The Great Escape: planet evolution post-main sequence

Last month Dimitri Veras and collaborators wrote a nice, and very thorough paper on what happens to planets when their parent stars die...

"The Great Escape: How Exoplanets and Smaller Bodies Desert Dying Stars",
Dimitri Veras, Mark C. Wyatt, Alexander J. Mustill, Amy Bonsor, John J. Eldridge
(MNRAS, arXiv:1107.1239)

National Geographic News Story: How Planets Can Survive a Supernova

The paper revisits an old topic I am particularly fond of: what happens to the planets orbiting a star as the star leaves the main sequence, become a giant, and sheds its mass enroute to becoming a white dwarf.

There are several aspects to the problem: close planets get engulfed by the envelope of the giant; planets further out are in a death race between the expansion of the envelope and the onset of tidal coupling between the planet and the envelope on one hand, and the expansion of the planetary orbit on the other hand (cf Rybicki and Denis); and, for outer planets, well beyond the radii at which interactions with the envelope are important, there is the orbital response of the planet to the mass loss. Radiation effects from the increased luminosity of the star may also become important (cf Villaver and Livio).

Mass loss, obviously, changes the gravitational force the planet feels from the star.

The effect of the mass loss is generally considered in two limits:

if the mass loss is sudden, "impulsive", the planet generally goes onto an eccentric orbit, possibly hyperbolic for mass loss of ~50% or more, depending on the original eccentricity of the orbit and where the planet is along the orbit. This scenario was considered by Blaauw in 1961 and is often referred to as a "Blaauw kick".

if the mass loss is slow, "adiabatic", then the solution is analytic, as the integrals of motion are conserved; this scenario has been considered extensively cf Parriott and Alcock and Debes and Sigurdsson in different contexts.

Schematic of outer planet orbit evolution as a star leaves the main sequence and undergoes mass loss (from Debes 2003).

The intermediate case, where the time scale for the mass loss is neither rapid, compared to the planetary orbital time scale, nor slow, compared to the orbital time scale, is a pain in the ass and is much neglected.
Hadjidemetriou considered the general solution in a series of papers, and now Dimitri and his collaborators have revisited the problem to consider explicit scenarios for stars of different mass and planets of different masses in a variety of orbits.

The paper considers a number of cases of current interest, including: planets around horizontal branch stars; effect of planets on planetary nebulae morphology; planets around white dwarfs; and, the prospect for dynamical ejection or stripping of planets in very wide orbits around stars.

Wheee....!

The paper recovers the limiting solutions, and explores in detail the variety of different outcomes for the intermediate case, some of which are surprising.
In particular, they find a series of solutions for moderately rapid mass loss, where the eccentricity of the planet grows secularly reaching e > 1.

ie the planets are ejected because the specific angular momentum grows, and not so much because of the decrease in binding energy.
That is interesting.

The principal focus of the paper is exploring the role of this effect in unbinding planet from their host star, especially in view of the recent observations which support the long held suspicion that there may be very large numbers of "free floaters" out there, cf Sumi et al.

A direct interpretation of the observation implies that there may well be more free floating planets than there are stars. If you think planets mostly form around stars, this would imply more than one planet ejected per star, on average, and the question then is how?
One method, which is effective if planets form "close packed" around typical stars, is for planet-planet dynamical interactions to eject some planets from their host stars; alternatively a substantial fraction of free floating planets may be ejected by the death throes of stars.
Both mechanisms must operate, which is dominant depends on details, such as the spacing and relative masses of planets when they are formed; how many planets are formed at large distances form their host stars; and, the number of planets formed around the more massive stars, compared to the less massive stars. The mechanism in Dimitri's paper is most effective for the more massive stars, and planets formed in wide orbits.
Scattering, of course, can also place planets in wide orbits, after formation further in, so the two mechanism are also complementary.

For really wild speculation, we can then consider whether there are circumstances in which free floating planets can sustain life, cf Stevenson 1999 and Debes and Sigurdsson 2007.