Is Earth Potentially a Captured Rogue Chthonian Planet?
A new multi-institutional paper with dozens of international authors appeared on Arxiv on March 20 that argued a fascinating evolutionary history of a particular exoplanet called TOI-849b. The authors embrace the Chthonian hypothesis of dense rocky planet development. During the Chthonian conversion, gas giants migrate from the outer reaches of a solar system into close proximity with their host star and effectively dissipate their outer layers evaporatively in the presence of the intense local stellar winds. Alternative explanations for the appearance of these dense rocky planets include impact events that suddenly remove their massive atmospheres.
The Chthonian hypothesis is not a new idea, however, though it may prove out as a much more common planetary evolution mechanism than the astrophysics community originally envisioned. The notion was first publicized by a French group led by Dr. G. Hebrard in the paper entitled, “Evaporation rate of hot Jupiters and formation of Chthonian planets.” Hebrard lays out the basic mechanism by which planets may lose their atmosphere in the vicinity of a host star, citing the paucity of Jupiter mass gas planets found within a particular critical distance of their hosts. He notes one exceptional such gas planet discovered the same year that is indeed losing hydrogen as it orbits the host star HD209458. Chthonians are proposed to be heated in their thermospheres by a combination of irradiance and tidal-sloshing type frictions within the atmosphere. I would also add to that greenhouse-like effects and Nernst-related pressure to temperature effects expected at the core of a giant planet.
In general, only 15% of exo-solar planets are found within the 0.1 AU distance from their parent stars. Hebrard uses the Chthonian principle of planetary evaporation to estimate planetary lifetime on the basis of mass and orbital distance. He suggests that this explains the scarcity of less-than-three-day period planets. The University of Washington researchers, Drs. D.C. Catling and K.J. Zahnle, have gone to great lengths to cross-catalogue the expected losses for various atmospheric gases. They find that in general, lighter, less-sticky gases like Hydrogen are the first to be lost during photoevaporation. The Chthonian process effectively selects for retainment of water and carbon dioxide, which are heavy and tend to rain back down upon condensing in the upper atmosphere. This implies that the Chtonian process when applied to a hot Jupiter will favor a watery giant world.
Lighter gases go first during a Chthonian transition. From Catling & Zahnle, 2009.
Rocky planets may also undergo evaporative transitions. Catling and Zahnle actually suggest that Earth itself may one day proceed toward a Venus-like appearance as a result of these long-term evaporative processes. Dr. J. Kasting made the assertion that Venus had perhaps lost an ocean’s worth of hydrogen over the course of mere tens of millions of years and has been developing the idea that the Earth and Venus are at different stages on a shared evolutionary time-line since as early as 1988. In general, there are a number of proposed mechanisms concerning migration of a gas-planet into the close proximity of a host star, including the loss of its own local satellites.
The idea that Earth-like planets are formed through evaporation of migrating gas or ice giants has also been investigated with mixed conclusions. The Austrian group led by Dr. M. Leitzinger at IGAM initially presented thermal mass loss calculations concerning the smallest transiting rocky exoplanets, and found evaporation unlikely to account for their appearance on account of insufficient time in the stellar evolution of the host star. For instance, the pressures needed for accumulation of an Earth-like core, around 100TPa, would require a gas giant at least 25 times the size of Jupiter. However, even a Uranus-like planet instantly placed into critical proximity of the host star should only produce a 7.6% mass-loss to the planet in question during the presumed lifetime of the star. It would thus require many star-lifetimes for an Earth-like planet to be formed from the Chthonian process. Hence, the only way that an Earth-like planet could have even possibly formed evaporatively from a gas giant is if it made stays in multiple star systems.
Do planets form and wander beyond solar systems?
Rogue planets like Cha 110913−773444 are another class of planets thought to be orphaned gas giants. It is suggested that such planets accrete out of dust and gas identically to stars but do not grow large enough to hail the pressures necessary for full luminous glow. Dr. G Fazio of the Harvard-Smithsonian indicates that the line between Brown Dwarf and Gas Giant is drawn by size or presumed formation history. The accretion disks often found surrounding these strange bodies are usually attributed to absorptions of dust and ice. Is it equally possible that the disc represents dissipation instead?—the discarded remains of a more massive earlier version of the Dwarf before it was debrided by countless violent exchanges with other celestial bodies in the course of its wandering? It is not apparent yet that such Rogue planets are regularly traded between star-systems and subsequently participate in serial evaporative events and the proposal may seem like a long shot, but on astronomical time scales the unlikely can become commonplace.
Rogue planets themselves are not a rare occurrence. New Brown Dwarfs are continually discovered. Dr. M.B. Lund of Cal Tech estimates that there may be between 2 and 40 billion such rogue exoplanets within our galaxy. The WFIRST microlensing experiments may detect even more candidates. Kavli Institute for Particle Astrophysics and Cosmology at Stanford University scientists have pushed this figure even further, indicating that there may be 100,000 nomad planets for every typical star system in the galaxy. It has been estimated that violent interaction of solar systems occurs quite regularly, on the order of millions of years.
What are the odds one of these Rogue Dwarves might be captured by a host star now and then and proceed to be evaporated through the Chthonian process? If that particular host star were to eventually expire or become tidally unstable, its Chthonian satellite may again wander the galaxy and later be subjected to another round of blistering stellar wind. Such a stepwise evolution may in theory be able to explain the appearance of our home planet although it stretches developmental timescales far beyond those presently accepted by the wider astronomical community.
