A newly discovered half-water, half-rock planet is straight out of science fiction

Since the 1990s, scientists have cataloged thousands of planets outside our solar system, called exoplanets. Some of them are massive and gaseous, while others are tiny and rocky like our homeworld. But recent analysis suggests some of these exoplanets could be denser and hold more water than previously thought, with big implications for extraterrestrial life.

There are four main types of exoplanets: Neptunian, gas giant, super-Earth and terrestrial. It is not easy to spot these planets directly, let alone understand what they are made of. One of the most proven methods of hunting exoplanets is called transit photometry, which basically involves pointing a telescope at a star and measuring the light as a planet passes by. A drop in brightness indicates that a planet is there.

But two astronomers, Rafael Luque of the University of Chicago and Enric Pallé of the Universidad de La Laguna in Spain, wanted to find the density of certain exoplanets. When they took a closer look at some of this transit data, they discovered something was wrong.

By analyzing data from a directory of 34 planets, Luque and Pallé discovered that some planets contained more water than previously thought. The new analysis would make these planets about 50% water and 50% rock, which would constitute a new class of exoplanets. In contrast, the Earth is almost entirely made of rock and contains far less than 1% water overall, even though its surface is covered with a large amount of water. Scientists believe that water is essential for life as we know it.

“It was a surprise to see evidence of so many water worlds orbiting the most common type of star in the galaxy,” Luque, the paper’s lead author, said in a statement. “This has huge implications for the search for habitable planets.” Their results were published in the journal Science.

The idea of ​​such a heavily waterlogged world was once the stuff of science fiction. A 1998 episode of “Star Trek: Voyager” featured the explorers visiting a planet composed entirely of water, resembling a spherical sea.

To make the discovery, Luque and Pallé looked at small planets transiting around red dwarf M stars — an extremely common type of star in the visible universe, but much smaller and cooler than our Sun. It is believed that when the planets around M dwarfs first form, they start out as spinning disks of dust and gas. Slowly they take on the marble shape we all know, but first form shields of hydrogen and helium called envelopes.

The husks slowly decay over time, eventually drifting through space. When estimating the mass and orbit of exoplanets, astronomers must take these envelopes into account. But when Luque and Pallé recalculated some of these measurements, they discovered that these envelopes probably don’t exist for some of these planets. Instead, they’re probably part rock and part water.


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However, these planets probably do not have oceans. They are too close to their stars, which means any surface water would immediately evaporate and transition to a supercritical gas phase – essentially, a super hot fluid in which the distinct liquid and gas phases do not exist. This would expand their radius as they swing around their sun.

“But we don’t see that in the samples,” Luque explained. “This suggests that the water is not in the form of a surface ocean.”

Instead, it is likely that water is embedded in the rock of these planets in equal proportions, possibly forming pockets below the surface. This may sound familiar: Europa, a moon of Jupiter, is believed to contain liquid water underground and, therefore, may support life. NASA hopes to send a probe to Europa in 2024 to search for signs of extraterrestrial life.

There are perhaps billions of planets in the universe. According to the Encyclopedia of Extrasolar Planets, 5172 exoplanets have been discovered so far, occupying 3816 planetary systems. Due to their relatively smaller size and the fact that, unlike stars, they generally do not generate their own light, planets are much harder to spot with telescopes than stars – which are estimated to be 200 miles away. billion billion, or 200 sextillion.

Compared to the large number of stars observed, 5172 exoplanets is a much smaller sample. But it continues to grow and new discoveries on these other worlds are regularly announced. Diamond rain can be quite common on some exoplanets, for example, while two Earth-like exoplanets were recently discovered 105 light-years away.

Do any of these exoplanets harbor extraterrestrial life? So far, scientists have no idea. But tools like the James Webb Space Telescope will make it easier to discover what exoplanets are really made of and whether they harbor at least the right materials to make life possible. So far, there are plenty of good candidates, but this new study could greatly add to that growing list.

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