Astronomers from NASA and the University of Washington have estimated total internal heating rates and depths to possible subsurface oceans for 17 planets that may be cold ocean planets — low-mass exoplanets with surface temperatures and/or densities that are consistent with icy surfaces and substantial water content. Like the icy moons in our outer Solar System, these planets may be astrobiologically significant worlds that harbor habitable environments beneath their icy surfaces.
This artist’s impression shows Proxima b orbiting Proxima Centauri, which at only 4.23 light-years is the closest star to our Solar System. The double star Alpha Centauri AB also appears in the image between the exoplanet and Proxima itself. Image credit: M. Kornmesser / ESO.
Ocean planets are a proposed class of low-density, terrestrial exoplanets with substantial liquid water layers.
They may exist in a variety of climactic states including ice free, partially ice covered, or completely frozen over at their surfaces.
“Our analyses predict that these 17 alien 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,” said Dr. Lynnae Quick, a researcher at NASA’s Goddard Space Flight Center.
“Thanks to the amount of internal heating they experience, all planets in our study could also exhibit cryovolcanic eruptions in the form of geyser-like plumes.”
Dr. Quick and colleagues considered conditions on 17 confirmed exoplanets that are roughly Earth-sized but less dense, suggesting that they could have substantial amounts of ice and water instead of denser rock.
Although the planets’ exact compositions remain unknown, initial estimates of their surface temperatures from previous studies all indicate that they are much colder than Earth, suggesting that their surfaces could be covered in ice.
The authors improved estimates of each exoplanet’s surface temperature by recalculating using the known surface brightness and other properties of Europa and Enceladus as models.
They also estimated the total internal heating in these exoplanets by using the shape of each exoplanet’s orbit to get the heat generated from tides and adding it to the heat expected from radioactive activity.
Surface temperature and total heating estimates gave the ice layer thickness for each exoplanet since the oceans cool and freeze at the surface while being heated from the interior.
Finally, they compared these figures to Europa’s and used estimated levels of geyser activity on Europa as a conservative baseline to estimate geyser activity on the exoplanets.
They predict that surface temperatures are colder than previous estimates by up to 33 degrees Celsius (60 degrees Fahrenheit).
An artist’s impression of the planetary system LHS 1140. Image credit: Sci.News.
Estimated ice shell thickness ranged from about 58 m (190 feet) for Proxima b and 1.6 km (1 mile) for LHS 1140b to 38.6 km (24 miles) for MOA-2007-BLG-192Lb, compared to Europa’s estimated average of 29 km (18 miles).
Estimated geyser activity went from just 8 kg per second for Kepler 441b to 290,000 kg per second for LHS 1140b and 6 million kg per second for Proxima b, compared to Europa at 2,000 kg per second.
“Since our models predict that oceans could be found relatively close to the surfaces of Proxima b and LHS 1140b, and their rate of geyser activity could exceed Europa’s by hundreds to thousands of times, telescopes are most likely to detect geological activity on these planets,” Dr. Quick said.
“This activity could be seen when the exoplanet passes in front of its star. Certain colors of starlight could be dimmed or blocked by water vapor from the geysers.”
“Sporadic detections of water vapor in which the amount of water vapor detected varies with time, would suggest the presence of cryovolcanic eruptions.”
“The water might contain other elements and compounds that could reveal if it can support life.”
“Since elements and compounds absorb light at specific signature colors, analysis of the starlight would let scientists determine the geyser’s composition and evaluate the exoplanet’s habitability potential.”
A paper on the findings was published in the Astrophysical Journal.
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Lynnae C. Quick et al. 2023. Prospects for Cryovolcanic Activity on Cold Ocean Planets. ApJ 956, 29; doi: 10.3847/1538-4357/ace9b6
