Credit: NASA / JPL-Caltech
Title: The Fate of Oceans on First-Generation Planets Orbiting White Dwarfs
Authors: Juliette Becker, Andrew Vanderburg, and Joseph R. Livesey
First Author’s Institution: University of Wisconsin–Madison
Status: Published in ApJ
Planets orbiting white dwarfs are particularly attractive targets for searches for biosignatures and, more speculatively, technosignatures, because their atmospheres are easier to detect due to their stars’ small sizes. We have yet to find a terrestrial planet orbiting a white dwarf, let alone one in the habitable zone, but searches are ongoing. Another important factor for habitability is the presence of water, and today’s article investigates whether a planet could retain an ocean through its star’s evolution and end up in the habitable zone, where life might exist.
Stellar Ocean Loss
To set the scene, let us imagine an ocean-bearing planet orbiting a Sun-like star evolving off the main sequence.
Even if the planet survives engulfment, it could easily lose its water
and become likely uninhabitable if the following steps occur: 1) high
surface temperatures evaporate the ocean into the atmosphere, 2)
high-energy photons dissociate the water molecules into hydrogen and
oxygen, and 3) those atoms escape into space and do not re-condensate.
As the star leaves the main sequence, the planet responds to changes
in the star’s size, brightness, and mass. The top four panels of Figure 1
show variations in stellar and planetary properties during this stage
of stellar evolution. During the asymptotic giant branch
phase, the star brightens considerably and expels ~30–80% of its mass,
causing the planet’s orbit to expand. X-ray and extreme-ultraviolet flux
from the star can cause the planet to lose atmospheric mass from
photoevaporation (i.e., when high-energy photons deposit sufficient
energy for particles to reach escape velocity). As the planet’s surface
temperature increases, the ocean could evaporate, creating a
predominantly water vapor atmosphere. If the extreme-ultraviolet flux is
sufficiently high such that oxygen and/or hydrogen escape the
atmosphere, the ocean is lost. The bottom panel of Figure 1 shows that
water retention becomes more difficult if the planet’s initial orbital
radius is small.
Tidal Ocean Loss
Large eccentricity helps drive a planet inwards, but it also stokes tidal heating that could increase the surface temperature. The exact effects on ocean evaporation and atmospheric mass loss are highly sensitive to how energy is dissipated. In general, tidal heating can result in Jeans escape, in which atmospheric particles reach sufficient thermal motion to escape into space. The authors find that while tidal heating is effective at evaporating the ocean into the atmosphere, it is less effective than the extreme-ultraviolet-driven mechanism at driving atmospheric mass loss.
Takeaways
There are a variety of factors that affect whether an ocean can be retained, including the planet’s initial orbital radius, the initial quantity of water, the stellar extreme-ultraviolet flux, the time at which the planet is scattered inwards, and the planet’s final orbital radius. To hold onto water, a planet must either start in a distant orbit (greater than 5–6 au for an Earth-sized ocean) or start with a massive quantity of water. Since large extreme-ultraviolet flux is required to drive water loss via photoevaporation, delaying inward scattering until the white dwarf cools aids ocean survival, as shown in Figure 2, which also shows that a larger final radius enhances water retention. If certain conditions are met, an ocean could be retained by a planet orbiting a white dwarf. This is an exciting finding for those searching for planets and signs of life around white dwarfs.
Original astrobite edited by William Smith
About the author, Kylee Carden:
