TOI-1853 b

TOI-1853 b is a hot, massive, and dense Neptune-sized exoplanet orbiting the orange dwarf star TOI-1853, located in the constellation Boötes about 545 light-years (167 parsecs) away from Earth. It was discovered by the Transiting Exoplanet Survey Satellite (TESS) in 2020 and confirmed in 2023. The planet orbits close to its host star with an orbital period of 1.24 days, which gives it a high temperature of about 1,480 K (1,210 °C; 2,200 °F). With a mass 73 times Earth's (77% of Saturn's mass) and a radius 90% of Neptune's, TOI-1853 b has a very high density between 9 and 10 g/cm3—nearly twice as dense as Earth and higher than that of steel. The extremely high density of TOI-1853 b implies it is nearly entirely made of solid rock and water with a thin atmosphere of hydrogen and helium, which makes it a mega-Earth instead of a gas giant. The high density of TOI-1853 b challenges the hypothesis that massive planets should form via pebble accretion. Two possible hypotheses have been proposed for the nature and origin of TOI-1853 b's density: it could either be a former gas giant whose atmosphere was stripped by its host star, or it could be a remnant of multiple collisions between super-Earths.

Discovery TESS first detected TOI-1853 b transiting its host star in early 2020. The planet was initially known as TOI-1853.01 until its confirmation in 2023. To confirm its planetary nature, a team of astronomers led by Luca Naponiello conducted follow-up observations using various telescopes from the ground. Naponiello's team observed additional transits by the planet in May and June 2020, searched for potential distant companions with high-resolution imaging in May–June 2020 and February 2021, and measured the planet's gravitational influence on its host star's radial velocity with Doppler spectroscopy between February 2021 and August 2022. After analyzing their results, Naponiello's team published their confirmation of TOI-1853 b in the journal Nature in August 2023.

Characteristics

Orbit and temperature TOI-1853 b orbits close to its host star with a semi-major axis of 0.0213 AU (3.19 million km; 1.98 million mi) and an orbital period of 1.24 days. At this distance from its host star, TOI-1853 b is heated to an equilibrium temperature of about 1,480 K (1,210 °C; 2,200 °F). As a Neptune-sized planet, TOI-1853 b's close orbital distance to its host star makes it a rare example of a hot Neptune planet in the Neptunian desert—a region of orbital periods shorter than 3.2 days where very few Neptune-sized planets have been found. The apparent rarity of planets in the Neptunian desert is thought to be caused by their host star's intense radiation stripping off their atmospheres and radii. The orbit of TOI-1853 b is expected to be close to circular, with an upper limit eccentricity of <0.03. The orbit of TOI-1853 b is inclined 84.7° with respect to the sky plane, which allows it to transit its host star from Earth's point of view. Viewed from Earth, TOI-1853 b takes about 1.19 hours to transit its host star. Although the planet's orbital inclination with respect to its host star's rotation axis is unknown, it is predicted that the planet's orbital inclination is aligned with its star's rotation due to tidal interactions. Tidal interactions between the planet and its star are also predicted to cause orbital decay; TOI-1853 is predicted to survive for at least 4 billion years into the future before it spirals into its host star.

Physical properties

TOI-1853 b is a Neptune-sized exoplanet with an unusually high mass and density. It is 73.2±2.7 times more massive than Earth (4.27× Neptune's mass or 76.9% Saturn's mass) and 3.46±0.08 times bigger than Earth in radius (89.5% Neptune's radius). This gives the planet a very high surface gravity of 60.1+3.8−3.6 m/s2 (6.13 times Earth's gravity) and a bulk density of 9.74+0.82−0.76 g/cm3—nearly twice as dense as Earth and denser than steel. This density is exceptionally high for a Neptune-sized giant planet; it implies that TOI-1853 b must be mostly solid and very rich in heavy elements—most likely in the form of rock and possibly ice—instead of gaseous hydrogen and helium like in typical gas giants. A 2026 study by Maxwell Kroft and colleagues proposed that TOI-1853 b belongs to a class of dense, Neptune-mass exoplanets called mega-Earths, which are defined as having radii between 2.1 and 5 Earth radii and densities greater than 5.5 g/cm3. According to Kroft et al., only 13 mega-Earths have been confirmed as of March 2026, with the most massive one being TOI-1853 b.

Hypothesized composition Although the composition of TOI-1853 b is unknown, it could be inferred from its density. In a 2023 study, Naponiello and colleagues proposed that TOI-1853 b's high density can be explained by two possible internal structures and compositions. One possible option has TOI-1853 b composed of 99% rock and metal and 1% atmosphere by mass. The other option has TOI-1853 b composed of 49.95% rock and metal, 49.95% water in the form of high-pressure ice and possibly supercritical fluid, and 0.1% atmosphere by mass. In both options, the thin atmosphere is assumed to be mostly hydrogen and helium (possibly containing steam if water-rich), and the rocky component of the planet is assumed to be differentiated into an iron core and a silicate mantle. The pressure of TOI-1853 b's deep interior is estimated to reach around 5,000 gigapascals (50 times the core–mantle boundary pressure of Earth), which is enough to cause most elements and their compounds to behave as metals. Naponiello et al. argued that a water-rich interior would be more likely if TOI-1853 b formed through planetary collisions. Transmission spectroscopy and secondary eclipse observations (when the planet passes behind its star) by the James Webb Space Telescope (JWST) may be able to provide insights into the composition of TOI-1853 b's atmosphere and possibly its interior. Transmission spectroscopy may be able to distinguish a thin, hydrogen-dominated atmosphere from a water-dominated one, while secondary eclipse spectroscopy may be able to detect carbon dioxide and constrain the planet's atmospheric metallicity.

Host star

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