle the interplay of these factors.

What are the implications of these findings for the potential habitability of rocky exoplanets around M dwarfs?

The findings regarding LTT 1445A b, particularly the likely absence of a thick atmosphere, have significant implications for the broader question of habitability around M dwarfs: Atmospheric retention challenges: The study reinforces the notion that maintaining substantial atmospheres on rocky planets close to M dwarfs can be challenging, especially given the intense stellar activity and potential for atmospheric stripping. This emphasizes the importance of considering factors beyond just the habitable zone's instellation levels. Diversity of outcomes: Despite similar instellation to other rocky exoplanets around M dwarfs, LTT 1445A b's likely lack of a thick atmosphere highlights the diversity of potential outcomes. This underscores the need for case-by-case studies to assess habitability rather than relying solely on broad generalizations. Focus on cooler, less irradiated planets: The findings suggest that planets with lower instellation levels, potentially further out in the habitable zone of M dwarfs, might stand a better chance of retaining atmospheres over long timescales. Future observations with JWST and other telescopes should prioritize such targets to explore the potential for habitable environments. While the results for LTT 1445A b might seem to paint a somewhat pessimistic picture, it's crucial to remember that this is just one data point in a vast and diverse exoplanetary landscape. The search for life beyond Earth requires a nuanced understanding of the complex interplay between stellar activity, planetary properties, and atmospheric evolution.
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