Astronomers discover an exoplanet that could be an 'ocean world'

The exoplanet has a water-rich envelope, a gas-rich envelope, or a mixture of both.
Sejal Sharma
Representative image of an exoplanet
Representative image of an exoplanet

Darryl Fonseka/iStock 

In 1990, human knowledge of exoplanets was limited to those revolving around the sun. But fast forward 33 years, and we now know that billions of ‘candidate’ exoplanets exist in our galaxy, the Milky Way, alone. And of these ‘candidate’ exoplanets, we have over 5,000 ‘confirmed’ exoplanets.

Astronomers and scientists just added another one to that list of ‘confirmed’ exoplanets.

A study published by a team of researchers has discovered a mini-Neptune exoplanet (less massive but resembling Neptune) - HD 207496b - orbiting around a young, bright K dwarf (stars which are smaller and redder than the sun) called HD-207496.

What is exoplanet HD 207496b?

The study found that HD 207496b has a density lower than Earth, indicating that it’s not entirely rocky and the celestial body could have a significant amount of water or gas, or both, in its composition.

To find whether the exoplanet contains gas or water (or both), the researchers explored the possible internal structure of HD 207496b assuming the possibilities of these two different compositions. 

One with a core and H/He (Hydrogen & Helium) envelope, meaning it could be a gaseous planet. And the second, whose atmosphere would have evaporated, is composed of a core, mantle, and a water layer, meaning an ocean planet.

Evaporation modeling revealed that if the exoplanet has a gas-rich atmosphere of hydrogen and helium, that state is temporary: Its star (HD-207496) will strip the exoplanet completely within 520 million years. It's also possible that the atmosphere has already gone, and HD 207496b is already a bare-ocean world.

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The study concluded that further observations of HD 207496b would allow researchers to distinguish between these two hypotheses. The researchers also note that an in-depth characterization of young exoplanets can give us insight into the physical processes that shape planetary systems.

The researchers obtained observations from High Accuracy Radial Velocity Planet Searcher (HARPS) on the European Southern Observatory's 3.6-meter telescope at La Silla Observatory in Chile and high-resolution imaging, which they combined with the Transit Exoplanet Survey Satellite (TESS) at the NASA Ames Research Center in California, United States.

Not long back, astronomers discovered twin exoplanets that could be water worlds - meaning a significant fraction of their mass is composed of water.

Study abstract:

Aims: We report the discovery and characterisation of the transiting mini-Neptune HD 207496 b (TOI-1099) as part of a large programme that aims to characterise naked core planets. Methods: We obtained HARPS spectroscopic observations, one ground-based transit, and high-resolution imaging which we combined with the TESS photometry to confirm and characterise the TESS candidate and its host star. Results: The host star is an active early K dwarf with a mass of 0.80 ̆ 0.04 Md , 0.769 ̆ 0.026 Rd radius, and a G magnitude of 8. We found that the host star is young, „ 0.52 Myr, allowing us to gain insight into planetary evolution. We derived a planetary mass of 6.1 ̆ 1.6 MC , a planetary radius of 2.25 ̆ 0.12 RC, and a planetary density of ρp “ 3.27`0.97 ́3. Conclusions: From internal structure modelling of the planet, we conclude that the planet has either a water-rich envelope, a gas-rich envelope, or a mixture of both. We have performed evaporation modelling of the planet. If we assume the planet has a gas-rich envelope, we find that the planet has lost a significant fraction of its envelope and its radius has shrunk. Furthermore, we estimate it will lose all its remaining gaseous envelope in„ 0.52 Gyr. Otherwise, the planet could have already lost all its primordial gas and is now a bare ocean planet. Further observations of its possible atmosphere and/or mass-loss rate would allow us to distinguish between these two hypotheses. Such observations would determine if the planet remains above the radius gap or if it will shrink and be below the gap.

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