A super dense neptunian planet for HARPS-N

An exoplanet with extraordinary characteristics, TOI-1853b, has been discovered by an international team of scientists led by the University of RomaTor Vergata and INAF. This object was initially identified in 2020 as a planet candidate by TESS, the NASA's Transiting Exoplanet Survey Satellite.

The confirmation of TOI-1853b's planetary nature and the measurements of its mass and density were made possible thanks to HARPS-N mounted at the Telescopio Nazionale Galileo.

HARPS-N is the ultra-precise and ultra-stable spectrograph of the TNG and thanks to its high accuracy radial velocity spectroscopic observations we now know that TOI-1853b physical properties challenge conventional theories of planetary formation and evolution.

TOI-1853b is extremely peculiar: it only takes 30 hours to complete one full revolution around its star and it is called “neptunian” because its radius is comparable to Neptune's radius, but its mass is about four times larger than Neptune’s (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).

TOI-1853b is located in the Bootes constellation, at a distance of 545 light-years from us, and its discovery, published in Nature, was made by an international team of researchers, led by Luca Naponiello (University of Rome Tor Vergata).

TOI-1853b falls 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% of the planet's mass. Or, another very fascinating hypothesis is that it may be composed half of rock and half of 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 (University of Rome Tor Vergata). Two different scenarios can describe its origin: a collisions between massive proto-planets in the original proto-stellar disk that may have removed almost all of the planet's atmosphere or, as alternative, 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.

“This is a really interesting discovery that places another tile in the quest for an understanding of planetary systems formation and evolution. It also shows that with HARPS-N the TNG keeps pushing strong: a powerful instrument in the hand of capable researchers take us at the forefront of scientific research.” adds A.Ghedina, TNG Director.

The research has been published in Nature: https://www.nature.com/articles/s41586-023-06499-2