Updated Parameters and a New Transmission Spectrum of HD 97658b

Abstract

Recent years have seen increasing interest in the characterization of sub-Neptune-sized planets because of their prevalence in the Galaxy, contrasted with their absence in our solar system. HD97658 is one of the brightest stars hosting a planet of this kind, and we present the transmission spectrum of this planet by combining four Hubble Space Telescope transits, 12 Spitzer/IRAC transits, and eight MOST transits of this system. Our transmission spectrum has a higher signal-to-noise ratio than those from previous works, and the result suggests that the slight increase in transit depth from wavelength 1.1-1.7 μm reported in previous works on the transmission spectrum of this planet is likely systematic. Nonetheless, our atmospheric modeling results are inconclusive, as no model provides an excellent match to our data. Nonetheless, we find that atmospheres with high C/O ratios (C/O0.8) and metallicities of 100×solar metallicity are favored. We combine the mid-transit times from all of the new Spitzer and MOST observations and obtain an updated orbital period of P=9.489295±0.000005, with a best-fit transit time center at T 0 =2456361.80690±0.00038 (BJD). No transit timing variations are found in this system. We also present new measurements of the stellar rotation period (34±2 days) and stellar activity cycle (9.6 yr) of the host star HD97658. Finally, we calculate and rank the Transmission Spectroscopy Metric of all confirmed planets cooler than 1000 K and with sizes between 1 R ⊕ and 4 R ⊕. We find that at least a third of small planets cooler than 1000 K can be well characterized using James Webb Space Telescope, and of those, HD97658b is ranked fifth, meaning that it remains a high-priority target for atmospheric characterization.

Transmission Spectroscopy for the Warm Sub-Neptune HD 3167c: Evidence for Molecular Absorption and a Possible High-metallicity Atmosphere

“…This is unsurprising given the long duration of the transit, resulting in individual visit durations of ∼8.5 hr after discarding the first HST orbit. A similar effect was noted by Guo et al (2020) and Colón et al (2020) for transit observations of HD97658b and KELT-11b, respectively. Those authors demonstrated how the broadband  R R p level varied significantly when different analytic expressions were assumed for the baseline trend, including linear, quadratic, logarithmic, and exponential functions of time.…”

“…The HST transmission spectrum presented here for HD3167c adds to a modest sample acquired to date for sub-Neptunes above the radius valley, with radii between 1.8 and 4 Å R . As shown in Figure 15, the other sub-Neptunes are GJ1214b (Kreidberg et al 2014), 55Cnce (Tsiaras et al 2016), HD97658b (Guo et al 2020), and K2-18b (Benneke et al 2019b). In addition to those shown in Figure 15, there have been a few spectra published for planets somewhat larger Already, the diversity predicted for the atmospheric properties of this population (e.g., Fortney et al 2013) is being borne out by observations, reminiscent of that which has been well documented for the hot Jupiters (e.g., Sing et al 2016).…”

“…In particular, statistically strong H 2 O detections have been made for HAT-P-11b (Fraine et al 2014), GJ3470b (Benneke et al 2019a), and K2-18b (Benneke et al 2019b). A more tentative H 2 O detection has recently been reported for HD106315c (Kreidberg et al 2020), while HD97658b shows indications of spectral features, although the interpretation remains uncertain (Guo et al 2020). For 55Cnce-which, with a radius of (Dai et al 2019), falls at the borderline of the super-Earth and sub-Neptune regimes- Tsiaras et al (2016) claimed the detection of a thick atmosphere containing HCN.…”

“…The Hubble Space Telescope (HST) has proven a productive facility for characterizing the atmospheres of transiting exoplanets. Observational studies released since 2019 alone include Arcangeli et al (2019), Benneke et al (2019aBenneke et al ( , 2019b, Chachan et al (2019), dos Santos et al (2019), Mikal-Evans et al (2019, Sing et al (2019), Spake et al (2019), Guo et al (2020), Wong et al (2020), Alam et al (2020), Fu et al (2020), Wakeford et al (2020), Carter et al (2020), Bruno et al (2020), Sotzen et al (2020), Carone et al (2020), Kreidberg et al (2020), and Colón et al (2020). Transmission spectroscopy measurements, made during primary transit, allow the composition of the day-night terminator atmosphere to be probed, while at other phases in the orbit when the irradiated dayside hemisphere is visible, the planetary emission can be constrained (for overviews, see Deming & Seager 2017;Deming et al 2019).…”

“…Transmission spectroscopy observations performed with HST provide a means of directly probing the atmospheric compositions for the most favorable sub-Neptunes. To date, there have been HST transmission spectra published for only four such targets with radii ∼1.8-4 Å R : GJ1214b (Berta et al 2012;Kreidberg et al 2014), HD97658b (Knutson et al 2014b;Guo et al 2020), 55Cnce (Tsiaras et al 2016), and K2-18b (Benneke et al 2019b). There have also been four HST transmission spectra published for planets with radii somewhat larger than Neptune ( -Å R 4 5 ): GJ436b (Knutson et al 2014a;Lothringer et al 2018), HAT-P-11b (Fraine et al 2014;Chachan et al 2019), GJ3470b (Ehrenreich et al 2014;Benneke et al 2019a), and HD106315c (Kreidberg et al 2020).…”