Hubble helps discover a new type of planet that is largely made of water

Researchers have found evidence of a new type of planet they’ve dubbed a “water world,” where water makes up a large portion of the entire planet. Discovered in a planetary system 218 light-years away, these worlds are unlike any planets in our solar system.

The team, led by Caroline Piaulet of the Institute for Exoplanet Research (iREx) at the University of Montreal, published a detailed study of a planetary system known as Kepler-138 in the journal Nature Astronomy on December 15.

Piaulet, a member of Bjorn Beneke’s research team at the University of Montreal, observed the exoplanets Kepler-138c and Kepler-138d using the NASA/ESA Hubble Space Telescope and NASA’s Spitzer Space Telescope. found that planets could be largely composed of water.

Artist’s illustration of the Kepler 138 planetary system

Water was not directly detected, but by comparing the sizes and masses of the planets with the models, they concluded that a significant part of their volume – even half of it – must consist of materials lighter than rock but heavier than hydrogen. or helium (which makes up the bulk of gas giant planets like Jupiter). The most common filter material is water.

“We previously thought that planets that were a little bigger than Earth were big balls of metal and rock, like spiraling versions of Earth, which is why we called them superplanets,” Beneke explained. “However, we have now shown that these two planets, Kepler-138 c and d, are very different in nature and that much of their entire volume is likely composed of water. This is the best evidence yet for water worlds, the type of planet that astronomers have hypothesized to exist. For a long time “.

With sizes more than three times the size of Earth and a mass more than twice as large, the density of planets C and D is much less than that of Earth. This is surprising because most of the planets slightly larger than Earth that have been studied in detail so far have all looked like rocky worlds like ours. The closest comparison, researchers say, would be some of the icy moons in the outer solar system that also consist largely of water surrounding a rocky core.

“Imagine larger versions of Europa or Enceladus, the water-rich moons that orbit Jupiter and Saturn, but come very close to their star,” Piaulet explained. “Instead of an ice surface, they contained large envelopes of water vapor.”

“The secure identification of an object as dense as the solar system’s icy moons, but much larger and larger, clearly demonstrates the great diversity of exoplanets,” added team member Jose Manuel Alminara from the University of Grenoble Alpes in France. “This is expected to be the result of a variety of formation and evolutionary processes.”

The researchers warn that the planets may not have oceans like those on Earth directly on the planet’s surface. The temperature in Kepler-138d’s atmosphere is probably above the boiling point of water, and we’d expect a thick, dense atmosphere made of steam on the planet. Only under this vaporous atmosphere can there be liquid water under high pressure, or even water in another phase that occurs at high pressures, called a supercritical liquid,” Piolet said.

The NASA/ESA/CSA James Webb Space Telescope will also facilitate valuable follow-up research. “Now that we have securely identified the ‘water world’ Kepler-138 d, the James Webb Space Telescope is the key to unveiling the atmospheric composition of such a strange object,” shares team member Daria Kupeshkina of the Austrian Academy of Sciences. “It will provide us with important information that enables us to compare the composition of the icy moons of the solar system with that of their larger, heavier counterparts outside the solar system.

Recently, another team at the University of Montreal found a planet called TOI-1452b that could be covered in an ocean of liquid water, but Webb would be needed to confirm that as well.

In 2014, data from NASA’s Kepler Space Telescope allowed astronomers to announce the discovery of three planets orbiting Kepler 138, a red dwarf star in the constellation Lyra. This was based on a measurable decrease in starlight as each planet momentarily passed in front of the star.

Benke and colleague Diana Dragomir, of the University of New Mexico, came up with the idea to re-observe the planetary system using the Hubble and Spitzer space telescopes between 2014 and 2016 to capture more transits of Kepler 138d, the third planet. In order to study the atmosphere.

Safe determination of an object as dense as the icy moons of the solar system, but larger and more massive, clearly shows the great variety of exoplanets, which are expected to be the result of a variety of processes of formation and evolution.
A new exoplanet in the system

While previous Kepler telescope observations had only shown three minor planets transiting around Kepler-138, Beaulet and her team were surprised to find that the Hubble and Spitzer observations required the presence of a fourth planet in the system, Kepler-138e.

This newly discovered planet is small and farther from its star than the other three, and it takes 38 days to complete an orbit. The planet is in its star’s habitable zone, a temperate zone where it receives just the right amount of heat from its cold star to be neither too hot nor too cold to allow liquid water to exist.

However, the nature of this newly discovered additional planet remains an open question because it does not appear to be transiting its host star. Observing the transit of an exoplanet has allowed astronomers to determine its size.

With Kepler-138 e now in the picture, the masses of previously known planets were again measured via the transit timing variance method, which involves tracking small differences in the exact moments a planet crosses in front of its star due to the gravitational pull of other nearby planets.
The researchers had another surprise: They found that the watery worlds Kepler-138 c and d are “twin” planets, roughly the same size and mass, when previously thought to be very different. On the other hand, the closest planet, Kepler-138 b, has been confirmed to be a small planet with the mass of Mars, and is one of the smallest exoplanets known to date.

“As our instruments and technologies become sensitive enough to find and study planets farther from their stars, we may start to find more of these water worlds,” Beneke concluded.

more information

The Hubble Space Telescope is an international collaboration project between the European Space Agency and NASA.

Link to Sciences Paper (PDF)

The study includes C. Piaulet (University of Montreal, Canada), B. Benneke (University of Montreal, Canada), JM Almenara (University of Grenoble Alpes, France), and D. Dragomir (University of New Mexico). , USA), HA Knutson (Caltech, USA), D. Thorngren (University of Montreal, Canada), MS Peterson (University of Montreal, Canada), IJM Crossfield (University of Kansas, USA), EM- R.. Kimpton (University of Maryland, USA), Dr. Kubishkina (Austrian Academy of Sciences, Austria), AW Howard (Caltech, USA), R. Angus (American Museum of Natural History, USA), and H. Isaacson (University of California – Berkeley, USA), LM Weiss (University of Notre Dame, USA), CA Beichman (Center for Infrared Analysis and Processing – Caltech, USA), JJ Fortney (University of California, USA), L. Fossati (Austrian Academy of Sciences, Austria), H. Lammer (Austrian Academy of Sciences, Austria), PR McCullough (Johns Hopkins University, USA; Space Telescope Science Institute, USA), C.V. Morley (University of Texas, USA) and I. Wong (MIT) Technology, USA; Fellow 51 Pegasi b).
Image credit: NASA, ESA, L. Hustak (STScI)

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