NASA: Some Ice-Rife Exoplanets Could Have Geysers and Habitable Oceans

A NASA study suggests that 17 exoplanets—worlds outside of our solar system—may have liquid water oceans beneath their icy shells, which would further the search for extraterrestrial life. Sometimes, as geysers, water from these oceans could burst through the ice crust. For the first time, estimates of the amount of geyser activity on these exoplanets were determined by the scientific team. They found two exoplanets that were close enough for telescopes to detect evidence of these eruptions.

Typically, exoplanets in a star's "habitable zone," or the distance at which temperatures permit liquid water to remain on their surfaces, are the focus of searches for extraterrestrial life. If an exoplanet has adequate internal heating, it could be able to support an ocean beneath its ice crust even though it is too far away and too cold. This is the case in our solar system, where the tides from the host planet's and its neighboring moons' gravitational pull cause the subsurface oceans of Europa, a moon of Jupiter, and Enceladus, a moon of Saturn, to exist.

If these subterranean oceans contain other essentials like an energy source and elements and compounds that are used in biological molecules, they might support life. Whole ecosystems on Earth flourish in total darkness near hydrothermal vents, which supply energy and nutrients, at the bottom of oceans.

According to Dr. Lynnae Quick of NASA's Goddard Space Flight Center in Greenbelt, Maryland, "our analyses predict that these 17 worlds may have ice-covered surfaces but receive enough internal heating from the decay of radioactive elements and tidal forces from their host stars to maintain internal oceans." All of the planets in our study may also show cryovolcanic eruptions in the form of plumes resembling geysers due to the degree of internal heating.” A paper on the research was published on October 4 in the Astrophysical Journal, with Quick serving as the lead author.

The team took into account the conditions on 17 confirmed exoplanets that are smaller than Earth but have lower densities, indicating that they might have significant amounts of water and ice rather than harder rock. The precise compositions of the planets are still unknown, but preliminary findings from earlier research on their surface temperatures all point to temperatures significantly lower than Earth's, raising the possibility that ice may be covering their surfaces.

The surface temperature estimates of each exoplanet were recalculated using models based on the known surface brightness and other characteristics of Europa and Enceladus, which led to improved estimates. Additionally, the team calculated the overall internal heating of these exoplanets by combining the heat expected from radioactive activity with the heat generated by tides derived from the orbital shape of each exoplanet. Since the oceans cool and freeze at the surface while being heated from the interior, estimates of the surface temperature and total heating provided the thickness of the ice layer for each exoplanet. Lastly, they contrasted these numbers with those of Europa, using the latter's estimated geyser activity levels as a conservative starting point for estimating the exoplanets' geyser activity.

According to their predictions, surface temperatures could be up to 60 degrees Fahrenheit (16 degrees Celsius) lower than earlier estimates. In comparison to Europa's estimated average of 18 miles (nearly 29 kilometers), the estimated ice shell thickness ranged from about 190 feet (58 meters) for Proxima Centauri b and one mile (1.6 kilometers) for LHS 1140 b to 24 miles (38.6 kilometers) for MOA 2007 BLG 192Lb. In comparison to Europa, which had geyser activity of 4,400 pounds per second (2,000 kilograms per second), the estimated geyser activity for Kepler 441b was only 17.6 pounds per second (about 8 kilograms per second), but for LHS 1140 b and Proxima Centauri b it was 639,640 pounds per second (290,000 kilograms/second) and 13.2 million pounds per second (six million kilograms/second).

Quick, who presented this research on December 12 at the American Geophysical Union meeting in San Francisco, California, stated, "Our models predict that oceans could be found relatively close to the surfaces of Proxima Centauri b and LHS 1140 b, and their rate of geyser activity could exceed Europa’s by hundreds to thousands of times." This means that telescopes are most likely to detect geological activity on these planets.

The exoplanet's motion may be observed when it passes in front of its star. The water vapor from the geysers could block or dull certain colors of starlight. According to Quick, sporadic observations of water vapor, where the amount of vapor detected changes over time, would point to the possibility of cryovolcanic eruptions. Other substances and elements in the water may indicate whether or not it can sustain life. Scientists could analyze the starlight to determine the composition of the geyser and assess the exoplanet's habitability potential because different elements and compounds absorb light at specific colors known as their "signature."

Strong telescopes that can measure light that an exoplanet reflects while orbiting its star may be able to detect geyser activity for planets like Proxima Centauri b that do not cross their stars from our point of view. At the exoplanet's surface, geysers would shoot icy particles, giving the object a dazzling, reflective appearance.

The Virtual Planetary Laboratory, a collaborator on the NASA Nexus for Exoplanet System Science coordination group, the University of Washington's Astrobiology Program, and NASA's Habitable Worlds Program provided funding for the study.