There can be as many water worlds as Earth in the Milky Way

On July 12, 2022, NASA released the first images it obtained James Webb Space Telescope, which were taken during the first six months of its operation. Among its many scientific goals, web It will look for smaller, rocky planets that orbit near their suns — particularly faint M-type stars (red dwarfs), which are the most common in the universe. This will help astronomers complete the census of exoplanets and gain a better understanding of the types of worlds out there. In particular, astronomers are curious to know how many terrestrial planets in our galaxy are actually “water worlds.”

These are rocky planets larger than Earth but with a lower density, which indicates that volatiles such as water make up a significant amount (up to half) of their mass. According to a recent study by researchers from the University of Chicago and the Astronomical Institute of the Canary Islands (IAC), water worlds may be as common as rocky “Earth-like” planets. These findings support the case for icy moon-like exoplanets in the Solar System (such as Europa) and could have important implications for future exoplanet studies and the search for life in our universe.

The study was led by researchers Raphael Luc, of the University of Chicago and the Astronomical Institute of Andalusia (IAA-CSIC) and Enrique Ballet, of the IAC and University of Laguna (ULL). Their findings, titled “Density, Not Radius, Separating Rocky, Water-rich Minor Planets Orbiting M Dwarf Stars,” appeared recently in the journal Sciences. For their study, Luque and Pallé analyzed the masses and radii of all 43 rocky exoplanets in the catalog of exoplanets orbiting M-type stars, which account for about 80% of the stars in the Milky Way.

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Average density distribution of planets around M stars, where different types of planets (rocky planets, water worlds, and miniature Neptunes) can be clearly distinguished. credit: IAC

This consists of combining transit data obtained by NASA Transiting a satellite to survey the outer planets (TESS) radial velocity measurements by the Carmen spectrometer on the 3.5-meter telescope at the Calar Alto Observatory in southern Spain. This allowed them to obtain accurate measurements of the sizes and masses of these planets, with which it is possible to limit the fraction of their mass (density). Their analysis found that a significant portion of the density was too light for its volume to consist entirely of rock.

For this reason, they concluded that these exoplanets should consist of half rock and half water or lighter particles (such as methane, ammonia and other volatile elements). As Luque explained in a recent IAC press release:

“We have discovered the first experimental evidence that there is a group of water worlds, and that they are in fact almost as abundant as Earth-like planets. We find that it is the density of a planet rather than its radius, as previously thought, that separates the dry planets from the wet planets. Earth is a dry planet, on the Although its surface is mostly covered with water, which gives it a very wet appearance. Water on Earth accounts for only 0.02% of its total mass, while in these watery worlds it represents 50% of the planet’s mass.”

However, planets orbiting M-type stars usually orbit so closely that they are gradually locked, with one of their sides constantly facing the Sun. At this distance, any water on the planet’s surface would likely exist in a supercritical gas phase, which increases their volumes. As a result, Locke and Ballet postulated that in this population, water is bound to rocks or in closed volumes below the surface, and not in the form of oceans, lakes, and rivers at the surface. These conditions are similar to what scientists have observed with icy moons in the outer solar system, such as Jupiter’s moon Europa and Saturn’s moon Titan.

An artistic impression of an aquatic world where half its mass is made up of water. Just like the Moon, a planet is connected to its star by tidal forces and always shows the same face to its host star. Credit: Pilar Montañez

As these planets are progressively confined to their suns, these planets may also have liquid oceans on their sun-facing side, but they have frozen surfaces everywhere else—colloquially known as “eyeball planets.” While astronomers have speculated about the existence of this class of exoplanets, these findings constitute the first confirmation of this new type of exoplanet. It also supports the growing state of water worlds that form outside the so-called “snow line” of stellar systems (the boundary beyond which volatile elements freeze solid), and then migrate near their star. Happy Ballet:

“We have discovered that small planets orbiting this type of star can be classified into three distinct groups: rocky planets very similar to Earth, planets half their masses made of water that we call water worlds, and small Neptunes with extended atmospheres of hydrogen and/or helium. The distribution of exoplanet sizes and densities is a consequence of the formation of planets at different distances from the star, not the result of the presence or absence of an atmosphere.”

“Because of the uncertainties in the masses and radii of our measurements, an individual planet can sometimes fit into more than one category (Earth, Water Realm, etc.)” added Locky. “When we observe a group of planets, as we have done here, we can highlight different patterns and different structures.”

According to the researchers, the next step will be to learn more about the internal structure of the water worlds. This entails knowing where the water is stored and whether these planets have weak atmospheres of detectable supercritical water vapor. This is similar to what astronomers have observed with Europa and other icy moons, which also contain weak atmospheres of water vapor and oxygen. The latter gas is produced by photolysis, where exposure to solar radiation causes water molecules to break down into oxygen and hydrogen (the latter is lost to space).

Artist’s impressions of two exoplanets in the TRAPPIST-1 system (TRAPPIST-1d & TRAPPIST-1f). Credit: NASA/JPL-Caltech

The James Webb Space Telescope (JWST) is ideally suited for conducting these surveys, thanks to its advanced range of infrared imagers and spectrometers. Several ground-based observatories such as the Very Large Telescope (ELT), the Giant Magellan Telescope (GMT), and the Thirty Meter Telescope (TMT) will be able to image these exoplanets directly with the help of their advanced spectrometers. These studies will obtain spectra from distant exoplanets and allow astronomers to characterize their atmosphere and surface properties like never before. As Loki emphasized:

“It is also essential to understand whether our finding also applies to groups of small planets around other types of stars. It is difficult to measure the exact masses of small planets around larger stars, but data will soon become available using the latest generation of ultra-stable spectrographs.”

Future surveys of rocky planets around M-type stars will include the nearest exoplanet outside the solar system. This is none other than Proxima b, a rocky planet located only 4.25 light-years away in the neighboring system of Proxima Centauri. Since its confirmation in 2016, scientists have sought to learn more about its composition to measure its potential habitability. In addition, TRAPPIST-1’s seven rocky planetary system will also be of interest, as scientists have speculated that some of these may be “dry eyeball” and “wet eyeball” planets.

The coming years are expected to be a time of profound discovery as the number of exoplanets reaches tens of thousands! With so many planets available to study, the search for life outside the solar system is also expected to accelerate (and perhaps even provide the first evidence of that!)

Further reading: Instituto de Astrofísica de CanariasAnd the Sciences

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