The UKCA Harding Essay Competition presents this year’s theme:
ICE, ICE, MAYBE?
Life in the Cold Universe
The UK Centre for Astrobiology at the University of Edinburgh is pleased to announce that Ruby Allen-Brown is this year’s winner!
Read her essay below:
Astrobiology’s Blind Spot: Does Starless Necessarily Mean Lifeless?
Traditionally, the habitable zone was a pivotal tool astrobiologists used to determine an exoplanet’s habitability: a conditional region encircling a star in which the temperature allows for liquid water to exist long-term on a planet’s surface. Accordingly, the existence of life on rogue, starless planets was not considered until decades later when scientists realized habitability is more complex. Even today, rogue planets are still overshadowed with discussions of life on glacial moons such as Europa dominating astrobiological speculations. Yet beneath their surfaces, the two share a perhaps surprising number of similarities; both may host subsurface oceans beneath kilometers of ice; both are driven by internal sources of heat rather than stellar; and they could both potentially support life in complete darkness under extreme pressures. Such parallels criticize the deeply rooted assumption that declares sunlight essential for life, setting the scene for a broader, more inclusive definition of habitability itself.
Rogue, or free-floating planets (FFPs) are defined as celestial bodies travelling through space without being gravitationally bound to a star. Like traditional planets, rogue planets are formed through gravitational accretion. Some are thought to have formed independently as failed stars that were unsuccessful in achieving nuclear ignition. Others may have formed as part of planetary systems and were then later ejected, either due to gravitational instabilities in the system or the parent star going supernova. FFPs are surprisingly abundant in the universe: they outnumber stars in the milky way at a ratio of around twenty to one according to current estimates. Furthermore, a significant number of these planets are believed to be Earth-sized. NASA’s Nancy Grace Roman Telescope could find up to 400 FFPs with Earth-like masses when it is set to launch in 2027.
Outwardly, rogue planets may seem utterly uninhabitable; their lack of a nearby star assures they receive no light or heat, leaving their surfaces bleak and frigid. Despite this, many rogue planets may still harbour subsurface oceans beneath a thick layer of ice as a result of several factors. To begin with, FFPs could maintain hydrogenrich atmospheres as they drift through the universe, trapping any internal heat generated and thus providing stable conditions within the planet’s oceans. If this heat is great enough to keep water in its liquid form, an environment suitable for life may be achievable for the rogue planet. Numerous sources could provide this heat including radioactive decay of isotopes such as uranium and thorium in the planet’s core; residual heat left behind from planetary formation; and even tidal heating if the rogue planet has its own moons. In 2011, researchers at the University of Chicago confirmed that rogue planets of Earth-like composition could maintain subglacial oceans if certain conditions are met and therefore may be considered habitable.
Even in total darkness on the ocean’s floor, hydrothermal vent systems on Earth allow life to exist. In the absence of sunlight, ecosystems cannot rely on photosynthesis to flourish but rather the conversion of minerals and chemicals into energy. Chemosynthetic communities like these could also exist on rogue planets if similar hydrothermal vent systems were to exist. It is unlikely that multi-cell organisms would ever develop in this environment with the absence of an oxygenrich atmosphere, however bacteria and archaea like those in Earth’s early history could thrive in the outlined conditions. Another important possibility to consider is that life in this subglacial environment could be fundamentally different from anything we have observed on Earth, challenging all previous anthropocentric assumptions made. If solvents such as ammonia or hydrocarbons were to exist, biochemistry itself as we know it could be completely redefined. As we have discovered from extremophiles on our own planet, life typically finds a way to exist even in the most inhospitable places – why couldn’t the same be true for FFPs?
The fundamental obstacle with the above hypothesis relates to the limited detection methods available: we cannot simply look for transits in front of a host star or detect atmospheric biosignatures if there is no light to analyse. Instead, gravitational microlensing is the leading tool. If a rogue planet happens to pass between Earth and a background star, its gravity will act as a lens, bending the star’s light as it curves around the planet. This magnified light acts as a telltale flare which astronomers can then detect and use to determine more information about the planet’s mass. Infrared surveys with instruments like JWST or WISE are another method of detecting FFPs. In particular, these surveys primarily discover younger planets that are still warm from their initial formation. The issue with these two techniques is that they both rely on specific circumstances to be met in order for detection to be made possible: in reality, the vast majority of rogue planets are already far too cold and ancient for infrared surveys, and microlensing events are not only rare, but also incredibly short-lived. Moreover, without light spectra we cannot discern any information about atmospheric composition, meaning potential biosignatures including oxygen or methane gases remain invisible with our current technology.
The existence of life on rogue planets would not only provoke scientific interest, but also a large philosophical discussion. For example, would such beings have any concept of time, light or the vast universe laying beyond the ice? Would they ever guess that their icy ceiling is not just ice, but entire galaxies waiting to be discovered? Or would they, like life in Earth’s own hydrothermal vents remain oblivious to the cosmos beyond? Such questions stretch both our scientific and philosophical boundaries, serving as a reminder that life may exist in ways completely unfamiliar to us. This is an important fact to remember as the astrobiology field continues to expand: in the past, anthropocentric expectations have only limited our capabilities. Going forward, it is vital we consider life in all its possible forms as the search continues.
Habitability has long been associated with the warmth of stars, exposing humanity’s bias in our search for extraterrestrial life. But the universe is not made in Earth’s image, and so we have no right to assume life should be either. Rogue planets’ exclusion in mainstream astrobiology is not a consequence of their lack of potential, but rather our lack of familiarity. If one were to pass through our solar system, it could offer a unique chance for exploration that is no longer distant and theoretical, but close, reachable and perhaps even revolutionary. Ultimately, starless does not have to mean lifeless – but only if we choose to see what astrobiology has long left underexplored.
Any queries: UKCA-info@ed.ac.uk
