Giant Planet Effects on Terrestrial Planet Formation and System Architecture

MNRAS 485, 541–549 (2019) doi:10.1093/mnras/stz385 Advance Access publication 2019 February 8 Giant planet effects on terrestrial planet formation and system architecture 1‹ 2 2,3 1 Anna C. Childs, Elisa Quintana, Thomas Barclay and Jason H. Steffen Downloaded from https://academic.oup.com/mnras/article-abstract/485/1/541/5309996 by NASA Goddard Space Flight Ctr user on 15 April 2020 1Department of Physics and Astronomy, University of Nevada, Las Vegas, NV 89154, USA 2NASA Goddard Space Flight Center, 8800 Greenbelt Road, Greenbelt, MD 20771, USA 3University of Maryland, Baltimore County, 1000 Hilltop Cir, Baltimore, MD 21250, USA Accepted 2019 February 4. Received 2019 January 16; in original form 2018 July 6 ABSTRACT Using an updated collision model, we conduct a suite of high-resolution N-body integrations to probe the relationship between giant planet mass and terrestrial planet formation and system architecture. We vary the mass of the planets that reside at Jupiter’s and Saturn’s orbit and examine the effects on the interior terrestrial system. We find that massive giant planets are more likely to eject material from the outer edge of the terrestrial disc and produce terrestrial planets that are on smaller, more circular orbits. We do not find a strong correlation between exterior giant planet mass and the number of Earth analogues (analogous in mass and semimajor axis) produced in the system. These results allow us to make predictions on the nature of terrestrial planets orbiting distant Sun-like star systems that harbour giant planet companions on long orbits – systems that will be a priority for NASA’s upcoming Wide-Field Infrared Survey Telescope (WFIRST) mission. Key words: giant planets – simulations – terrestrial planet formation. picture of terrestrial planet formation that addresses this inability is 1 INTRODUCTION the ‘Pebble Accretion’ model where 100- to 1000-km bodies accrete Terrestrial planet formation is typically separated into three stages submetre-sized pebbles to form terrestrial planets (Lambrechts & due to the different physical processes that are involved in each Johansen 2012; Levison et al. 2015). Most studies that utilize N- period (Lissauer 1993; Righter & O’Brien 2011a). The initial body integrators, however, model the late stage of planet formation. stage involves the condensation of solids from the gas disc that The physics involved considers only the interactions of planetes- grow until they reach kilometre-sized planetesimals (Chiang & imals and embryos as purely gravitational and, until recently, Youdin 2010). The middle stage focuses on the agglomeration assumed only relatively trivial collisions – either completely elastic of small planetesimals (Moon-sized bodies) into embryos (Mars- or completely inelastic. These commonly used collision models sized bodies; Weidenschilling 1977;Rafikov2003). The final stage limit the accuracy of simulations of terrestrial planet formation, follows the collisional evolution of embryos, the giant impact as real collision outcomes are more complex (Chambers 2013; phase, that ultimately yields planets (Chambers 2001; Schlichting, Haghighipour, Maindl & Schaefer 2017). An accurate collision Warren & Yin 2012). model is needed to probe the properties of planetary systems that When modelling the late stage of planet formation, we assume depend on the collision history, such as their final architecture, that all the gas in the disc has been dispersed. The lifetime of composition, magnetic field, moon system, atmosphere, and internal a protoplanetary gas disc depends on the dispersal mechanisms structure (Elser et al. 2011; Jacobson et al. 2017;Lock&Stewart of the gas, but for solar-type stars the lifetime of the gas disc 2017). is approximately 2–3 Myr (Alexander, Clarke & Pringle 2006; Leinhardt & Stewart (2012) developed a prescription for more Pfalzner, Steinhausen & Menten 2014). After the gas disc dissipates, realistic collisions that includes fragmentation. Chambers (2013) the growth of protoplanets ensues via gravitational collisions that implemented this fragmentation model into the N-body integrator yield accretion and erosion events (Wetherill 1995; Righter & Mercury to allow for collisions that result in partial accretion or O’Brien 2011b). erosion. With this fragmentation code, Quintana et al. (2016) studied A shortcoming of the core-accretion model is its inability to how giant impacts affect terrestrial planet formation in the presence replicate the mass and formation time-scale differences between of Jupiter and Saturn. They found that the final architecture of the the Earth and Mars (Raymond et al. 2009). A less conventional terrestrial planetary system is comparable to systems formed using a