Astronomers discover strange twin alien planets might be water worlds

Two planets found by NASA's retired Kepler Space Telescope may be made mostly of water, according to new research.

The two exoplanets, dubbed Kepler-138c and Kepler-138d, orbit a star located about 218 light-years away from Earth in the constellation Lyra. Scientists intrigued by 2014 data from the Kepler Space Telescope decided to revisit the two planets using the Hubble Space Telescope and NASA's retired Spitzer Space Telescope in hopes of better understanding the distant worlds and what they're made of. And surprisingly, the answer might be mostly water.

"We previously thought that planets that were a bit larger than Earth were big balls of metal and rock, like scaled-up versions of Earth, and that's why we called them super-Earths," Björn Benneke, a planetary astrophysicist at the Université de Montréal in Canada and a co-author on the new research, said in a statement (opens in new tab).

"However, we have now shown that these two planets, Kepler-138c and d, are quite different in nature: a big fraction of their entire volume is likely composed of water," he said. "It is the first time we observe planets that can be confidently identified as water worlds, a type of planet that was theorized by astronomers to exist for a long time."

Related: 10 amazing exoplanet discoveries

An artist's depiction of an exoplanet with water vapor in its atmosphere. (Image credit: ESA/Hubble, M. Kornmesser)

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.

(The similarity of the two worlds was a surprise in and of itself, since scientists had expected the planets to be quite different.)

The low density suggested that as much as half of each world is made up of something heavier than the hydrogen and helium of gas giants, but lighter than the rock scientists had expected to find — leaving water a real possibility.

The researchers suggest that the two planets might be a little bit like icy moons of the outer solar system, which can hide an ocean of liquid water below an icy shell and above a rocky core. However, the Kepler worlds would be much, much hotter than these moons, with little ice to be found.

"Imagine larger versions of Europa or Enceladus, the water-rich moons orbiting Jupiter and Saturn, but brought much closer to their star," Caroline Piaulet, a Ph.D. student at the Université de Montréal and lead author on the new research, said in the statement. "The temperature in Kepler-138c's and Kepler-138d's atmospheres is likely above the boiling point of water, and we expect a thick, dense atmosphere made of steam on these planets."

An artist's depiction of the exoplanet Kepler-138d compared to Earth. (Image credit: Benoit Gougeon, Université de Montréal)

(Underneath all that steam, however, the scientists say that there might be liquid water, or possibly even water so hot and under so much pressure that it has the density of a liquid but flows like a gas, a state called a supercritical fluid.)

The exotic worlds aren't the only surprise the researchers found in the Hubble and Spitzer data. Astronomers had originally used the Kepler observations to discover three planets, announced in 2014. Benneke was particularly interested in the outermost world, Kepler-138d, prompting the Hubble and Spitzer observations that occurred between 2014 and 2016.

That data points to a fourth planet, which would be dubbed Kepler-138e, a small world orbiting the star every 38 days at a distance that could permit liquid water to exist at its surface, the scientists said. (Kepler-138b and c orbit every 10 and 14 days respectively.)

The additional observations also suggested that the innermost planet, Kepler-138b, is about the size of Mars, which would make it one of the smallest of the more than 5,000 exoplanets scientists have found to date.

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.

The research is described in a paper (opens in new tab) published Thursday (Dec. 15) in the journal Nature Astronomy.

Email Meghan Bartels at mbartels@space.com or follow her on Twitter @meghanbartels. Follow us on Twitter @Spacedotcom and on Facebook.

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Meghan Bartels
Senior Writer

Meghan is a senior writer at Space.com and has more than five years' experience as a science journalist based in New York City. She joined Space.com in July 2018, with previous writing published in outlets including Newsweek and Audubon. Meghan earned an MA in science journalism from New York University and a BA in classics from Georgetown University, and in her free time she enjoys reading and visiting museums. Follow her on Twitter at @meghanbartels.

  • rod
    "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.
    Reply
  • rod
    Space.com reported on the possible water world TOI-1452 b also.

    Possible water world spotted orbiting an alien star, https://forums.space.com/threads/possible-water-world-spotted-orbiting-an-alien-star.57343/
    Reply
  • rod
    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.
    Reply