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The radius valley, i.e. a dearth of planets with radii between 1.5 and 2 Earth radii, provides insights into planetary formation and evolution. Using homogenously revised planetary parameters from Kepler 1-min short cadence light curves, we remodel transits of 72 small planets mostly orbiting low-mass stars, improving the precision and accuracy of planet parameters. By combining this sample with a similar sample of planets around higher mass stars, we determine the depth of the radius valley as a function of stellar mass. We find that the radius valley is shallower for low-mass stars compared to their higher mass counterparts. Upon comparison, we find that theoretical models of photoevaporation underpredict the number of planets observed inside the radius valley for low-mass stars: with decreasing stellar mass, the predicted fraction of planets inside the valley remains approximately constant whereas the observed fraction increases. We argue that this provides evidence for the presence of icy planets around low-mass stars. Alternatively, planets orbiting low-mass stars undergo more frequent collisions and scatter in the stars' high-energy output may also cause planets to fill the valley. We predict that more precise mass measurements for planets orbiting low-mass stars would be able to distinguish between these scenarios. [ABSTRACT FROM AUTHOR]

In this work, we develop a rocky planet interior model and use it to investigate the evolution of catastrophically evaporating rocky exoplanets. These planets, detected through the dust tails produced by evaporative outflows from their molten surfaces, can be entirely destroyed in a fraction of their host star's lifetime. To allow for the major decrease in mass, our interior model can simultaneously calculate the evolution of the pressure and density structure of a planet alongside its thermal evolution, which includes the effects of conduction, convection and partial melting. We first use this model to show that the underlying planets are likely to be almost entirely solid. This means that the dusty tails are made up of material sampled only from a thin dayside lava pool. If one wishes to infer the bulk compositions of rocky exoplanets from their dust tails, it is important to take the localized origin of this material into account. Secondly, by considering how frequently one should be able to detect mass loss from these systems, we investigate the occurrence of sub-Earth mass exoplanets, which is difficult with conventional planet detection surveys. We predict that, depending on model assumptions, the number of progenitors of the catastrophically evaporating planets is either in line with, or higher than, the observed population of close-in (substellar temperatures around 2200 K) terrestrial exoplanets. [ABSTRACT FROM AUTHOR]

Catastrophically evaporating rocky planets provide a unique opportunity to study the composition of small planets. The surface composition of these planets can be constrained via modelling their comet-like tails of dust. In this work, we present a new self-consistent model of the dusty tails: we physically model the trajectory of the dust grains after they have left the gaseous outflow, including an on-the-fly calculation of the dust cloud's optical depth. We model two catastrophically evaporating planets: KIC 1255 b and K2-22 b. For both planets, we find the dust is likely composed of magnesium–iron silicates (olivine and pyroxene), consistent with an Earth-like composition. We constrain the initial dust grain sizes to be ∼ 1.25–1.75 μm and the average (dusty) planetary mass-loss rate to be ∼ 3 |$\, M_{\oplus } \mathrm{Gyr^{-1}}$|⁠. Our model shows that the origin of the leading tail of dust of K2-22 b is likely a combination of the geometry of the outflow and a low radiation pressure force to stellar gravitational force ratio. We find the optical depth of the dust cloud to be a factor of a few in the vicinity of the planet. Our composition constraint supports the recently suggested idea that the dusty outflows of these planets go through a greenhouse effect–nuclear winter cycle, which gives origin to the observed transit depth time variability. Magnesium–iron silicates have the necessary visible-to-infrared opacity ratio to give origin to this cycle in the high mass-loss state. [ABSTRACT FROM AUTHOR]

