Key Takeaways:

  1. Astronomers have identified two exoplanets, Kepler-138 c and Kepler-138 d, as the first true water worlds with oceans estimated to be 500 times deeper than Earth’s.
  2. These planets are slightly larger than Earth but have densities between rocky planets and gas giants.
  3. The planets orbit a red dwarf star called Kepler-138, located 218 light-years away.
  4. Data from the Hubble and Spitzer space telescopes provided crucial evidence for these water worlds.
  5. These planets, while similar in composition to Earth’s oceans, have significantly different conditions, with high temperatures and pressure.

In a revelation that defies conventional understanding of celestial bodies, astronomers announced in December 2022 the identification of two unprecedented exoplanets termed “water worlds.” These exoplanets, Kepler-138 c and Kepler-138 d, are marginally larger than Earth but exhibit densities that fall between rocky planets like ours and the gas giants in our solar system. It is estimated that their global oceans reach depths of at least 1,000 miles, making them over 500 times deeper than Earth’s oceans.

These remarkable worlds orbit a red dwarf star named Kepler-138, situated 218 light-years away in the Lyra constellation. Although scientists had speculated about the existence of such global water worlds, these two planets represent the first concrete evidence of their presence. This breakthrough was made possible through data obtained from the Hubble and retired Spitzer space telescopes.

Lead researcher Caroline Piaulet, from the Trottier Institute for Research on Exoplanets at the University of Montreal, likened these planets to enlarged, warmer versions of the ocean moons in our solar system, such as Europa and Enceladus. These moons are much smaller than planets and possess ice-covered oceans. In contrast, Kepler-138 c and d are characterized by immense water-vapor envelopes.

The peculiar densities of these planets perplexed scientists. While they are slightly larger than Earth, they lack the density expected of rocky planets. Instead, a significant portion of their volume is believed to be composed of water. This discovery challenges the previous classification of such planets as “super-Earths.”

Comparison of the interior structures of Earth and Kepler-138 d. Image via Benoit Gougeon (University of Montreal)/ Hubblesite.

Although these water worlds exhibit oceans akin to Earth’s, they differ significantly. The evidence points to higher temperatures and immense pressure, potentially erasing the clear boundary between ocean and atmosphere. Kepler-138 d’s atmosphere, for instance, is anticipated to consist of steam due to its scorching temperatures.

Despite their intriguing characteristics, Kepler-138 c and d are not located within the habitable zone conducive to conventional forms of liquid water. Their atmospheres are excessively heated, creating steamy conditions with liquid water existing at extreme pressures beneath.

The Kepler-138 system also encompasses two additional planets. Kepler-138 b, similar in size to Mars, is rocky and diminutive. Kepler-138 e, positioned farthest from the star, lies just within the inner edge of the habitable zone. While its size is uncertain, it is projected to surpass Kepler-138 b in mass. Unlike the other three planets, Kepler-138 e’s orbit does not facilitate easy study due to its non-transiting configuration.

This groundbreaking discovery challenges prior assumptions about super-Earths, suggesting that not all of them are rocky. Kepler-138 c and d provide crucial evidence for the existence of water worlds and open the door to further exploration of such unique celestial bodies.

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