Mysterious exoplanets: perhaps the first “water worlds” ever discovered in space

Mysterious exoplanets
The first “water worlds” may have been discovered in space

12/22/2022 at 4:16 PM

At a distance of 218 light-years, a research team has discovered what is believed to be the first exoplanets ever to be completely covered in water. These are slightly larger than Earth, but are believed to be covered by vast oceans. Against these, terrestrial seas may act like ponds.

The more planets outside our solar system are discovered, the more amazed researchers become. Orbiting the red dwarf star Kepler-138, 218 light-years away, are two other unusual species that may be the first “water worlds” ever discovered in deep space. At least, this is the conclusion reached by an international research team after extensive observations of the two celestial bodies. Water takes up about 50 percent of the planets’ volume, which is about 11 percent of the mass, the scientists wrote in the journal. “Natural Astronomy”.

The two planets were discovered in 2014 using the Kepler space telescope. When seen from Earth, it passes in front of its star regularly – every 14 days and every 23 days – and thus dims its brightness slightly. Using these transits, researchers can determine the orbital period and size of the planets. In order to determine its mass, they must also measure how much the planets “vibrate” with the force of their gravity on their central star.

Caroline Piaulet of the University of Montreal in Canada and her team have been observing Kepler-138 for several years with the Hubble and Spitzer space telescopes and with the Large Keck Reflector Telescope in Hawaii. They succeeded in determining the sizes and masses of the planets Kepler-138c and d, which were the most accurate to date. Each is 1.5 Earths in diameter, and these are called super-Earths.

Many of these planets are already known to astronomy, and are larger versions of the rocky inner planets of our solar system. But what surprised the research team was the mass of the planets, which are only 2.1 and 2.3 times that of Earth. If the two planets were composed almost entirely of rocks like Earth, they would be expected to be about 3.4 Earth masses.

A bowl of water 2000 km thick

So much of these planets must be made of lighter-than-rock material—and Piaollet and her colleagues conclude that water is the best candidate for this, based on modeling of planetary structure. Accordingly, Kepler 138c and d has an iron core and a mantle of rocks, similar to Earth’s. But this is surrounded by a shell of water about 2,000 kilometers thick. For comparison: the average depth of the oceans on Earth is only 4 km.

“It’s the first time we’ve seen planets that we can safely call water worlds,” Piaulet says. However, it is likely not a deep ocean similar to the terrestrial seas. “The temperature on this planet is higher than the boiling point of water,” says Piaulet. “So we would expect a dense atmosphere of water vapor.” But the pressure eventually increases so far down that water can melt even at these temperatures. The researcher continues: “It is possible that the water is supercritical as well.” Then water will be as dense as a liquid, but will have the same mobility as gas.

The two superplanets aren’t the only Kepler-138 planets. The first observations with the Kepler telescope have already shown another celestial body about half the size of Earth in a very close orbit around the star. Beaulet and her team discovered another planet about half the mass of Earth in distant orbit. However, because the planet Kepler-138e does not pass in front of its star as seen from Earth, its size and thus composition remains unknown. This planet is of interest to researchers because it is located in the habitable zone: its surface temperature is a comfortable 20 degrees Celsius.