Exoplanets with substantial hydrogen/helium atmospheres have been discovered in abundance, many residing extremely close to their parent stars. The extreme irradiation levels that these atmospheres ...experience cause them to undergo hydrodynamic atmospheric escape. Ongoing atmospheric escape has been observed to be occurring in a few nearby exoplanet systems through transit spectroscopy both for hot Jupiters and for lower-mass super-Earths and mini-Neptunes. Detailed hydrodynamic calculations that incorporate radiative transfer and ionization chemistry are now common in one-dimensional models, and multidimensional calculations that incorporate magnetic fields and interactions with the interstellar environment are cutting edge. However, comparison between simulations and observations remains very limited. While hot Jupiters experience atmospheric escape, the mass-loss rates are not high enough to affect their evolution. However, for lower-mass planets, atmospheric escape drives and controls their evolution, sculpting the exoplanet population that we observe today.
Observations of some exoplanets have detected atmospheric escape driven by hydrodynamic outflows, causing the exoplanets to lose mass over time.
Hydrodynamic simulations of atmospheric escape are approaching the sophistication required to compare them directly to observations.
Atmospheric escape sculpts sharp features into the exoplanet population that we can observe today; these features have recently been detected.
Unveiling the planet population at birth Rogers, James G; Owen, James E
Monthly Notices of the Royal Astronomical Society,
05/2021, Volume:
503, Issue:
1
Journal Article
Peer reviewed
Open access
ABSTRACT
The radius distribution of small, close-in exoplanets has recently been shown to be bimodal. The photoevaporation model predicted this bimodality. In the photoevaporation scenario, some ...planets are completely stripped of their primordial H/He atmospheres, whereas others retain them. Comparisons between the photoevaporation model and observed planetary populations have the power to unveil details of the planet population inaccessible by standard observations, such as the core mass distribution and core composition. In this work, we present a hierarchical inference analysis on the distribution of close-in exoplanets using forward models of photoevaporation evolution. We use this model to constrain the planetary distributions for core composition, core mass, and initial atmospheric mass fraction. We find that the core-mass distribution is peaked, with a peak-mass of ∼4M⊕. The bulk core-composition is consistent with a rock/iron mixture that is ice-poor and ‘Earth-like’; the spread in core-composition is found to be narrow ($\lesssim 16{{\ \rm per\ cent}}$ variation in iron-mass fraction at the 2σ level) and consistent with zero. This result favours core formation in a water/ice poor environment. We find the majority of planets accreted a H/He envelope with a typical mass fraction of $\sim 4{{\ \rm per\ cent}}$; only a small fraction did not accrete large amounts of H/He and were ‘born-rocky’. We find four times as many super-Earths were formed through photoevaporation, as formed without a large H/He atmosphere. Finally, we find core-accretion theory overpredicts the amount of H/He cores would have accreted by a factor of ∼5, pointing to additional mass-loss mechanisms (e.g. ‘boil-off’) or modifications to core-accretion theory.
A new piece of evidence supporting the photoevaporation-driven evolution model for low-mass, close-in exoplanets was recently presented by the California-Kepler Survey. The radius distribution of the ...Kepler planets is shown to be bimodal, with a "valley" separating two peaks at 1.3 and 2.6 R⊕. Such an "evaporation valley" had been predicted by numerical models previously. Here, we develop a minimal model to demonstrate that this valley results from the following fact: the timescale for envelope erosion is the longest for those planets with hydrogen/helium-rich envelopes that, while only a few percent in weight, double its radius. The timescale falls for envelopes lighter than this because the planet's radius remains largely constant for tenuous envelopes. The timescale also drops for heavier envelopes because the planet swells up faster than the addition of envelope mass. Photoevaporation therefore herds planets into either bare cores (∼1.3 R⊕), or those with double the core's radius (∼2.6 R⊕). This process mostly occurs during the first 100 Myr when the stars' high-energy fluxes are high and nearly constant. The observed radius distribution further requires the Kepler planets to be clustered around 3 M⊕ in mass, born with H/He envelopes more than a few percent in mass, and that their cores are similar to the Earth in composition. Such envelopes must have been accreted before the dispersal of the gas disks, while the core composition indicates formation inside the ice line. Lastly, the photoevaporation model fails to account for bare planets beyond ∼30-60 days; if these planets are abundant, they may point to a significant second channel for planet formation, resembling the solar system terrestrial planets.
Abstract
The population of small, close-in exoplanets is bifurcated into super-Earths and sub-Neptunes. We calculate physically motivated mass–radius relations for sub-Neptunes, with rocky cores and ...H/He-dominated atmospheres, accounting for their thermal evolution, irradiation, and mass loss. For planets ≲10
M
⊕
, we find that sub-Neptunes retain atmospheric mass fractions that scale with planet mass and show that the resulting mass–radius relations are degenerate with results for “water worlds” consisting of a 1:1 silicate-to-ice composition ratio. We further demonstrate that our derived mass–radius relation is in excellent agreement with the observed exoplanet population orbiting M dwarfs and that planet mass and radii alone are insufficient to determine the composition of some sub-Neptunes. Finally, we highlight that current exoplanet demographics show an increase in the ratio of super-Earths to sub-Neptunes with both stellar mass (and therefore luminosity) and age, which are both indicative of thermally driven atmospheric escape processes. Therefore, such processes should not be ignored when making compositional inferences in the mass–radius diagram.
