Artist rendering of a water world exoplanet. NASA predicts that quite a few exist in the galaxies but none has been confirmed. Two new candidates have been put forward. (The Cosmic Companion)

Among the most intriguing types of exoplanet expected to be orbiting distant stars is the  “water world,” planets that are liquid to a far, far greater extent than on Earth.

Astronomers have theorized the existence of such planets and several candidates have been put forward, though not confirmed.  But the logic is strong enough for NASA scientists to conclude there are likely many of them in our galaxy.

Now two new potential water worlds have been proposed in a planetary system 218 light years away.

Using both the Hubble Space Telescope and data from the retired Spitzer Space Telescope, a team from Montreal has identified  the planets circling a red dwarf star.  Water, they propose, may well make up a significant portion of the planets.

Though the telescopes can’t directly observe the planets’ surfaces, other paths to identifying a water world are known.  By determining the planets’ densities through measurements of their weight and radii (and then volume), these planets — which would normally be described as “super-Earths because of their size — are lighter than rock worlds but heavier than gas-dominated ones.

The large international team, led by Caroline Piaulet of the Institute for Research on Exoplanets (iREx) at the University of Montreal, published a detailed study of the planetary system, known as Kepler-138, this month in the journal Nature Astronomy.

The results are quite surprising because most of the planets just slightly larger than Earth that have been studied in detail so far all seemed to be rocky worlds like ours. The closest comparison, say researchers, would be some of the icy moons in the outer solar system that are also largely composed of liquid water surrounding a rocky or icy core.

“Imagine larger versions of Europa or Enceladus, the water-rich moons orbiting Jupiter and Saturn, but brought much closer to their star,” explained Piaulet. “Instead of an icy surface, they would harbor large water-vapor envelopes.”

Researchers caution the planets may not have oceans like those on Earth directly at the planet’s surface.

“The temperature in Kepler-138d’s atmosphere is likely above the boiling point of water, and we expect a thick, dense atmosphere made of steam on this planet. Only, under that steam atmosphere there could potentially be liquid water at high pressure, or even water in another phase that occurs at high pressures, called a supercritical fluid,” Piaulet said.

In this illustration, exoplanet Kepler-138 d is in the foreground and Kepler-138 c is on the left. In the background, Kepler 138 b is passing in front of its parent star. The low densities of Kepler-138 c and Kepler-138 d indicate that they must be composed largely of water. (NASA, ESA, Leah Hustak (STScI)

The two possible water worlds, Kepler-138c and d, are not located in their sun’s the habitable zone, the area around a star where temperatures would allow liquid water on the surface of a rocky planet. But in the Hubble and Spitzer data, researchers additionally found evidence for a new planet in the system, Kepler-138e, in the habitable zone.

This newly found planet is small and farther from its star than the three others, taking 38 days to complete an orbit. The nature of this additional planet, however, remains an open question because it does not seem to transit its host star. Observing the exoplanet’s transit would have allowed astronomers to determine its size.

With Kepler-138e now in the picture, the masses of the previously known planets were measured again via the transit timing-variation method, which consists of tracking small variations in the precise moments of the planets’ transits in front of their star caused by the gravitational pull of other nearby planets.

The researchers had another surprise: They found that the two water worlds Kepler-138c and are “twin” planets, with virtually the same size and mass, while they were previously thought to be drastically different. The closer-in planet, Kepler-138b, on the other hand, is confirmed to be a small Mars-mass planet, one of the smallest exoplanets known to date.

“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 of these water worlds,” Benneke concluded.

An artist’s rendering of a hot and humid Hycean planet — a water world surrounded by a thick hydrogen atmosphere that has been proposed by researchers at the University of Cambridge . (Amanda Smith, University of Cambridge).

The question of whether a water (or ocean) world can support life is hotly debated.

On the one hand, NASA has long argued that the path to finding potential extraterrestrial life is to “follow the water.”  So a planet with vast amounts of water would appear to be a good candidate for habitability.  Life as we know it requires liquid water, a source of energy, and nutrients, and all three key requirements can potentially be satisfied within some of these bodies.  And fish, after all, live in a liquid environment.

A 2018 paper  in the Astrophysical Journal from researchers at the University of Chicago and Pennsylvania State University argued just that there is a decent chance that water worlds are indeed candidates for biology.  The group conducted more than 1,000 simulations of conditions on a wide variety of possible water worlds and found that conditions sometimes arose that could support life.

But others argue that a planet or moon needs some rocky land to be deemed habitable.

This is due not only to the inferred need for some dry land to form large molecules during the origin of life process, but also because the temperature of planets is regulated by cycles that need exposed minerals. Over long timescales, our planet cools itself by drawing down greenhouse gases into minerals and warms itself up by releasing them via volcanoes.

Also there is a potential phosphorus problem.  The element is essential the create the backbone of  life as we know it, and it become available through weathering — the process by which exposed elements and minerals are washed out of the rocks holding them.  This is how phosphorus became available on Earth.

And regarding Kepler 138c and -138d in particular,  Piaulet did caution that they might not have oceans of liquid water on their surface at all because they are so close to their sun.  Instead, they would have envelope of water vapor around them..

So water worlds are intriguing as a type of exoplanet and are avidly sought and researched as a result.  But many, many conditions have to be met before a particular water world might be considered habitable.