An international research team – including astronomers from Graz – has identified an exoplanet whose solid core is completely surrounded by an ocean in the vastness of space. The “water world” is surrounded by a 2000 km deep ocean and an ephemeral atmosphere.
The central star of this oceanic planet is about 200 light-years from our sun. As a so-called red dwarf, Kepler-138 is considerably smaller and cooler than our sun. The star, which lies outside our solar system, is surrounded by several planets, Lucca Fossati, a co-author from the Institute for Space Research (IWF) in Graz, told APA. It is unlikely that there is life there.
Observed with Hubble and Spitzer
They were discovered in 2014 and observed by the Hubble (pictured below) and Spitzer Space Telescopes. Now something surprising has been found in the planet Kepler-138d, which is about 1.5 times the size of Earth, in this planetary system, the scientists report in the journal Nature Astronomy.
To date, about 5000 exoplanets (celestial bodies outside our solar system, note) are known. Exoplanets with less than 1.7 Earth radii have so far been assigned to the categories of rocky planets or mini-Neptune: Rocky planets usually orbit their star in very narrow orbits, meaning that over time they lose their atmospheres due to the influence of stellar radiation.
The small, gaseous mini-Neptunes, on the other hand, accumulated so much hydrogen gas during their formation that they could no longer release it into space, just like the large gas planets in the outer solar system. According to the Graz Space Institute, the same applies to so-called Hycean planets, hypothetical celestial bodies surrounded by a hot water ocean and a correspondingly very hydrogen-rich atmosphere.
Planet with very special conditions
The conditions for Kepler-138d are peculiar: According to the IWF researchers’ simulations, it would be almost impossible for the planet to have a hydrogen shell: it would have been lost to space within ten million years. Since its age is estimated to be about one to 2.7 billion years, it cannot be a mini-Neptune or Hycean planet.
Comparing the exoplanet’s size and mass with simulation models, the team of astronomers concluded that a significant portion of its volume should consist of matter lighter than rock but heavier than hydrogen or helium: the most abundant of these possible substances would be water.
“Coupled modeling of the planetary interior and atmospheric structure supported the assumption that the Kepler-138d Earth-rocky core is most likely surrounded by a 2,000-kilometer-deep water ocean and a volatile atmosphere,” Fossati summarizes. The ocean on Kepler-138d would therefore be 500 times deeper than the average ocean on Earth.
Life on Kepler-138d rather unlikely
So-called oceanic planets have been hypothesized for some 20 years. Kepler-138d now appears to be the first exoplanet to actually fit the designation. “Due to the enormous amount of water, there is a good chance that ice or even water in another phase – a so-called supercritical fluid – will form under high pressure. That would prevent the emergence of life as we know it. That means that life would could be found on Kepler-138d rather unlikely, the researchers believe.
Source: Krone
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