Title: The Fate of Oceans on First-Generation Planets Orbiting White Dwarfs
Authors: Juliette Becker, Andrew Vanderburg, Joseph Livesey
First Author’s Institution: Department of Astronomy, University of Wisconsin-Madison, Madison, WI 53706, USA
Status: Accepted to the Astrophysical Journal [open access]
Towards the ends of their lives, stars like the Sun are destined to expand into red giants, expel their outer layers, and leave behind their Earth-sized cores, white dwarfs. What happens to any planets during this stage of stellar evolution is far more uncertain. Planets sufficiently close to their host star are expected to be engulfed as the star expands into a red giant, including our very own Earth. However, some planets can survive or perhaps even form during white dwarf formation, and we know of a handful of planets and planet candidates around white dwarfs from transits, direct imaging, and mid-infrared excess.
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 paper 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 (AGB), the star brightens considerably and expels ~30-80% of its mass, causing the planet’s orbit to expand. X-ray and extreme ultraviolet (XUV) flux from the star can cause the planet to lose atmospheric mass from photoevaporation (i.e., 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 XUV flux is sufficiently high such that oxygen and/or hydrogen escape the atmosphere, the ocean is lost. The bottom panel of Figure 1 shows water retention becomes more difficult if the planet’s initial orbital radius is small.
Tidal Ocean Loss
Not only does the ocean have to survive the aforementioned complications, but the planet needs to end up in the habitable zone despite starting far away from the star. The planet must be perturbed to achieve high eccentricity and then tidally circularize its orbit in the white dwarf habitable zone (~0.01 AU). Planet-planet scattering (i.e., dynamical interactions between planets in a multi-planet system) is the most plausible mechanism to drive a planet inwards. These interactions could be delayed substantially after white dwarf formation. Since white dwarfs cool and emit less XUV flux over time, delaying inward scattering could enhance water retention.
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 XUV-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 XUV 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 XUV flux is required to drive water loss via photoevaporation, delaying inward scattering for when the white dwarf cools aids ocean survival, as shown in Figure 2, which also shows 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.
Astrobite edited by William Smith
Featured image credit: NASA/JPL-Caltech