ABSTRACT
Recently, K2 and TESS have discovered transiting planets with radii between ∼5 and 10 R⊕ around stars with ages <100 Myr. These young planets are likely to be the progenitors of the ...ubiquitous super-Earths/sub-Neptunes, which are well studied around stars with ages ≳1 Gyr. The formation and early evolution of super-Earths/sub-Neptunes are poorly understood. Various planetary origin scenarios predict a wide range of possible formation entropies. We show how the formation entropies of young (∼20–60 Myr), highly irradiated planets can be constrained if their mass, radius, and age are measured. This method works by determining how low-mass an H/He envelope a planet can retain against mass-loss, this lower bound on the H/He envelope mass can then be converted into an upper bound on the entropy. If planet mass measurements with errors ≲20 per cent can be achieved for the discovered young planets around DS Tuc A and V1298 Tau, then insights into their origins can be obtained. For these planets, higher measured planet masses would be consistent with the standard core-accretion theory. In contrast, lower planet masses (≲6–7 M⊕) would require a ‘boil-off’ phase during protoplanetary disc dispersal to explain.
ABSTRACT We show that, for a low-mass planet that orbits its host star within a few tenths of an AU (like the majority of the Kepler planets), the atmosphere it was able to accumulate while embedded ...in the protoplanetary disk may not survive unscathed after the disk disperses. This gas envelope, if more massive than a few percent of the core (with a mass below ), has a cooling time that is much longer than the timescale on which the planet exits the disk. As such, it could not have contracted significantly from its original size, of the order of the Bondi radius. So a newly exposed protoplanet would be losing mass via a Parker wind that is catalyzed by the stellar continuum radiation. This represents an intermediate stage of mass-loss, occurring soon after the disk has dispersed, but before the EUV/X-ray driven photoevaporation becomes relevant. The surface mass-loss induces a mass movement within the envelope that advects internal heat outward. As a result, the planet atmosphere rapidly cools down and contracts, until it has reached a radius of the order of 0.1 Bondi radius, at which time the mass-loss effectively shuts down. Within a million years after the disk disperses, we find a planet that has only about 10% of its original envelope, and a Kelvin-Helmholtz time that is much longer than its actual age. We suggest that this "boil-off" process may be partially responsible for the lack of planets above a radius of in the Kepler data, provided planet formation results in initial envelope masses of tens of percent.
Inspired by the Kepler mission's planet discoveries, we consider the thermal contraction of planets close to their parent star, under the influence of evaporation. The mass-loss rates are based on ...hydrodynamic models of evaporation that include both X-ray and EUV irradiation. We find that only low mass planets with hydrogen envelopes are significantly affected by evaporation, with evaporation being able to remove massive hydrogen envelopes inward of ~0.1 AU for Neptune-mass objects, while evaporation is negligible for Jupiter-mass objects. Moreover, most of the evaporation occurs in the first 100 Myr of stars' lives when they are more chromospherically active. We construct a theoretical population of planets with varying core masses, envelope masses, orbital separations, and stellar spectral types, and compare this population with the sizes and densities measured for low-mass planets, both in the Kepler mission and from radial velocity surveys. This exercise leads us to conclude that evaporation is the driving force of evolution for close-in Kepler planets. In fact, some 50% of the Kepler planet candidates may have been significantly eroded. Evaporation explains two striking correlations observed in these objects: a lack of large radius/low density planets close to the stars and a possible bimodal distribution in planet sizes with a deficit of planets around 2 R sub(+ in circle). Planets that have experienced high X-ray exposures are generally smaller than this size, and those with lower X-ray exposures are typically larger. A bimodal planet size distribution is naturally predicted by the evaporation model, where, depending on their X-ray exposure, close-in planets can either hold on to hydrogen envelopes ~0.5%-1% in mass or be stripped entirely. To quantitatively reproduce the observed features, we argue that not only do low-mass Kepler planets need to be made of rocky cores surrounded with hydrogen envelopes, but few of them should have initial masses above 20 M sub(+ in circle) and the majority of them should have core masses of a few Earth masses.
Snow lines can be thermally unstable Owen, James E
Monthly notices of the Royal Astronomical Society,
07/2020, Volume:
495, Issue:
3
Journal Article
Peer reviewed
ABSTRACT
Volatile species in protoplanetary discs can undergo a phase change from vapour to solid. These ‘snow lines’ can play vital roles in planet formation at all scales, from dust coagulation to ...planetary migration. In the outer regions of protoplanetary discs, the temperature profile is set by the absorption of reprocessed stellar light by the solids. Further, the temperature profile sets the distribution of solids through sublimation and condensation at various snow lines. Hence, the snow line position depends on the temperature profile and vice versa. We show that this coupling can be thermally unstable, such that a patch of the disc at a snow line will produce either runaway sublimation or condensation. This thermal instability arises at moderate optical depths, where heating by absorption of reprocessed stellar light from the disc’s atmosphere is optically thick, yet cooling is optically thin. Since volatiles in the solid phase drift much faster than volatiles in the vapour phase, this thermal instability results in a limit cycle. The snow line progressively moves in, condensing volatiles, before receding, as the volatiles sublimate. Using numerical simulations, we study the evolution of the carbon monoxide (CO) snow line. We find the CO snow line is thermally unstable under typical disc conditions and evolves inwards from ∼50 to ∼30 au on time-scales from 1000 to 10 000 yr. The CO snow line spends between ${\sim}10{{\ \rm per\ cent}}\,\mathrm{ and}\,50{{\ \rm per\ cent}}$ of its time at smaller separations, where the exact value is sensitive to the total opacity and turbulent viscosity. The evolving snow line also creates ring-like structures in the solid distribution interior to the snow line. Multiple ring-like structures created by moving snow lines could potentially explain the substructures seen in many ALMA images.
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