Understanding atmospheric escape in close-in exoplanets is critical to interpreting their evolution. We map out the parameter space over which photoevaporation and core-powered mass-loss dominate atmospheric escape. Generally, the transition between the two regimes is determined by the location of the Bondi radius (i.e. the sonic point of core-powered outflow) relative to the penetration depth of extreme ultra-violet (XUV) photons. Photoevaporation dominates the loss when the XUV penetration depth lies inside the Bondi radius (R XUV < R B) and core-powered mass-loss when XUV radiation is absorbed higher up in the flow (R B < R XUV). The transition between the two regimes occurs at a roughly constant ratio of the planet's radius to its Bondi radius, with the exact value depending logarithmically on planetary and stellar properties. In general, core-powered mass-loss dominates for lower gravity planets with higher equilibrium temperatures, and photoevaporation dominates for higher gravity planets with lower equilibrium temperatures. However, planets can transition between these two mass-loss regimes during their evolution, and core-powered mass-loss can 'enhance' photoevaporation over a significant region of parameter space. Interestingly, a planet that is ultimately stripped by core-powered mass-loss has likely only ever experienced core-powered mass-loss. In contrast, a planet that is ultimately stripped by photoevaporation could have experienced an early phase of core-powered mass-loss. Applying our results to the observed super-Earth population suggests that it contains significant fractions of planets where each mechanism controlled the final removal of the H/He envelope, although photoevaporation appears to be responsible for the final carving of the exoplanet radius valley. [ABSTRACT FROM AUTHOR]

Disc accretion rate onto low mass protostar FU Ori suddenly increased hundreds of times 85 yr ago and remains elevated to this day. We show that the sum of historic and recent observations challenges existing FU Ori models. We build a theory of a new process, Extreme Evaporation (EE) of young gas giant planets in discs with midplane temperatures of ≳ 30 000 K. Such temperatures are reached in the inner 0.1 AU during thermal instability bursts. In our 1D time-dependent code the disc and an embedded planet interact through gravity, heat, and mass exchange. We use disc viscosity constrained by simulations and observations of dwarf novae instabilities, and we constrain planet properties with a stellar evolution code. We show that dusty gas giants born in the outer self-gravitating disc reach the innermost disc in a ∼ O (104) yr with radius of ∼10 R J. We show that their EE rates are |$\gtrsim 10^{-5} {\rm {\rm M}_{\odot }}$|  yr−1; if this exceeds the background disc accretion activity then the system enters a planet-sourced mode. Like a stellar secondary in mass-transferring binaries, the planet becomes the dominant source of matter for the star, albeit for ∼ O (100) yr. We find that a ∼6 Jupiter mass planet evaporating in a disc fed at a time-averaged rate of |$\sim 10^{-6} {\rm {\rm M}_{\odot }}$|  yr−1 appears to explain all that we currently know about FU Ori accretion outburst. More massive planets and/or planets in older less massive discs do not experience EE process. Future FUOR modelling may constrain planet internal structure and evolution of the earliest discs. [ABSTRACT FROM AUTHOR]

Atmospheric mass-loss is known to play a leading role in sculpting the demographics of small, close-in exoplanets. Knowledge of how such planets evolve allows one to 'rewind the clock' to infer the conditions in which they formed. Here, we explore the relationship between a planet's core mass and its atmospheric mass after protoplanetary disc dispersal by exploiting XUV photoevaporation as an evolutionary process. Historically, this inference problem would be computationally infeasible due to the large number of planet models required; however, we use a novel atmospheric evolution emulator which utilizes neural networks to provide three orders of magnitude in speedup. First, we provide a proof of concept for this emulator on a real problem by inferring the initial atmospheric conditions of the TOI-270 multi-planet system. Using the emulator, we find near-indistinguishable results when compared to the original model. We then apply the emulator to the more complex inference problem, which aims to find the initial conditions for a sample of Kepler, K2 , and TESS planets with well-constrained masses and radii. We demonstrate that there is a relationship between core masses and the atmospheric mass they retain after disc dispersal. This trend is consistent with the 'boil-off' scenario, in which close-in planets undergo dramatic atmospheric escape during disc dispersal. Thus, it appears that the exoplanet population is consistent with the idea that close-in exoplanets initially acquired large massive atmospheres, the majority of which is lost during disc dispersal, before the final population is sculpted by atmospheric loss over 100 Myr to Gyr time-scales. [ABSTRACT FROM AUTHOR]

