Astronomers Find a Waterworld Planet With Deep Oceans in the Habitable Zone

An exoplanet only 100 light-years from Earth appears to be the best candidate yet for a sloshy, water-covered ocean world.

It’s known as TOI-1452b, and measurements of its size and mass indicate a density profile consistent with a global liquid ocean. Scientists believe that such worlds exist, but have yet to find one conclusively.

We’ll need to wait for James Webb Space Telescope observations to study the exoplanet’s atmosphere and make a more confident determination about the nature of TOI-1452b, but the initial reports are very intriguing.

“This paper reports the discovery and characterization of the transiting temperate exoplanet TOI-1452b,” writes a team of researchers led by astronomer Charles Cadieux of the University of Montreal in Canada in a paper published in The Astronomical Journal.

“The results of our interior modeling and the fact that the planet receives modest irradiation make TOI-1452b a good candidate water world.”

It’s easy to see why TOI-1452b evaded detection until now, in spite of its relative closeness to the Solar System.

The exoplanet was discovered orbiting one of a pair of small, dim red dwarfs separated by only 97 astronomical units. The stars are so close together that they appear to be one.

The TESS exoplanet-hunting telescope, on the other hand, is sensitive enough to detect regular, faint dips in starlight that indicate an object passing between us and its host star on a regular basis; these passages across the star are known as transits. The researchers then built their own highly sensitive instrument to detect exoplanet transits at Canada’s Mont Mégantic Observatory.

Both telescopes’ observations confirmed the presence of an exoplanet orbiting one of the stars in the TOI-1452 binary.

By looking at how much light the star emits

and how much it dims when the exoplanet passes in front, the researchers were able to ascertain that the exoplanet is relatively small, coming in at 1.672 times the size of Earth – what we call a super-Earth.

It’s on an 11-day orbit with its star, which seems insanely close to us, with our comparatively leisurely 365-day orbit. However, because the star is so cool and dim compared to the Sun, this places the exoplanet bang in the middle of the star’s temperate zone. Both telescopes’ observations confirmed the presence of an exoplanet orbiting one of the stars in the TOI-1452 binary. This is not so far away from the star that any liquid water on its surface would freeze, nor is it so close that the water would evaporate due to the star’s heat.

Waterworld Planet
An artistic impression of potential water world TOI-1452b. (Benoit Gougeon/Université de Montréal)

The researchers then examined the star more closely. They focused on its radial velocity, or how it moves due to the gravitational influence of the exoplanet. Because any two bodies in a system orbit a mutual center of gravity, the star moves around slightly in its orbital arrangement with TOI-1452b.

Changes in the star’s light reveal this motion and, even better, allow astronomers to calculate the mass of the orbiting body by measuring the strength of that movement. As a result, they calculated TOI-1452b’s mass to be 4.82 times that of Earth.

And this is where things start to get interesting.

When you know the size and mass of an object, you can calculate its average density. For TOI-1452b, that density is 5.6 grams per cubic centimeter, and that’s very close to Earth’s density of 5.5 grams per cubic centimeter. However, a density similar to that of Earth for a larger object indicates that the object is made of a lighter material, according to the researchers.

“TOI-1452b is one of the best candidates for an ocean planet that we have found to date,” Cadieux says. “Its radius and mass suggest a much lower density than what one would expect for a planet that is basically made up of metal and rock, like Earth.”

They modeled the exoplanet’s interior composition and determined that as much as 30 percent of its mass could be water.

That’s a lot of wet stuff. Water makes up less than 1% of Earth’s mass; TOI-1452b’s composition appears to be closer to that of Jupiter’s water moon Europa and Saturn’s water moon Enceladus.

However, based on the measurements we have, we cannot say for certain what TOI-1452b is made of. This is where Webb comes in.

Remember how the exoplanet passes between us and its star? If the exoplanet has an atmosphere, some of the star’s light will pass through it. Webb is sensitive enough to detect the difference in that light in sufficient detail for scientists to detect what is in the exoplanet’s atmosphere.

If TOI-1452b is a water world, Webb has the best chance of finding it.

“Our observations with the Webb Telescope will be essential to better understanding TOI-1452b,” says astronomer René Doyon of the University of Montreal. “As soon as we can, we will book time on Webb to observe this strange and wonderful world.”

The research has been published in The Astronomical Journal.

READ MORE: Found: The first exoplanet outside of our Milky Way

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