The NASA study broadens the exploration for extraterrestrial life by proposing that 17 exoplanets may possess oceans of liquid water beneath icy surfaces, a crucial component for supporting life. These oceans could potentially give rise to geysers, as calculated by the science team in a groundbreaking effort, marking the first estimates of geyser activity on these distant worlds. Notably, two of these exoplanets are identified as being close enough for potential observation of these eruptions through telescopes, adding a new dimension to the search for signs of life beyond our solar system.
While the conventional quest for extraterrestrial life centers on exoplanets within a star's habitable zone, where temperatures permit liquid water, a NASA study highlights the possibility of distant and cold exoplanets harboring oceans beneath icy crusts through sufficient internal heating. Drawing parallels to our solar system, moons like Europa (Jupiter's moon) and Enceladus (Saturn's moon) showcase this phenomenon, possessing subsurface oceans sustained by tidal heating from the gravitational interactions with their host planets and neighboring moons.
The potential for life within the subsurface oceans of these exoplanets hinges on meeting essential conditions, including a viable energy supply and the presence of elements and compounds crucial for biological molecules. Drawing parallels with Earth's ecosystems thriving near hydrothermal vents in complete darkness, these distant oceans could harbor life if the necessary components are present. Dr. Lynnae Quick from NASA's Goddard Space Flight Center in Maryland emphasizes that the 17 studied exoplanets, though possessing ice-covered surfaces, likely maintain internal oceans due to sufficient internal heating from the decay of radioactive elements and tidal forces from their host stars.
The research suggests that these planetary bodies could also experience cryovolcanic eruptions, adding a dynamic aspect to their potential habitability. Quick is the lead author of the research paper published in the Astrophysical Journal on October 4.
The study delved into the conditions of 17 Earth-sized exoplanets with lower density, hinting at substantial ice and water content rather than denser rock. While the exact compositions of these planets remain unknown, prior studies suggest colder surface temperatures compared to Earth, indicating the likelihood of ice-covered surfaces. To refine estimates, the team utilized models based on Europa and Enceladus, recalculating each exoplanet's surface temperature and assessing total internal heating.
By factoring in tidal forces and radioactive activity, they determined ice layer thickness, considering the cooling and freezing at the surface juxtaposed with interior heating. The comparison to Europa served as a baseline for estimating geyser activity on the studied exoplanets, providing valuable insights into their potential geological characteristics.
The study predicts significantly colder surface temperatures, up to 60 degrees Fahrenheit (33 degrees Celsius) lower than previous estimates. Estimated ice shell thickness varies, ranging from 190 feet (58 meters) for Proxima Centauri b to 24 miles (38.6 kilometers) for MOA 2007 BLG 192Lb, compared to Europa's average of 18 miles (almost 29 kilometers). Geyser activity estimates also show substantial variations, from 17.6 pounds per second for Kepler 441b to 13.2 million pounds per second for Proxima Centauri b. Dr. Lynnae Quick highlights that the models suggest oceans could exist relatively close to the surfaces of Proxima Centauri b and LHS 1140 b, with geyser activity potentially surpassing Europa's by significant margins.
These findings increase the likelihood of detecting geological activity on these exoplanets using telescopes, as presented by Quick at the American Geophysical Union meeting in San Francisco on December 12.
The anticipated geological activity on these exoplanets, such as cryovolcanic eruptions, could be observable when the planet transits its star. Water vapor from the geysers might selectively block or dim specific colors of starlight, leading to sporadic detections that vary over time. Dr. Lynnae Quick notes that such occurrences would strongly indicate the presence of cryovolcanic activity. The water expelled from these geysers may contain elements and compounds crucial for life, and their signature absorption of specific colors of light could be analyzed to assess the exoplanet's habitability potential.
For planets like Proxima Centauri b, not crossing their stars from our perspective, powerful telescopes capable of measuring reflected light could detect geyser activity, making the exoplanet appear exceptionally bright and reflective. The research, contributing valuable insights into potential habitable conditions, was funded by NASA's Habitable Worlds Program, the University of Washington's Astrobiology Program, and the Virtual Planetary Laboratory, a member of the NASA Nexus for Exoplanet System Science coordination group.
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