There Could be as Many Water Worlds as Earths in the Milky Way

On July 12th, 2022, NASA released the first images acquired by the James Webb Space Telescope, which were taken during its first six months of operation. Among its many scientific objectives, Webb will search for smaller, rocky planets that orbit closer to their suns – especially dimmer M-type (red dwarf) stars, 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 that exist out there. In particular, astronomers are curious about how many terrestrial planets in our galaxy are actually “water worlds.”

These are rocky planets that are larger than Earth but have a lower density, which suggests that volatiles like water make up a significant amount (up to half) of their mass-fraction. According to a recent study by researchers from the University of Chicago and the Instituto de Astrofísica de Canarias (IAC), water worlds may be just as common as “Earth-like” rocky planets. These findings bolster the case for exoplanets that are similar to icy moons in the Solar System (like Europa) and could have significant implications for future exoplanet studies and the search for life in our Universe.

The study was led by the researchers Rafael Luque, of the University of Chicago and the Instituto de Astrofísica de Andalucía (IAA-CSIC) and Enric Pallé, of the IAC and the University of La Laguna (ULL). Their results, titled “Density, not radius, separates rocky and water-rich small planets orbiting M dwarf stars,” recently appeared in the journal Science. For the sake of their study, Luque and Pallé analyzed the masses and radii of all 43 rocky exoplanets in the exoplanet catalog that orbit M-type stars, which account for about 80% of stars in the Milky Way.

The distribution of the mean densities of the planets around M stars, in which the different types of planets (rocky planets, water-worlds, and mini-Neptunes) can be clearly distinguished. Credit: IAC

This consisted of combining transit data obtained by NASA’s Transiting Exoplanet Survey Satellite (TESS) with radial velocity measurements by the CARMENES spectrograph on the 3.5 m telescope at the Calar Alto Observatory in southern Spain. This allowed them to obtain accurate measurements of the sizes and masses of these planets, from which their mass-fraction (densities) could be constrained. Their analysis found that a significant portion had densities that were too light in relation to their size to be composed entirely of rock.

For this reason, they deduced that these exoplanets must be composed of half rock and half water or other lighter molecules (such as methane, ammonia, and other volatile elements). As Luque explained in a recent IAC press release:

“We have discovered the first experimental proof that there is a population of water worlds, and that they are in fact almost as abundant as Earth-like planets. We found that it is the density of a planet and not its radius, as was previously thought, which separates dry planets from wet ones. The Earth is a dry planet, even though its surface is mostly covered in water, which gives it a very wet appearance. The water on Earth is only 0.02% of its total mass, while in these water worlds it is 50% of the mass of the planet.”

However, planets around M-type stars typically orbit so closely that they are tidally locked, where one side is constantly facing toward its sun. At this distance, any water on the planet’s surface would likely exist in a supercritical gas phase, increasing their sizes. As a result, Luque and Pallé theorized that in this population, water is bound to the rock or in closed volumes below the surface, not in the form of oceans, lakes, and rivers on 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.

Artist’s impression of a water world, where half of its mass consists of water. Just like the Moon, the planet is bound to its star by tidal forces and always shows the same face to its host star. Credit: Pilar Montañés

Given that they are tidally locked to their suns, these planets may also have liquid oceans on their sun-facing side but frozen surfaces everywhere else – colloquially known as “eyeball planets.” While astronomers have speculated about the existence of this class of exoplanet, these findings constitute the first confirmation for this new type of exoplanet. They also bolster the growing case for water worlds that form beyond the so-called “snow line” in star systems (the boundary beyond which volatile elements freeze solid), then migrate closer to their star. Said Pallé:

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

“Because of the uncertainties in the masses and radii of our measurements, an individual planet could sometimes fit into more than one category (terrestrial, water world, etc.),” added Luque. “It is when we observe a population of planets as we have done here that we can bring out the patterns of distinct, different compositions.”

According to the researchers, the next step will be to learn more about the internal structure of water worlds. This entails finding out where the water is stored and if these planets have tenuous atmospheres of detectable supercritical water vapor. This is similar to what astronomers have observed with Europa and other icy moons, which also has tenuous atmospheres of water vapor and oxygen. This latter gas results from photolysis, where exposure to solar radiation causes water molecules to break into oxygen and hydrogen (the latter of which 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 to conduct these surveys, thanks to its advanced suite of infrared imagers and spectrometers. Several ground-based observatories like the Extremely Large Telescope (ELT), the Giant Magellan Telescope (GMT), and the Thirty Meter Telescope (TMT) will be able to directly image these exoplanets with the help of their coronographs and advanced spectrometers. These studies will obtain spectra from distant exoplanets and allow astronomers to characterize their atmospheres and surface features like never before. As Luque stressed:

“It is also fundamental to understand if our discovery also applies to the populations of small planets around other types of stars. It is more difficult to measure the exact masses of small planets around larger stars, but the data should soon become available using the newest generation of ultra-stable spectrographs.”

Future surveys of rocky planets around M-type stars will include the closest exoplanet beyond the Solar System. This is none other than Proxima b, a rocky planet located just 4.25 light-years away in the neighboring system of Proxima Centauri. Since it was confirmed in 2016, scientists have sought to learn more about its composition to gauge its potential habitability. In addition, the seven-rocky-planet system of TRAPPIST-1 will also be of interest, as scientists have speculated that some of these planets may be “dry eyeball” and “wet eyeball” planets.

The coming years are expected to be a time of profound discovery as the census of exoplanets reaches the tens of thousands! With so many planets available for study, the search for life beyond the Solar System is also expected to accelerate (and maybe even provide the first evidence of it!)

Further Reading: Instituto de Astrofísica de Canarias, Science