Water Beyond Earth: Known Planets and Stars Where Liquid Water Shapes Habitability

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🌍 Exoplanets With Detected Water

K2‑18b: A Water Vapor Pioneer

K2‑18b is the first super‑Earth in the habitable zone with confirmed water vapor in its atmosphere, discovered by University College London and confirmed by Hubble observations.
Read more on NASA.


TRAPPIST‑1e: Earth-like Density in the Habitable Zone

Climate models suggest TRAPPIST‑1e may retain several Earth-ocean equivalents of water. It is one of the most Earth-like exoplanets in terms of density and orbital position.
More on Wikipedia.

Kepler‑138c & d: True “Water Worlds”

Kepler‑138c and d may contain over 50% water by volume – the strongest evidence to date for exoplanets dominated by water.
Source: CNN Science.

LHS 1140 b: An Ocean Planet within 50 Light‑Years

This planet is potentially composed of 10–20% water and has an atmosphere with temperatures near 20 °C. JWST findings point to a rich, nitrogen-based environment.
See more: Forbes Science.

💧 Water in Protoplanetary Disks and Stars

In young stars like DG Tau and PDS 70, astronomers have detected water vapor in planet-forming zones—suggesting future planets may form with abundant water.
Sources: NASA Goddard.

🌌 Why Water Matters More Than Ever

  • Climate Regulation: Without liquid water, CO₂ can build up and overheat the atmosphere.
  • Heat Management: Water moderates planetary temperatures and supports weather systems.
  • Solvent for Life: It enables biochemical reactions necessary for life.

🎓 Universities and Experts

  • Tokyo Institute of Technology: Discovered K2‑155d with Earth-like surface temperatures.
  • University of Montreal: Leading research on Kepler-138 and LHS 1140 b.

💬 Expert Quotes

“It is the best evidence yet for water worlds…” — Björn Benneke on Kepler‑138 planets
“LHS 1140 b could well be our best bet… to one day indirectly confirm liquid water on the surface of an alien world” — Charles Cadieux

📊 Summary Table

Planet Distance Water Status Habitability Potential
K2‑18 b ~110 ly Water vapor, ocean likely Hycean world, promising
TRAPPIST‑1 e ~40 ly Liquid water possible Top Earth-like candidate
Kepler‑138 c/d ~218 ly >50% water composition Likely water-dominated worlds
LHS 1140 b ~49 ly Oceanic, with atmosphere Excellent future target

🔗 Sources

  • NASA Exoplanet Archive
  • CNN Space
  • JPL NASA
  • Hycean Planets - Wikipedia

Published on July 23, 2025 | Original content for Natural World 50

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