A recently discovered exoplanet has posed an interesting problem for astronomers.
It’s called TOI-332b, and its physical properties and orbital distance from its star are difficult to explain with current theory of planetary formation. With so few such exoplanets detected so far in the Milky Way, this could explain why fast orbiting worlds the size of Neptune seem so rare.
“Here we present the detection and characterization of TOI-332b, an ultrashort-period planet with unusually high density located deep in the Neptunian desert,” write a team led by astrophysicist Ares Osborn of the University of Warwick in the UK in a study. preprint available on arXiv.
“TOI-332b is a very interesting addition to our current findings from the Neptunian desert and a case study for testing the theory of planet formation.”
Humanity has discovered and confirmed over 5,500 exoplanets to date (i.e. extrasolar planets, worlds outside the solar system), and we are seeing some interesting patterns emerging.
Some properties of exoplanets, such as radius or distance from their star, are relatively common; some, however, are much rarer. Understanding why we have discovered so many of some types of worlds, and so few of others, could be a clue to how planetary systems form and evolve.
An area of scarcity is known as the Neptunian Desert. We found surprisingly few Neptune-sized exoplanets orbiting close to their stars. In fact, as Osborn and their colleagues point out, there are only a handful of square exoplanets in this breach.
The recently discovered TOI-332b, orbiting an orange dwarf star 727 light-years away, fits perfectly inside. Measurements of the effects it has on the light of its host star reveal that it has a radius 3.2 times that of Earth (Neptune is 3.88 Earth radii) and orbits the star once every 18.72 hours.
These properties alone would make TOI-332b an object of interest, but there is more. The gravitational effect on its host star allowed Osborn and his team to measure the mass of the exoplanet. It reaches 57.2 land masses. Neptune is 17.15 Earth masses.
This means that TOI-332b has a whopping density of 9.6 grams per cubic centimeter. It is one of the densest masses of Neptune exoplanets ever discovered. Neptune itself has a density of 1.64 grams per cubic centimeter. That of the Earth is 5.51 grams. TOI-332b is, on average, denser than iron.
Modeling suggests the exoplanet has a huge, dense iron core, with a rocky mantle and a thin atmosphere of hydrogen and helium. But for a nucleus the size of the one inferred from TOI-332b, one would expect a vast, thick, and extended atmosphere, similar to that of Jupiter.
Which begs the very interesting question: where did TOI-332b’s atmosphere go?
For a world near its star, processes of photoevaporation – where the star’s extreme radiation causes the atmosphere to evaporate and leak – should occur. But photoevaporation alone cannot explain the loss of an atmosphere of this size. In fact, that can’t explain most of it.
Other processes include planetary migration, which would have seen the atmosphere thinning out as the exoplanet moved a greater distance towards its host star, and the subsequent warming of the interior due to gravitational changes in the orbit of the exoplanet.
It is also possible that a giant impact with another planet-sized object blew up most of TOI-332b’s atmosphere. Or TOI-332b just never created an atmosphere to begin with.
Anyway, it will take a closer look to understand why the TOI-332b is the way it is.
“This unusual planet tests what we currently understand about planet formation; how such a giant core exists without a gaseous envelope remains an unanswered question,” the researchers write.
“Further observations are needed to potentially unravel the formation history of TOI-332b and its current characteristics.”
The research was accepted in the Royal Astronomical Society Monthly Noticesand is available on arXiv.
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