An international team of scientists led by the Tor Vergata University of Rome and by INAF identified the new planet thanks to NASA's TESS satellite and characterized with the Galileo National Telescope.
It is called TOI-1853b and is extremely peculiar: every 30 hours it completes one full revolution around its star (it takes Earth one year to complete one full revolution around the Sun), it has a radius comparable to Neptune's (3.5 Earth radii, hence the name) but a mass about four times larger (73 Earth masses).
For this reason, it holds the record for the highest density among Neptunian exoplanets known to date (about 10 g/cm3, twice the density of Earth). Distant 545 light-years, TOI-1853b is located in the Bootes constellation, and its discovery, published in Nature ( "A super-massive Neptune-sized planet” ), was made by an international team of researchers, led by Luca Naponiello, 31 years old, a PhD student in astrophysics at Tor Vergata University of Rome and first author of the paper. Several researchers from the National Institute of Astrophysics (INAF) made key contributions to the study.
TOI-1853b is located in the so-called 'Neptunian desert', a near-stellar region devoid of Neptune-sized planets: here the planets receive strong irradiation from the star, so they cannot retain their evaporating gaseous atmospheres, thus leaving exposed a solid core much smaller than Neptune.
"Based on theories of planetary formation and evolution, it was not expected that such a planet could exist and so close to its star", Naponiello comments. "The density of TOI-1853b is too high for it to be a classical Neptunian-type planet and, consequently, it must be extremely rich in heavy elements".
Its presence in the 'Neptunian desert' is, therefore, yet another mystery to clarify. Its exact composition is not known. Naponiello adds, "We expect TOI-1853b to be mostly rocky and surrounded by a small gaseous envelope of hydrogen and helium that makes up at most 1 percent of the planet's mass. Or, another very fascinating hypothesis is that it may be composed of half rock and half water ice. Given the planet's high temperature (about 1500 degrees Kelvin), in the latter case TOI-1853b could have an atmosphere rich in water vapor".
"Its origin is also a mystery since none of the theoretical models of planetary formation predict that a planet with such characteristics could exist," says Luigi Mancini, a professor in the Department of Physics at the Tor Vergata University of Rome and second author of the paper. "However, numerical simulations we have conducted under extreme scenarios suggest that its origin may be due to collisions between massive proto-planets in the original proto-stellar disk". "Such collisions," Naponiello continues, "may have removed almost all of the planet's atmosphere, which would explain its small size and large density, as if only the planet's bare core remained".
As an alternative to the planetary collision scenario, according to the researchers, the planet might initially have been a gas giant like Jupiter or more massive, and would have assumed a very elliptical orbit as a result of dynamic instabilities due to gravitational interactions with other planets. This would have led it to make very close encounters with its star, which would have caused it to lose its outer atmospheric layers and would, at the same time, have circularized and stabilized its orbit at its current distance from the star.
"At the moment, we cannot distinguish which of the two formation scenarios is the more plausible one, but we will continue to observe this planet to figure it out. We also cannot rule out that subsequent theoretical studies from this exceptional discovery may lead to new formation models for very massive Neptunian planets", comments Aldo Bonomo, a researcher at INAF Turin and co-author of the paper.
TOI-1853b was initially identified in 2020 as a planetary candidate by NASA's Transiting Exoplanet Survey Satellite (TESS) with the transit method, that is, by observing the periodic light dimming of its star produced by the planet's passage in front of it. Confirmation of TOI-1853b's planetary nature and measurement of its mass and density were made possible by radial velocity spectroscopic observations obtained by the team with the HARPS-N (High Accuracy Radial Velocity Planet Searcher for the Northern hemisphere) spectrograph at the Telescopio Nazionale Galileo (TNG), located on the island of La Palma in the Canary Islands. These observations allowed the planet's gravitational signal on the motion of its star to be revealed and characterized with high accuracy.
"HARPS-N has now been operational at TNG for more than 10 years (with its first light obtained in March 2012). It is one of the few state-of-the-art instruments available to the astronomical community to measure with high accuracy the masses and densities of extrasolar planets, in some cases even Earth-sized ones", concludes Alessandro Sozzetti, first investigator at INAF Turin and co-author of the paper. "As in this case, new discoveries and measurements often bring more questions than answers”.
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