An Empirical Mass–Radius Relation for Cool Giant Planets

Thorngren, Daniel; Marley, Mark S.; Fortney, Jonathan J.

doi:10.3847/2515-5172/ab4353

Cited by 17 publications

(11 citation statements)

References 42 publications

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“…We can do this because the mass-radius relation is nearly flat for the giant planets at the upper edge of the Neptune desert (e.g. Chen & Kipping 2017;Owen & Lai 2018;Thorngren et al 2019). This is because giant planets at the upper edge of the desert are well-approximated by P ∝ ρ 2 polytropes for which the radius does not depend on mass (Stevenson 1982).…”

Section: The Upper Neptune Desert Is Stable Against Mass Lossmentioning

confidence: 99%

The Upper Edge of the Neptune Desert Is Stable Against Photoevaporation

Vissapragada¹,

Knutson²,

Greklek-McKeon³

et al. 2022

Preprint

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Transit surveys indicate that there is a deficit of Neptune-sized planets on close-in orbits. If this "Neptune desert" is entirely cleared out by atmospheric mass loss, then planets at its upper edge should only be marginally stable against photoevaporation, exhibiting strong outflow signatures in tracers like the metastable helium triplet. We test this hypothesis by carrying out a 12-night photometric survey of the metastable helium feature with Palomar/WIRC, targeting seven gas-giant planets orbiting K-type host stars. Eight nights of data are analyzed here for the first time along with reanalyses of four previously-published datasets. We strongly detect helium absorption signals for WASP-69b, HAT-P-18b, and HAT-P-26b; tentatively detect signals for WASP-52b and NGTS-5b; and do not detect signals for WASP-177b and WASP-80b. We interpret these measured excess absorption signals using grids of isothermal Parker wind models to derive mass-loss rates, which are typically around 10 10 − 10 11 g s −1 . Our empirically constrained mass-loss rates are in good agreement with predictions from the 1D hydrodynamical outflow code ATES for all planets except WASP-52b and WASP-80b, where our data suggest that the outflows are much smaller than predicted. Excluding these two planets, the outflows for the rest of the sample are consistent with a mean energy-limited outflow efficiency of ε = 0.41 +0.16 −0.13 . Even when we make the relatively conservative assumption that gas-giant planets experience energy-limited outflows at this efficiency for their entire lives, photoevaporation would still be too inefficient to carve the upper boundary of the Neptune desert. We conclude that this feature of the exoplanet population is a pristine tracer of giant planet formation and migration mechanisms.

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Section: The Upper Neptune Desert Is Stable Against Mass Lossmentioning

confidence: 99%

The Upper Edge of the Neptune Desert Is Stable Against Photoevaporation

Vissapragada¹,

Knutson²,

Greklek-McKeon³

et al. 2022

Preprint

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“…Unfortunately, there are no published measurements of the mass of V1298 Tau d. We therefore ran three separate model grids for representative masses of 20 M ⊕ , 50 M ⊕ , and 100 M ⊕ . The latter mass is a rough empirical upper limit from Thorngren et al (2019), who studied an ensemble of older gas giant planets cooler than 1000 K. Planets in their sample with radii comparable to V1298 Tau d have M  100M ⊕ , so we take this as a conservative upper limit on the mass of planet d. On the low-mass end, previous studies of the V1298 system (David et al 2019a(David et al , 2019b have suggested that these planets may have masses closer to those of the sub-Neptune population observed by Kepler (i.e., 1-10 M ⊕ ). This idea is supported by the current ensemble of TTV measurements for this system, which appear to favor M d < 10M ⊕ (Livingston et al 2021, in preparation).…”

Section: Atmospheric Escape Modelingmentioning

confidence: 99%

A Search for Planetary Metastable Helium Absorption in the V1298 Tau System

Vissapragada¹,

Stefánsson

Greklek-McKeon³

et al. 2021

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Early in their lives, planets endure extreme amounts of ionizing radiation from their host stars. For planets with primordial hydrogen and helium-rich envelopes, this can lead to substantial mass loss. Direct observations of atmospheric escape in young planetary systems can help elucidate this critical stage of planetary evolution. In this work, we search for metastable helium absorption—a tracer of tenuous gas in escaping atmospheres—during transits of three planets orbiting the young solar analog V1298 Tau. We characterize the stellar helium line using HET/HPF, and find that it evolves substantially on timescales of days to months. The line is stable on hour-long timescales except for one set of spectra taken during the decay phase of a stellar flare, where absoprtion increased with time. Utilizing a beam-shaping diffuser and a narrowband filter centered on the helium feature, we observe four transits with Palomar/WIRC: two partial transits of planet d (P = 12.4 days), one partial transit of planet b (P = 24.1 days), and one full transit of planet c (P = 8.2 days). We do not detect the transit of planet c, and we find no evidence of excess absorption for planet b, with ΔR b/R ⋆ < 0.019 in our bandpass. We find a tentative absorption signal for planet d with ΔR d/R ⋆ = 0.0205 ± 0.054, but the best-fit model requires a substantial (−100 ± 14 minutes) transit-timing offset on a two-month timescale. Nevertheless, our data suggest that V1298 Tau d may have a high present-day mass-loss rate, making it a priority target for follow-up observations.

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“…Consequently, strong constraints were derived for short-period gas giants where they had 95% completeness for periods ≤ 3 yrs with a minimum companion mass of ∼ 0.75 M J , which corresponds to a HJ radius of approximately 1.25 R J (Chen & Kipping 2017). Given that the majority of the range is for cool Jupiters, we consider radius as a step function using the Chen & Kipping (2017) for periods < 10 days and Thorngren et al (2019) for periods > 10 days. Within the HJ range, we expect the planets to be inflated thereby increasing the radius.…”

Section: Comparison To Previous Workmentioning

confidence: 99%

Searching For Transiting Planets Around Halo Stars. II. Constraining the Occurrence Rate of Hot Jupiters

Boley,

Wang,

Zinn

et al. 2021

Preprint

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Jovian planet formation has been shown to be strongly correlated with host star metallicity, which is thought to be a proxy for disk solids. Observationally, previous works have indicated that jovian planets preferentially form around stars with solar and super solar metallicities. Given these findings, it is challenging to form planets within metal-poor environments, particularly for hot Jupiters that are thought to form via metallicity-dependent core accretion. Although previous studies have conducted planet searches for hot Jupiters around metal-poor stars, they have been limited due to small sample sizes, which are a result of a lack of high-quality data making hot Jupiter occurrence within the metalpoor regime difficult to constrain until now. We use a large sample of halo stars observed by TESS to constrain the upper limit of hot Jupiter occurrence within the metal-poor regime (-2.0 ≤ [Fe/H] ≤ -0.6). Placing the most stringent upper limit on hot Jupiter occurrence, we find the mean 1-σ upper limit to be 0.18 % for radii 0.8 -2 R Jupiter and periods 0.5 − 10 days. This result is consistent with previous predictions indicating that there exists a certain metallicity below which no planets can form.

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An Empirical Mass–Radius Relation for Cool Giant Planets

Cited by 17 publications

References 42 publications

The Upper Edge of the Neptune Desert Is Stable Against Photoevaporation

The Upper Edge of the Neptune Desert Is Stable Against Photoevaporation

A Search for Planetary Metastable Helium Absorption in the V1298 Tau System

Searching For Transiting Planets Around Halo Stars. II. Constraining the Occurrence Rate of Hot Jupiters

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