The Effects of Giant Impacts on Exoplanetary Systems

Insights into planet formation from debris disks: II. Giant impacts in extrasolar planetary systems

Authors:

Wyatt et al

Abstract:

Giant impacts refer to collisions between two objects each of which is massive enough to be considered at least a planetary embryo. The putative collision suffered by the proto-Earth that created the Moon is a prime example, though most Solar System bodies bear signatures of such collisions. Current planet formation models predict that an epoch of giant impacts may be inevitable, and observations of debris around other stars are providing mounting evidence that giant impacts feature in the evolution of many planetary systems. This chapter reviews giant impacts, focussing on what we can learn about planet formation by studying debris around other stars. Giant impact debris evolves through mutual collisions and dynamical interactions with planets. General aspects of this evolution are outlined, noting the importance of the collision-point geometry. The detectability of the debris is discussed using the example of the Moon-forming impact. Such debris could be detectable around another star up to 10Myr post-impact, but model uncertainties could reduce detectability to a few 100yr window. Nevertheless the 3% of young stars with debris at levels expected during terrestrial planet formation provide valuable constraints on formation models; implications for super-Earth formation are also discussed. Variability recently observed in some bright disks promises to illuminate the evolution during the earliest phases when vapour condensates may be optically thick and acutely affected by the collision-point geometry. The outer reaches of planetary systems may also exhibit signatures of giant impacts, such as the clumpy debris structures seen around some stars.

There are two Populations of SuperEarths

Correlations between compositions and orbits established by the giant impact era of planet formation

Authors:

Dawson et al

Abstract:

The giant impact phase of terrestrial planet formation establishes connections between super-Earths' orbital properties (semimajor axis spacings, eccentricities, mutual inclinations) and interior compositions (the presence or absence of gaseous envelopes). Using N-body simulations and analytic arguments, we show that spacings derive not only from eccentricities, but also from inclinations. Flatter systems attain tighter spacings, a consequence of an eccentricity equilibrium between gravitational scatterings, which increase eccentricities, and mergers, which damp them. Dynamical friction by residual disk gas plays a critical role in regulating mergers and in damping inclinations and eccentricities. Systems with moderate gas damping and high solid surface density spawn gas-enveloped super-Earths with tight spacings, small eccentricities, and small inclinations. Systems in which super-Earths coagulate without as much ambient gas, in disks with low solid surface density, produce rocky planets with wider spacings, larger eccentricities, and larger mutual inclinations. A combination of both populations can reproduce the observed distributions of spacings, period ratios, transiting planet multiplicities, and transit duration ratios exhibited by Kepler super-Earths. The two populations, both formed in situ, also help to explain observed trends of eccentricity vs. planet size, and bulk density vs. method of mass measurement (radial velocities vs. transit timing variations).

How Common are Thea-like Impacts on Terrestrial Worlds?

Giant Impacts on Earth-like Worlds

Authors:

Quintana et al

Abstract:

The late stages of terrestrial planet formation are dominated by giant impacts that collectively influence the growth, dynamical stability, composition and habitability of any planets that form. Hitherto, numerical models designed to explore these late stage collisions have been limited in two major ways. First, nearly all N-body models have assumed that two-body collisions lead to perfect accretion. Second, many of these studies lack the large number of realizations needed to account for the chaotic nature of these N-body systems. In this article we perform hundreds of simulations of late stage terrestrial planet formation using an N-body algorithm that includes fragmentation and hit-and-run collisions. We performed 140 simulations of planet accretion around a Sun-like star with Jupiter and Saturn analogs with and without this new collision model. We find that when fragmentation is included, the final planets formed are similar to those formed in the perfect-accretion model in terms of mass and number, however the paths towards building these planets are significantly different. Over 90% of the fragmentation simulations produced an Earth-analog and we parameterized the impacts onto these planets in terms of their specific impact energies. Only 15 of our 164 Earth-analogs experienced an impact that was energetic enough to strip an entire atmosphere. To strip about half of an atmosphere requires energies comparable to the Moon-forming giant impact, and almost all Earth-analogs received at least one impact that met this criteria and received on average 3.0 of these giant impacts during the 2 Gyr simulations. The median time of the final giant impact was 43 Myr after the start of the simulations, leading us to conclude that the time-frame of the Moon-forming impact is typical amongst planetary systems around Sun-like stars.

10% of Exoplanetary Systems are Predicted to be Like our Solar System

Warm Debris Disks Produced by Giant Impacts During Terrestrial Planet Formation

Authors:

Genda et al

Abstract:

In our solar system, Mars-sized protoplanets frequently collided with each other during the last stage of terrestrial planet formation called the giant impact stage. Giant impacts eject a large amount of material from the colliding protoplanets into the terrestrial planet region, which may form debris disks with observable infrared excesses. Indeed, tens of warm debris disks around young solar-type stars have been observed. Here, we quantitatively estimate the total mass of ejected materials during the giant impact stages. We found that ∼0.4 times the Earth's mass is ejected in total throughout the giant impact stage. Ejected materials are ground down by collisional cascade until micron-sized grains are blown out by radiation pressure. The depletion timescale of these ejected materials is determined primarily by the mass of the largest body among them. We conducted high-resolution simulations of giant impacts to accurately obtain the mass of the largest ejected body. We then calculated the evolution of the debris disks produced by a series of giant impacts and depleted by collisional cascades to obtain the infrared excess evolution of the debris disks. We found that the infrared excess is almost always higher than the stellar infrared flux throughout the giant impact stage (∼100 Myr) and is sometimes ∼10 times higher immediately after a giant impact. Therefore, giant impact stages would explain the infrared excess from most observed warm debris disks. The observed fraction of stars with warm debris disks indicates that the formation probability of our solar system-like terrestrial planets is approximately 10%