Recent observations identify a valley in the radius distribution of small exoplanets, with planets in the range 1.5−2.0 R ⊕ significantly less common than somewhat smaller or larger planets. This valley may suggest a bimodal population of rocky planets that are either engulfed by massive gas envelopes that significantly enlarge their radius, or do not have detectable atmospheres at all. One explanation of such a bimodal distribution is atmospheric erosion by high-energy stellar photons. We investigate an alternative mechanism: the luminosity of the cooling rocky core, which can completely erode light envelopes while preserving heavy ones, produces a deficit of intermediate sized planets. We evolve planetary populations that are derived from observations using a simple analytical prescription, accounting self-consistently for envelope accretion, cooling and mass loss, and demonstrate that core-powered mass loss naturally reproduces the observed radius distribution, regardless of the high-energy incident flux. Observations of planets around different stellar types may distinguish between photoevaporation, which is powered by the high-energy tail of the stellar radiation, and core-powered mass loss, which depends on the bolometric flux through the planet's equilibrium temperature that sets both its cooling and mass-loss rates.

Some recently discovered short-period Earth to Neptune sized exoplanets (super Earths) have low observed mean densities which can only be explained by voluminous gaseous atmospheres. Here, we study the conditions allowing the accretion and retention of such atmospheres. We self-consistently couple the nebular gas accretion onto rocky cores and the subsequent evolution of gas envelopes following the dispersal of the protoplanetary disk. Specifically, we address mass-loss due to both photoevaporation and cooling of the planet. We find that planets shed their outer layers (dozens of percents in mass) following the disk's dispersal (even without photo-evaporation), and their atmospheres shrink in a few Myr to a thickness comparable to the radius of the underlying rocky core. At this stage, atmospheres containing less particles than the core (equivalently, lighter than a few % of the planet's mass) can be blown away by heat coming from the cooling core, while heavier atmospheres cool and contract on a timescale of Gyr at most. By relating the mass-loss timescale to the accretion time, we analytically identify a Goldilocks region in the mass-temperature plane in which low-density super Earths can be found: planets have to be massive and cold enough to accrete and retain their atmospheres, while not too massive or cold, such that they do not enter runaway accretion and become gas giants (Jupiters). We compare our results to the observed super-Earth population and find that low-density planets are indeed concentrated in the theoretically allowed region. Our analytical and intuitive model can be used to investigate possible super-Earth formation scenarios.

Observations from the last decade have indicated the existence of a general class of superluminous supernovae (SLSNe), in which the peak luminosity exceeds 10 44 erg s −1 . Here we focus on a subclass of these events, where the light curve is also tens of days wide, so the total radiated energy is order 10 51 erg. If the origin of these SLSNe is a core-collapse-driven explosion of a massive star, then the mechanism which converts the explosion energy into radiation must be very efficient (much more than in typical core collapse SNe, where this efficiency is of order one percent). We examine the scenario where the radiated luminosity is due to efficient conversion of kinetic energy of the ejected stellar envelope into radiation by interaction with an optically thick, pre-existing circumstellar material (CSM), presumably the product of a steady wind from the progenitor. We base the analysis on a simple, numerically solved, hydrodynamic diffusion model, which allows us to identify the qualitative behavior of the observable light curves, and to relate them to the parameters of the wind. We specifically show that a wide and superluminous supernova requires the mass of the relevant wind material to be comparable to that of ejected material from the exploding progenitor. We find the wind parameters which explain the peak luminosity and width of the bolometric light curves of three particular SLSNe, namely, SN 2005ap, SN 2006gy, and SN 2010gx, and show that they are best fitted with a wind that extends to a radius of order 10 15 cm. These results serve as an additional indication that at least some SLSNe are powered by interaction of the ejected material with a steady wind of similar mass.

Some extrasolar giant planets in close orbits-"hot Jupiters"-exhibit larger radii than that of a passively cooling planet. The extreme irradiation L eq these hot Jupiters receive from their close in stars creates a thick isothermal layer in their envelopes, which slows down their convective cooling, allowing them to retain their inflated size for longer. This is yet insufficient to explain the observed sizes of the most inflated planets. Some models invoke an additional power source, deposited deep in the planet's envelope. Here we present an analytical model for the cooling of such irradiated, and internally heated gas giants. We show that a power source L dep , deposited at an optical depth τ dep , creates an exterior convective region, between optical depths L eq /L dep and τ dep , beyond which a thicker isothermal layer exists, which in extreme cases may extend to the center of the planet. This convective layer, which occurs only for L dep τ dep > L eq , further delays the cooling of the planet. Such a planet is equivalent to a planet irradiated with L eq (1 + L dep τ dep /L eq ) β , where β ≈ 0.35 is an effective power-law index describing the radiative energy density as function of the optical depth for a convective planet U ∝ τ β . Our simple analytical model reproduces the main trends found in previous numerical works, and provides an intuitive understanding. We derive scaling laws for the cooling rate of the planet, its central temperature, and radius. These scaling laws can be used to estimate the effects of tidal or Ohmic dissipation, wind shocks, or any other mechanism involving energy deposition, on sizes of hot Jupiters.

We present L'-band imaging of the PDS 70 planetary system with Keck/NIRC2 using the new infrared pyramid wave front sensor. We detected both PDS 70 b and c in our images, as well as the front rim of the circumstellar disk. After subtracting off a model of the disk, we measured the astrometry and photometry of both planets. Placing priors based on the dynamics of the system, we estimated PDS 70 b to have a semimajor axis of-+ 20 4 3 au and PDS 70 c to have a semimajor axis of-+ 34 6 12 au (95% credible interval). We fit the spectral energy distribution (SED) of both planets. For PDS 70 b, we were able to place better constraints on the red half of its SED than previous studies and inferred the radius of the photosphere to be 2-3R Jup. The SED of PDS 70 c is less well constrained, with a range of total luminosities spanning an order of magnitude. With our inferred radii and luminosities, we used evolutionary models of accreting protoplanets to derive a mass of PDS 70 b between 2 and 4 M Jup and a mean mass accretion rate between 3×10 −7 and 8×10 −7 M Jup /yr. For PDS 70 c, we computed a mass between 1 and 3 M Jup and mean mass accretion rate between 1×10 −7 and 5×10 −7 M Jup /yr. The mass accretion rates imply dust accretion timescales short enough to hide strong molecular absorption features in both planets' SEDs. Unified Astronomy Thesaurus concepts: Exoplanet formation (492); Exoplanet atmospheres (487); Orbit determination (1175); Exoplanet dynamics (490); Coronagraphic imaging (313);