The Cosmic Shoreline: The Evidence that Escape Determines which Planets Have Atmospheres, and what this May Mean for Proxima Centauri B

By K. Zahnle, David C. Catling. 2017

207 citations recorded by the source corpus.

The planets of the solar system are neatly divided between those with atmospheres and those without when arranged by insolation (I) and escape velocity ( ). The dividing line goes at . Exoplanets with reported masses and radii are shown to crowd against the extrapolation of the solar system trend, making a metaphorical cosmic shoreline that unites all the planets. The relation may implicate thermal escape. We therefore address the general behavior of hydrodynamic thermal escape models ranging from Pluto to highly irradiated extrasolar giant planets (EGPs). Energy-limited escape is harder to test because copious XUV radiation is mostly a feature of young stars, and hence requires extrapolating to historic XUV fluences ( ) using proxies and power laws. An energy-limited shoreline should scale as , which differs distinctly from the apparent relation. Energy-limited escape does provide good quantitative agreement to the highly irradiated EGPs. Diffusion-limited escape implies that no planet can lose more than 1% of its mass as H2. Impact erosion, to the extent that impact velocities can be estimated for exoplanets, fits a shoreline. The proportionality constant is consistent with what the collision of comet Shoemaker–Levy 9 showed us we should expect of modest impacts in deep atmospheres. With respect to the shoreline, Proxima Centauri b is on the metaphorical beach. Known hazards include its rapid energetic accretion, high impact velocities, its early life on the wrong side of the runaway greenhouse, and Proxima Centauri’s XUV radiation. In its favor is a vast phase space of unknown unknowns.

Subfields: Astronomy and Astrophysics, Instrumentation

View Paper: https://www.semanticscholar.org/paper/55477017285289cd8f4d603479db2a7570615f5f

How researchers see this paper

The researchers whose trusted networks rank it highest, strongest advocate first. Every researcher gets a different order.

In Astronomy and Astrophysics

1. J. Owen
2. B. Fulton
3. E. Petigura
4. Timothy Morton
5. Andrew W. Howard
6. G. Marcy
7. Yanqin Wu
8. J. Winn

In Instrumentation

1. J. Owen
2. E. Petigura
3. B. Fulton
4. Timothy Morton
5. Andrew W. Howard
6. G. Marcy
7. J. Winn
8. Yanqin Wu

Citations in the corpus

• 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…”
• The Evaporation Valley in the Kepler Planets (2017, background): “…density play a primary role in controlling a planet’s evolution by setting the escape velocity (Owen & Wu 2013; Lopez & Fortney 2013; Owen & Morton 2016; Zahnle & Catling 2017), allowing one to break the compositional degeneracies in individual systems by statistical modelling (Owen & Morton 2016).”
• High Tide or Riptide on the Cosmic Shoreline? A Water-rich Atmosphere or Stellar Contamination for the Warm Super-Earth GJ 486b from JWST Observations (2023, background): “However, given the high activity of most M-dwarf stars (e.g., Peacock et al. 2019), their planets are subject to extremeUV radiation regimes that may remove any significant atmosphere through escape processes (e.g., Kasting & Pollack 1983; Airapetian et al. 2017, 2020; Zahnle & Catling 2017).”