Astronomers discover strange twin planets might be water worlds

"The two exoplanets, dubbed Kepler-138c and Kepler-138d, orbit a star located about 218 light-years away from Earth in the constellation Lyra."...The astronomers can't be sure yet that these planets are indeed watery; they haven't directly detected the substance on these worlds. But the researchers were able to calculate the density of each planet: Each has about three times the volume of Earth but only twice the mass, making them much less dense than our own world."

Good to point out here concerning this water world report. Other exoplanets were reported with likely water world too.

An extrasolar world covered in water?, https://www.sciencedaily.com/releases/2022/08/220824103051.htm

Ref - TOI-1452 b: SPIRou and TESS Reveal a Super-Earth in a Temperate Orbit Transiting an M4 Dwarf, https://iopscience.iop.org/article/10.3847/1538-3881/ac7cea, 12-August-2022.

Exoplanet atmospheres (iac.es), 133 are listed with some type of atmosphere, the reported water worlds are not listed.
 
I dug a bit more into this water world report for Kepler-138 c and d. The paper cited in the space.com is good. The exoplanet.eu cite for these two exoplanets shows some properties.

The Extrasolar Planet Encyclopaedia — Kepler-138 c (exoplanet.eu)

The Extrasolar Planet Encyclopaedia — Kepler-138 d (exoplanet.eu)

138 d shows a calculated temperature 431 K. Taking a good swim (I like to do this) could make for a very interesting and warm swim :) I also note this about M dwarf star behavior. Proxima Centauri b is a subject of much study now.

Revisiting the space weather environment of Proxima Centauri b, https://arxiv.org/abs/2211.15697

My notes. From the PDF report. "1. INTRODUCTION Extensive research has been directed toward understanding the conditions of close-in planets orbiting M dwarfs. These planets are by far the most abundant kind of detected exoplanet orbiting in the temperature based definition of the habitable zone (HZ). Due to the low luminosity of M dwarfs, their HZ resides very close to the host star (e.g. Kopparapu et al. 2013, 2014 Shields et al. 2016). Low mass stars are typically magnetically more active than higher mass stars, and remain active for much longer (e.g. Reiners & Basri 2008, Wright et al. 2011a, Jackson et al. 2012, Cohen & Drake 2014, Davenport et al. 2019). The associated coronal and chromospheric integrated high-energy radiation can evaporate planetary atmospheres and poses a risk for close-in exoplanets...Proxima Centauri b (Proxima b hereafter) is a rocky planet orbiting in the \habitable zone" of Proxima Centauri, our closest neighboring star at only 1.3 parsecs from Earth (Anglada-Escude et al. 2016). Detailed and realistic magnetohydrodynamic (MHD) simulations predicted that Proxima b should experience stellar wind pressures four orders of magnitude larger than the solar wind pressure experienced at Earth, together with strong variations of this pressure on timescales as short as a day (Garraffo et al. 2016b). Such simulations also predicted that planets around M dwarfs like Proxima b will sufer from intense Joule heating (Cohen et al. 2014), severe atmospheric loss (Dong et al. 2017, Garcia-Sage et al. 2017)...The average wind pressures for the three cases fall in the range 100-300 times the solar wind pressure at Earth."

My note. It is apparent that Proxima Centauri b may not be a habitable exoplanet with the host star magnetic field causing problems and intense stellar wind and radiation blasting the exoplanet. Defining habitable zones around red dwarf stars has more issues for astrobiology to find life, somewhere out there it seems. At the moment, Kepler-138 system I did not find information on concerning host star stellar winds and erosion or surface temperature other than 138 d showing 431 K. The space.com report ends, "The researchers hope that the Kepler-138 planets won't be the last to offer puzzles like these. "As our instruments and techniques become sensitive enough to find and study planets that are farther from their stars, we might start finding a lot more water worlds like Kepler-138c and d," Benneke said."

Exoplanets orbiting red dwarf stars may not be good places to live at and water worlds remain to be confirmed too.