KEPLER PLANETS: A TALE OF EVAPORATION

By J. Owen, Yanqin Wu. 2013

398 citations recorded by the source corpus.

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⊕. 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⊕ and the majority of them should have core masses of a few Earth masses.

Subfields: Astronomy and Astrophysics, Instrumentation

View Paper: https://www.semanticscholar.org/paper/cde975408110fb66a47cd710bb9df09dde2c0d32

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In Astronomy and Astrophysics

1. J. Owen
2. Jon Fortney
3. C. Mordasini
4. Yanqin Wu
5. Timothy Morton
6. E. Petigura
7. B. Fulton
8. Sheng Jin

In Instrumentation

1. J. Owen
2. Timothy Morton
3. E. Petigura
4. C. Mordasini
5. B. Fulton
6. Yanqin Wu
7. E. Gaidos
8. Jon Fortney

Citations in the corpus

• SUPER-EARTH ATMOSPHERES: SELF-CONSISTENT GAS ACCRETION AND RETENTION (2015, methodology): “In addition to this spontaneous mass shedding, gas envelopes are also vulnerable to evaporation by high-energy stellar photons (see, e.g. Lopez et al. 2012; Owen & Wu 2013).”
• Revised Radii of Kepler Stars and Planets Using Gaia Data Release 2 (2018, background): “Hot Super-Earth “Desert” Our revised radius and irradiance values confirm the existence of a deficit or “desert” of super-Earth to Neptune-size planets at high irradiance (Owen & Wu 2013), i.e. with 2.2”
• The California-Kepler Survey. IV. Metal-rich Stars Host a Greater Diversity of Planets (2017, background): “This gap was predicted by various groups who modeled the erosion erosion of envelopes (e.g. Lopez & Fortney 2013; Owen & Wu 2013; Jin et al. 2014; Chen & Rogers 2016).”
• DENSITIES AND ECCENTRICITIES OF 139 KEPLER PLANETS FROM TRANSIT TIME VARIATIONS (2013, background): “Such a correlation for Kepler candidates has been noted in Wu & Lithwick (2013) and Owen & Wu (2013).”
• MAKE SUPER-EARTHS, NOT JUPITERS: ACCRETING NEBULAR GAS ONTO SOLID CORES AT 0.1 AU AND BEYOND (2014, background): “Over the course of ∼100 Myrs, X-rays from host stars can photoevaporate super-Earth envelopes from initial GCRs of ∼0.4 down to final GCRs of ∼0.01–0.1, with the precise evolution depending on stellocentric distance and core mass (Owen & Wu 2013, their Figure 8).”
• The California-Kepler Survey. III. A Gap in the Radius Distribution of Small Planets (2017, background): “Photoevaporation Photoevaporation provides a possible mechanism to produce a gap in the radius distribution, even if the initial radius distribution was continuous (Owen & Wu 2013).”
• Core-powered mass-loss and the radius distribution of small exoplanets (2017, background): “…(see also Ikoma & Hori 2012; Owen & Wu 2016) a process of spontaneous mass loss that is essentially different from (and complementary to) the high-energy photoevaporation that is usually discussed in the literature (Murray-Clay et al. 2009; Lopez et al. 2012; Owen & Jackson 2012; Owen & Wu 2013).”
• THE RADIUS DISTRIBUTION OF PLANETS AROUND COOL STARS (2013, background): “…systems have been measured from the effects of mutual gravitational interactions (e.g. Cochran et al. 2011, Lithwick et al. 2012, Nesvorny et al. 2013) providing insight into the composition and atmospheric evolution of these planets (Wu & Lithwick 2013; Owen & Wu 2013; Lopez et al. 2012).”
• An asteroseismic view of the radius valley: stripped cores, not born rocky (2017, background): “Numerical models empirically give shallower slopes than analytic models for the same evaporation models, e.g. a slope of P−0.12 is found from numerical models, for an analytical slope of P−0.16 (Owen & Wu 2013).”
• The California-Kepler Survey. V. Peas in a Pod: Planets in a Kepler Multi-planet System Are Similar in Size and Regularly Spaced (2017, background): “Photo-evaporation (e.g., Lopez et al. 2012; Owen & Wu 2013; Zahnle & Catling 2017) could produce such an effect, since the expectation from photo-evaporation is that inner planets should be smaller than outer planets (assuming identical core masses), and that this effect should be more pronounced…”
• They are small worlds after all: revised properties of Kepler M dwarf stars and their planets (2015, background): “However, there are theoretical expectations that planets on wider orbits will retain envelopes of low molecular weight volatiles and have larger radii (e.g., Owen & Wu 2013; Luger et al. 2015).”
• MOST 1.6 EARTH-RADIUS PLANETS ARE NOT ROCKY (2014, background): “To leading order, mass loss would have a more pronounced effect on the radius distribution of planets (the relative number of planets at each radii, Owen & Wu 2013) than on the fraction of planets at specified size that are sufficiently dense to be rocky.”
• Accurate Empirical Radii and Masses of Planets and Their Host Stars with Gaia Parallaxes (2016, background): “This may simply be due to the small sample size, but it has been suggested that the lowest mass planets at high insolation may have had their atmospheres photoevaporated (Owen & Wu 2013).”
• HOW ROCKY ARE THEY? THE COMPOSITION DISTRIBUTION OF KEPLER’S SUB-NEPTUNE PLANET CANDIDATES WITHIN 0.15 AU (2014, background): “Indeed, if photoevaporation plays a significant role in shaping this irradiated planet population, planets which are shown to exclusively lie in the radius-flux “occurrence valley” predicted by Lopez & Fortney (2013) and Owen & Wu (2013) are likely such water worlds.”
• UNDERSTANDING THE MASS–RADIUS RELATION FOR SUB-NEPTUNES: RADIUS AS A PROXY FOR COMPOSITION (2013, background): “…mass is critical for understanding how volatile rich planets accrete their initial H/He envelope (Bodenheimer et al. 2000; Ikoma & Hori 2012) and whether they can retain it against X-ray and EUV driven photo-evaporation (Lopez et al. 2012; Lopez & Fortney 2013; Owen & Jackson 2012; Owen & Wu 2013).”