During the formation of planetary systems around stars, gravitational interactions with gas giant planets can cause some planets or planetesimals to enter hyperbolic orbits and get ejected into interstellar space. These objects will wander the dark and vast expenses of interstellar space as rouge planets. Several months ago, I wrote an article about the possibilities that a wandering planet in interstellar space can have a dense and high pressure atmosphere of hydrogen gas which can create a greenhouse effect that is strong enough such that liquid water can be maintained on the planet’s surface by the planet’s geothermal heat flux alone, making the planet potentially habitable. In this article, I explore the possibilities in which a planet wandering in interstellar space can be potentially habitable from an alternative mechanism that was recently published in a paper entitled “The Steppenwolf: A Proposal for a Habitable Planet in Interstellar Space”. This paper explores whether an Earth-like planet wandering in interstellar space can be potentially habitable by sustaining a subglacial ocean of liquid water.
In this case, an Earthlike planet means that the planet has a total mass and a water mass fraction that is within one order of magnitude of the Earth’s, and the planet also has approximately the same composition as the Earth. A rouge Earth-like planet wandering in interstellar space and harboring a subglacial ocean of liquid water is referred to in the paper as a Steppenwolf planet. Such a planet will have a layer of ice on top of a subglacial ocean of liquid water where the radiogenic geothermal heat flux from the interior of the planet prevents the liquid water from freezing solid. Furthermore, the overlying layer of ice acts an insulating layer which prevents the radiogenic geothermal heat flux from escaping too quickly into space, thereby trapping sufficient heat energy to sustain the subglacial ocean of liquid water.
The radiogenic geothermal heat flux is produced from the decay of radioisotopes in the interior of the planet and it is estimated to be sufficient to keep the subglacial ocean of liquid water from freezing for the duration of a few billion years. For instance, the Earth has a current average geothermal heat flux of 0.087 watts per square meter of the Earth’s surface and since this geothermal heat flux decays with time, the Earth is estimated to have a geothermal heat flux of twice the current value at around 3 billion years ago. After a few billion years, the decline in the amount of radioisotopes makes the rate of radiogenic heating insufficient to provide enough warmth to keep the subglacial ocean of liquid water from freezing. Hence, a Steppenwolf planet will have a habitable lifetime that is comparable to planets that are found in the traditional habitable zones of Sun-like stars.
The steady-state thickness of the layer of ice on a Steppenwolf planet depends on the amount of radiogenic geothermal heat flux being radiated by the planet. A greater heat flux will allow for a thinner steady-state layer of ice to exist above the subglacial ocean of liquid water while a lower heat flux will result in a thicker layer of ice. If the heat flux is too low, the resulting steady-state thickness of the layer of ice will be greater than the depth of the ocean and no subglacial ocean of liquid water will be possible in this case. In addition, volcanoes on continents or islands that rise above the layer of ice on a Steppenwolf planet can emit significant quantities of carbon dioxide, leading to the eventual formation of a thick cryo-atmospheric layer of carbon dioxide. Such a layer of carbon dioxide can raise the temperature at the top surface of the layer of ice and enable a significantly reduced thickness for the steady-state layer of ice overlying the subglacial ocean of liquid water.
Without a cryo-atmospheric layer of carbon dioxide, a Steppenwolf planet with the same radioisotope composition, age and water mass fraction as the Earth will have to be at least 3.5 times more massive than Earth in order to sustain a subglacial ocean of liquid water. In contrast, a Steppenwolf planet with ten times the water mass fraction of the Earth and with a thick cryo-atmospheric layer of carbon dioxide will require a mass of only 0.3 times the mass of the Earth for it to sustain a subglacial ocean of liquid water. The transport of heat up the layer of ice on a Steppenwolf planet can be assumed to be conductive in nature since any transport of heat by convecting ice is expected to occur only in the lower and warmer regions of the ice layer where it will be capped by an overlying lid of stagnant conducting ice.
Steppenwolf planets will be very challenging to detect as they wander through the dark and immense voids of interstellar space and produce no light of their own. However, if a Steppenwolf planet were to loiter close enough to our Sun, it could make its presence know by detecting the sunlight being reflected off its surface. For instance, a planned wide-field survey telescope known as the Large Synoptic Survey Telescope (LSST) will be able to detect a Steppenwolf planet out to a maximum distance of around 1000 astronomical units where one astronomical unit is the average distance of the Earth from our Sun. It should be known that only surveys that observe large regions of the sky continuously will be likely to discover any Steppenwolf planets because such planets can be anywhere in the sky. Finally, the discovery of any potentially habitable Steppenwolf planets will be rather exciting because it can mean that potentially habitable worlds are truly ubiquitous in the universe.