Ultrashort period planets offer a window into the poorly understood interior composition of exoplanets through material evaporated from their rocky interiors. Among these objects are a class of disintegrating planets, observed when their dusty tails transit in front of their host stars. These dusty tails are thought to originate from dust condensation in thermally driven winds emanating from the sublimating surfaces of these planets. Existing models of these winds have been unable to explain their highly variable nature and have not explicitly modelled how dust forms in the wind. Here, we present new radiation-hydrodynamic simulations of the winds from these planets, including a minimal model for the formation and destruction of dust, assuming that nucleation can readily take place. We find that dust forms readily in the winds, a consequence of large dust grains obtaining lower temperatures than the planet's surface. As hyphothesized previously, we find that the coupling of the planet's surface temperature to the outflow properties via the dust's opacity can drive time-variable flows when dust condensation is sufficiently fast. In agreement with previous work, our models suggest that these dusty tails are a signature of catastrophically evaporating planets that are close to the end of their lives. Finally, we discuss the implications of our results for the dust's composition. More detailed hydrodynamic models that self-consistently compute the nucleation and composition of the dust and gas are warranted in order to use these models to study the planet's interior composition. [ABSTRACT FROM AUTHOR]

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 ∼4 M ⊕. 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 2 σ 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. [ABSTRACT FROM AUTHOR]

The Sun shows a ∼10 per cent depletion in refractory elements relative to nearby solar twins. It has been suggested that this depletion is a signpost of planet formation. The exoplanet statistics are now good enough to show that the origin of this depletion does not arise from the sequestration of refractory material inside the planets themselves. This conclusion arises because most sun-like stars host close-in planetary systems that are on average more massive than the Sun's. Using evolutionary models for the protoplanetary discs that surrounded the young Sun and solar twins, we demonstrate that the origin of the depletion likely arises due to the trapping of dust exterior to the orbit of a forming giant planet. In this scenario, a forming giant planet opens a gap in the gas disc, creating a pressure trap. If the planet forms early enough, while the disc is still massive, the planet can trap ≳100 M⊕ of dust exterior to its orbit, preventing the dust from accreting on to the star in contrast to the gas. Forming giant planets can create refractory depletions of |$\sim 5{-}15{{\ \rm per\ cent}}$|⁠ , with the larger values occurring for initial conditions that favour giant planet formation (e.g. more massive discs that live longer). The incidence of solar twins that show refractory depletion matches both the occurrence of giant planets discovered in exoplanet surveys and 'transition' discs that show similar depletion patterns in the material that is accreting on to the star. [ABSTRACT FROM AUTHOR]

The photoevaporation model is one of the leading explanations for the evolution of small, close-in planets and the origin of the radius-valley. However, without planet mass measurements, it is challenging to test the photoevaporation scenario. Even if masses are available for individual planets, the host star's unknown EUV/X-ray history makes it difficult to assess the role of photoevaporation. We show that systems with multiple transiting planets are the best in which to rigorously test the photoevaporation model. By scaling one planet to another in a multitransiting system, the host star's uncertain EUV/X-ray history can be negated. By focusing on systems that contain planets that straddle the radius-valley, one can estimate the minimum masses of planets above the radius-valley (and thus are assumed to have retained a voluminous hydrogen/helium envelope). This minimum mass is estimated by assuming that the planet below the radius-valley entirely lost its initial hydrogen/helium envelope, then calculating how massive any planet above the valley needs to be to retain its envelope. We apply this method to 104 planets above the radius gap in 73 systems for which precise enough radii measurements are available. We find excellent agreement with the photoevaporation model. Only two planets (Kepler -100c and 142c) appear to be inconsistent, suggesting they had a different formation history or followed a different evolutionary pathway to the bulk of the population. Our method can be used to identify TESS systems that warrant radial-velocity follow-up to further test the photoevaporation model. The software to estimate minimum planet masses is publicly available at https://github.com/jo276/EvapMass. [ABSTRACT FROM AUTHOR]