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A recent study concluded that superplanets are not remnants of Neptune small, posing a challenge to our understanding of how exoplanets form.
The small planets Neptune and super-Earth, which are four times the size of our planet, are the most common exoplanets orbiting stars outside our solar system.
Until now, super-terrestrial planets have been considered the rocky cores of young Neptune whose gaseous atmosphere was blown away.
And in a new study published in The Astrophysical Journal, astronomers at McGill University show that some of these exoplanets never had a gaseous atmosphere to begin with, shedding new light on their mysterious origins.
One theory says that most exoplanets are born as a small Neptune, but some are stripped of their gas coatings by radiation from host stars, leaving behind only a dense rock core. This theory predicts that our galaxy has very few Earth-sized and smaller exoplanets. However, recent observations show that this may not be the case.
To find out more, astronomers used simulations to track the evolution of these mysterious exoplanets. The model used thermodynamic calculations based on the mass of the rock cores, their distance from host stars and the heat of the surrounding gas.
“Unlike previous theories, our study shows that some exoplanets can never create a gaseous atmosphere,” said co-author Eve Lee, assistant professor in the physics department at McGill University and the McGill Space Institute.
The results indicate that not all super-terrestrial planets are small remnants of Neptune. Instead, the exoplanets were formed by a single distribution of rocks and spawned a rotating disc of gas and dust around their host stars.
“Some of the rocks produced gas shells, while others appeared and remained as rocky as super-Earth,” she added.
The planets are thought to form in a rotating disk of gas and dust around the stars. Rocks larger than the moon have enough gravitational pull to attract surrounding gas and form an envelope around its core. Over time, this gaseous crust cools and contracts, creating space for more surrounding gas to be drawn out and the exoplanet to expand. After the entire crust has cooled to the same temperature as the surrounding nebula gas, the crust cannot shrink and growth stops.
For smaller nuclei, these coatings are very small, so they remain rocky exoplanets. The distinction between Super-Earth and miniature Neptune comes from the ability of these rocks to grow and retain coatings of gas.
“Our findings help explain the origin, and possibly the spread, of the two groups of exoplanets,” says Lee. Using the theory proposed in the study, we can finally decipher how rocky exoplanets are common.
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