GJ 1252b: A Hot Terrestrial Super-Earth with No Atmosphere

Abstract

In recent years, the discovery of increasing numbers of rocky, terrestrial exoplanets orbiting nearby stars has drawn increased attention to the possibility of studying these planets’ atmospheric and surface properties. This is especially true for planets orbiting M dwarfs, whose properties can best be studied with existing observatories. In particular, the minerological composition of these planets and the extent to which they can retain their atmospheres in the face of intense stellar irradiation both remain unresolved. Here, we report the detection of the secondary eclipse of the terrestrial exoplanet GJ 1252b, obtained via 10 eclipse observations using the Spitzer Space Telescope’s IRAC2 4.5 μm channel. We measure an eclipse depth of 149 − 32 + 25 ppm, corresponding to a dayside brightness temperature of 1410 − 125 + 91 K. This measurement is consistent with the prediction for a bare rock surface. Comparing the eclipse measurement to a large suite of simulated planetary spectra indicates that GJ 1252b has a surface pressure of ≲10 bar, i.e., substantially thinner than the atmosphere of Venus. Assuming energy-limited escape, even a 100 bar atmosphere would be lost in <1 Myr, far shorter than our gyrochronological age estimate of 3.9 ± 0.4 Gyr. The expected mass loss could be overcome by mantle outgassing, but only if the mantle’s carbon content were >7% by mass—over two orders of magnitude greater than that found in Earth. We therefore conclude that GJ 1252b has no significant atmosphere. Model spectra with granitoid or feldspathic surface composition, but with no atmosphere, are disfavored at >2σ. The eclipse occurs just +1.4 − 1.0 + 2.8 minutes after orbital phase 0.5, indicating e cos ω = +0.0025 − 0.0018 + 0.0049 , consistent with a circular orbit. Tidal heating is therefore likely to be negligible with regard to GJ 1252b’s global energy budget. Finally, we also analyze additional, unpublished TESS transit photometry of GJ 1252b, which improves the precision of the transit ephemeris by a factor of 10, provides a more precise planetary radius of 1.180 ± 0.078 R ⊕, and rules out any transit-timing variations with amplitudes ≳1 minute.

A rocky exoplanet classification method and its application to calculating surface pressure and surface temperature

“…Using our classification method, we categorise GJ 1252b as a close-in "Mars-analogue" exoplanet likely to have a thin water-less atmosphere. Comparing our analogue adjusted temperature of 1011 ± 107 K to the Crossfield et al (2022) dayside brightness temperature of 1410 +91 −125 K, we fall within 2𝜎 of their dayside GJ 1252b surface temperature. Despite using different assumptions, our primary classification model is consistent with other work using different techniques, giving us confidence in our approach.…”

“…This is consistent with the findings from Kreidberg et al (2019) of a small atmosphere susceptible to erosion by stellar winds and thus likely being barerock with low bond-albedo. Additionally, we compared our results to the Crossfield et al (2022) observations of GJ 1252b. While our model assumes a fast-rotating exoplanet, unlike the slow-rotating GJ 1252b, when run through our models, the results agree with Crossfield et al (2022) that GJ 1252b is a hot rocky exoplanet with no significant atmosphere.…”

“…Additionally, we compared our results to the Crossfield et al (2022) observations of GJ 1252b. While our model assumes a fast-rotating exoplanet, unlike the slow-rotating GJ 1252b, when run through our models, the results agree with Crossfield et al (2022) that GJ 1252b is a hot rocky exoplanet with no significant atmosphere. Using our classification method, we categorise GJ 1252b as a close-in "Mars-analogue" exoplanet likely to have a thin water-less atmosphere.…”

Is the Hot, Dense Sub-Neptune TOI-824 b an Exposed Neptune Mantle? Spitzer Detection of the Hot Dayside and Reanalysis of the Interior Composition

“…The ESM is a metric which yields high values for high-S/N targets that are most favorable for emission spectroscopy. For all planets, we show the ESM assuming the zero-albedo and full-heat- Kreidberg et al 2019;Crossfield et al 2020;Zieba et al 2022;Crossfield et al 2022). For GJ 436 b and GJ 3470b at 4.5 μm, we show the brightness temperature upper bounds as triangles (Stevenson et al 2010;Benneke et al 2019b).…”

“…Exoplanets with reported secondary-eclipse measurements are shown in bold (Alonso 2018). Small planets with available Spitzer 3.6 and 4.5 μm thermal emission measurements are labeled and are shown with the reported Spitzer dayside brightness temperatures (diamonds with error bars; Demory et al 2016;Kreidberg et al 2019;Crossfield et al 2020;Zieba et al 2022;Crossfield et al 2022). For GJ 436 b and GJ 3470b at 4.5 μm, we show the brightness temperature upper bounds as triangles(Stevenson et al 2010;Benneke et al 2019b).…”