trivial collision model, but the collision history and accretion time-scales of these systems differed significantly – giving insight E-mail: [email protected] into a different formation process. Examining the high-resolution C 2019 The Author(s) Published by Oxford University Press on behalf of the Royal Astronomical Society 542 A. C. Childs et al. collision history of a planet also has implications for habitability. Table 1. The exterior giant planet masses and integration times used in our Tracking the fragments in a system allows us to examine how simulations. For reference, Jupiter is 318 M⊕ and Saturn is 95 M⊕. the by-products of collisions interact with the rest of the system and affect planet formation. Further analysis of these interactions Mass at Saturn’s orbit Mass at Jupiter’s orbit Time will determine if they result in impacts that are catastrophic to the (M⊕)(M⊕)(Myr) planet’s habitable properties – stripping the atmosphere, heating the 95 318 500 planet, fragmenting the planet, etc. 75 225 5 When considering the Solar system, not only is a high-resolution 50 150 500 Downloaded from https://academic.oup.com/mnras/article-abstract/485/1/541/5309996 by NASA Goddard Space Flight Ctr user on 15 April 2020 collision study needed to accurately explore the properties of the 30 90 5 terrestrial planets, but the effects of the giant planets must be 15 45 5 accounted for as well. Previous studies have shown the important role Jupiter played in the formation of our terrestrial system. Horner & Jones (2008) showed that Jupiter shielded the earth of planets are on longer orbital periods, and what determines the from a high rate of giant impacts from the asteroid belt. More habitability of Earth-like worlds. This mission aims to resolve the generally, Levison & Agnor (2003) found that different exterior discrepancy on giant planet occurrence rates. giant planet environments produce terrestrial planets of different Here we explore the extent to which exterior giant planets sizes and orbits. Additionally, Raymond et al. (2014) found that affect the formation and evolution, and the final architecture of changing the eccentricity of Jupiter and Saturn will affect the orbital the terrestrial planet system. We use a suite of high-resolution and mass distributions, and water delivery to the terrestrial planets. N-body simulations with five different giant planet environments Other studies suggest that exterior giant planets may be necessary with giant planets at Jupiter’s and Saturn’s current orbit. Each suite for the development of life on Earth analogues (Horner & Jones of simulations has scaled down masses for Jupiter and Saturn – 2010b). Horner et al. (2009) found through numerical studies that maintaining the 3:1 mass ratio between the body at Jupiter’s orbit exterior giant planets promote the delivery of volatiles to planets in and the body at Saturn’s orbit. We use exterior giant planets that ∼ 3 1 1 1 the habitable zone of the system. Additionally, Horner, Gilmore & are 4 , 2 , 4 ,and 7 the mass of Jupiter and Saturn. Because Suzuki Waltham (2015) argue that giant planets are needed to induce et al. (2016) argue that cold Neptunes are likely the most common climate change on planets in the habitable zone, similar to the role type of planets beyond the snow line, the smallest mass used in Jupiter plays in the Milankovitch cycles on the Earth. Considering our giant planet systems is a Neptune-sized body. The integration the influence that the exterior giant planets had on the Solar system, time and the varied masses used for Saturn and Jupiter are listed the question follows: what would our terrestrial planets be like if in Table 1. If we can constrain the role that exterior giant planets Jupiter and Saturn did not grow large enough to accrete significant play in terrestrial planet formation, we can predict how common amounts of gas before the gas disc dissipated, and therefore had Solar system-like planetary systems are in conjunction with the much smaller masses? occurrence rates of planets on long orbits from WFIRST. Along these same lines, thousands of diverse exoplanet systems have been discovered by the Kepler mission. Although Kepler is not sensitive to planets on longer orbital periods, Foreman-Mackey et al. 2 SIMULATIONS (2016) developed an automated method to predict the occurrence rates of giant planets with long orbital periods, even if only one or The fragmentation code developed by Chambers (2013) (adopting two transits were observed. Their probabilistic model estimates the the collision model from Leinhardt & Stewart 2012) allows for occurrence rate for exoplanets on orbital periods from 2 to 25 yr, several of collision outcomes. These include the following: ∼ with a radius between 0.1–1RJ to be 2.00 per G/K dwarf star. (i) A collision

Load more