Astronomers generally divide the planets in our solar system into two types: rocky worlds and gas giants. But according to a new study of planets in other galaxies in our galaxy, there is a third kind of world, made up of about 50 percent water and 50 percent rock. And such a water-rich world is an exciting place for astronomers to test their hypotheses about what enables a planet to support life.
“In our lifetime, we may be able to say something scientifically proven for the first time about habitability on other planets,” said Rafael Luque, a postdoctoral researcher at the University of Chicago who is the lead author of the new study published Thursday. in the news Science. “And that’s a big, big step.”
In recent years, astronomers have been rapidly discovering new planets orbiting stars outside our own, called exoplanets. More than 5,000 exoplanets have been discovered and confirmed to date. But figuring out exactly what those worlds look like — and thus whether or not they’re habitable — from light-years away is a difficult feat.
Most exoplanets have been discovered using the so-called transit method, which indirectly identifies a planet by observing how the star’s light dims slightly as the planet passes in front of it. Astronomers can also infer an exoplanet’s radius from how much starlight it blocks. Scientists have used that information to compare these alien worlds to the planets in our own solar system as a way of imagining what they might look like. A planet with the same radius as Earth, for example, is considered quite rocky.
But in orbit around many red dwarf stars, which are by far the most common stars in our galaxy, there is a kind of planet that has no analogue in our solar system. Based on their radii, these worlds fit the size gap between Earth and Neptune.
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The thought among astronomers has long been that those minor planets fall into two categories: some were thought to be “super-Earth” and others as “mini-Neptune.” This idea was reinforced by the observation of a shortage of exoplanets with a radius about 1.6 times that of Earth, which is called a “ray valley,” explains Ravi Kopparapu, a planetary scientist at NASA’s Goddard Space Flight Center. who was not involved in the new study. The way a star’s radiation erodes a planet’s atmosphere, he says, would explain that hole in the rays.
By that logic, “super-Earths,” which were on the smaller side of that jet valley, were left with a very thin atmosphere and a largely exposed rocky surface. “Mini-Neptunes”, on the other hand, had retained thick, swollen atmospheres and therefore these gaseous planets had larger radii.
But there may be other ways to build an exoplanet to have those rays. Since they have no analogues in our solar system, these worlds could really be alien. So to find out what materials these distant planets might consist of, Luque and his collaborator Enric Pallé set out to determine their density.
Density isn’t something that can be measured directly from that far away, but with a planet’s mass and radius, it’s a simple calculation (mass divided by volume equals density). The researchers used the radius and mass measurements of 34 newly discovered planets by the Transiting Exoplanet Survey Satellite (TESS), which launched in 2018, to collect a sample of densities for these mysterious small exoplanets.
[Related: We may be underestimating how many cold, giant planets are habitable]
Based on their calculations, the jet valley is not what separates the different types of planets orbiting red dwarf stars. It’s density. And they extrapolated that those exoplanets could be one of three types of worlds: rocky, gaseous or, the new type, wetland.
“We can think of Earth as a water-rich planet, but the water on Earth is only 0.02 percent of its total weight,” says Luque. The density of these distant water worlds, meanwhile, indicates that about half of their mass consists of water.
But don’t start imagining a world with a rocky core and a deep ocean of water sloshing on it, exposed to space, Luque says. “What we saw in our sample is that this water can’t be on the surface,” he says. “The water may be trapped below the surface or maybe mixed with the magma, but it won’t be in the form of deep, deep oceans — at least not on the surface.”
The closest analogs we have in our own solar system with such water-rich worlds are the moons of Jupiter and Saturn. For example, Europa, one of Jupiter’s moons, has a deep ocean sloshing beneath a global water ice shell.
These exoplanets are unlikely to have a water ice shell, Luque says, because being much closer to their star — all the water on the surface would evaporate. At least that’s on the sun-facing side of the planet. These worlds do not rotate on their axis to have a day and night cycle like the Earth does. Instead, there is a permanent light and dark side. But, Luque says, maybe there’s an area where the light and dark sides meet, in a sort of perpetual twilight, where the surface temperature can be just right to keep liquid water stable.
When looking for habitable worlds, astronomers usually use liquid water as a guideline. That’s because it’s essential to life as we know it (that is, life on Earth, because it’s the only life we know so far).
“We only have one template of life in this universe, so we use that as a template to find life elsewhere,” Kopparapu says. But stable liquid water isn’t all it takes to make a place habitable by that definition, and just because a place is capable of supporting life doesn’t mean anything lives there, he adds.
To investigate the habitability of these distant worlds, astronomers will turn to tools such as the recently launched James Webb Space Telescope (JWST), which can look into the chemistry of exoplanet atmospheres to reveal more details about their composition. With telescopes like JWST, astronomers will look for water vapor to confirm the presence of H2O, as well as gases such as methane, oxygen, carbon dioxide, nitrogen and more found in Earth’s atmosphere.
“We are finding more and more evidence that there are many potentially habitable worlds. Our earth is not unique’, says Kopparapu. He uses an analogy: “When you move to a new neighborhood and you want to introduce yourself to your neighbors, you may see a lot of houses, but not a lot of people. So we find a lot of houses. Now we just need to find people.”