Astronomers using NASA’s Transiting Exoplanet Survey Satellite have discovered a sub-Neptune exoplanet orbiting TOI-220, a bright and old K-type dwarf star located 290 light-years away in the constellation of Pictor.
An artist’s impression of the sub-Neptune exoplanet TOI-220b and its host star. Image credit: Sci-News.com.
“Among all the confirmed exoplanets, one class of particular interest are the small-sized planets — with radii less than 5 Earth radii — orbiting bright stars with periods shorter than 10 days,” said Dr. Sergio Hoyer of Aix Marseille University and his colleagues.
“Within this parameter space there is the so-called Neptunian desert, which is a region that presents a significant paucity of hot/highly irradiated planets with respect to the overall planet population.”
“This deficit could be seen as evidence of photoevaporation and/or tidal disruptions or of core-powered atmospheric mass loss mechanism.”
“Currently, of all the known planets in the Neptunian desert only a very small number have precisely measured masses, preventing a comprehensive understanding of the formation history of these objects,” they added.
“On the other side, there is a population of sub-Neptune size planets exposed to a milder stellar irradiation that usually present a gas-rich envelope with equilibrium temperatures below 1,000 K (727 degrees Celsius, 1,341 degrees Fahrenheit).”
The newly-discovered planet is about 3 times larger than Earth and 13.8 times more massive, implying a density of 2.73 g/cm3.
Designated TOI-220b, the alien world orbits its parent star every 10.7 days at a distance of 0.09 AU.
Dr. Hoyer and co-authors estimate the planet’s temperature to be about 806 K (533 degrees Celsius, 991 degrees Fahrenheit).
The transit signals of TOI-220b were initially detected by NASA’s Transiting Exoplanet Survey Satellite.
The follow-up radial velocity observations with the HARPS spectrograph on ESO’s La Silla 3.6-m telescope confirmed their planetary nature.
“The low density of this strongly irradiated planet could be explained with a water-rich atmosphere that reaches the supercritical phase at its base, with a silicate mantle and an iron core as its bulk,” the astronomers said.
“As an alternative the planet might have a solid core surrounded by a thick hydrogen-helium (H/He) atmosphere.”
“Future work should explore this possibility but also a composition with a well-mixed water and H/He atmosphere, since these are the most common volatiles that can form low-mass planets.”
The discovery of TOI-220b is described in a paper to be published in the Monthly Notices of the Royal Astronomical Society.
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S. Hoyer et al. 2021. TOI-220b: a warm sub-Neptune discovered by TESS. MNRAS, in press; arXiv: 2105.01944
