Scientists from the University of Montreal took a closer look at the planetary system of the star Kepler-138, which is located 218 light-years from Earth. The researchers used observations made by Cosmic Telescope Hubble and the now defunct Spitzer Space Telescope. Your results prac described in the journal Nature Astronomy.
Two aquatic worlds far from everything we know
Research shows that Kepler-138 c and Kepler-138 d, or two – previously discovered by the Kepler Space Telescope – exoplanets are made up of more than half of water. While not directly discovered, data on the size, mass, and density of these objects suggest so.
Astronomers were able to determine that the two planets are twins that are similar to each other, but significantly different from terrestrial planets (such as Earth). Kepler-138c and Kepler-138d have twice the mass of our planet, but twice the density. The researchers compared the data with the models and concluded that planets can be neither rocky nor gaseous.
Half, or a little over half, of the building material of both planets is something lighter than rock but heavier than hydrogen and helium (these two elements are the main building blocks of gas giants). By far the most likely candidate in this case is water.
Giant ocean and clouds of steam
The researchers suggest that Kepler-138c and Kepler-138d likely cover the global ocean with water, and that the planets are also surrounded by an atmosphere of water vapor. As he writes in his press release NASAobjects “like no planet in our solar system” and are the best candidates for representatives of oceanic planets (or so-called water worlds), the existence of which has long been predicted.
Two planets made mostly of water have been discovered. “I’m like nothing in the solar system” photo. Benoit Gougeon (University of Montreal
However, there are many indications that the water on these planets is not like that in Earth’s seas. Kepler-138 cid orbits their star at short distances, and the prevailing temperatures are much higher than on Earth, exceeding the boiling point of water. Probably most of the water on exoplanets is active border liquid and gaseous state or in the supercritical state (in which the boundary between gas and liquid disappears). Only below can there be liquid water under enormous pressure.
As the researchers calculated, Kepler-138d has a metallic rock “core” that is just under half the exoplanet’s volume. Above it extends a global “ocean” of water and gas with a depth of about 2,000 meters. km (for comparison, the average depth of the oceans on Earth is “only” 4 km). Above is a layer of gaseous atmosphere composed mostly of water vapour.
The temperature in Kepler-138d’s atmosphere is likely higher than the boiling point of water, and we expect a thick, water vapor-dense atmosphere on this planet. Only under this gaseous atmosphere can there be potentially high pressure liquid water and even water in another high pressure phase, called the supercritical state
– explains Caroline Piaulet of the University of Montreal, who led the research.
A brand new exoplanet has been found in the habitable zone
This is not the end of the discoveries made by the Montreal team of researchers. Only three planets orbiting the star Kepler-138 have been known so far. In addition to the Kepler-138 c and c planets, there was also a planet b, which – as has now been confirmed – has the mass of Mars and is one of the smallest exoplanets known to us. However, all three objects are too close to their star for life to exist.
Now, however, scientists have found evidence of the fourth planet of this system – Kepler-138 E. It is located farthest from the star – in the so-called habitable zone, where liquid water is found and there are conditions favorable for the development of life. Unfortunately this planet cannot be observed with the transit method, i.e. while passing in front of the star (this is the most common method for discovering and studying exoplanets).
The scientists were only able to determine that Kepler-138e is a small rocky planet that orbits its star every 38 days. More observations are needed to find out subject nothing more.
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