TOI-1853 b

Lore & Background

TOI-1853 b was first detected by TESS in early 2020 as TOI-1853.01. A team led by Luca Naponiello conducted follow-up observations using ground-based telescopes, observing additional transits in May and June 2020, searching for companions with high-resolution imaging in 2020–2021, and measuring radial velocity variations from February 2021 to August 2022. The planet's confirmation was published in the journal Nature in August 2023.

The planet orbits its host star at a distance of 0.0213 AU with a period of 1.24 days, placing it in the Neptunian desert—a region of short orbital periods where few Neptune-sized planets are found. Its equilibrium temperature is about 1480 K. The orbit is expected to be nearly circular, with an upper eccentricity limit of 0. The planet's high density implies it is composed almost entirely of solid rock and water, with only a thin hydrogen-helium atmosphere. Two possible internal structures have been proposed: one with 99% rock and metal and 1% atmosphere, and another with roughly equal parts rock/metal and high-pressure water ice, plus a trace atmosphere.

Reader's Guide

TOI-1853 b is significant because its high density challenges the standard pebble accretion model for planet formation, which predicts that massive planets should accrete substantial gas and become gas giants. Its density is so high that it is classified as a mega-Earth—one of only 13 such planets confirmed, and the most massive among them. Two competing hypotheses explain its nature: it could be a former gas giant whose atmosphere was stripped by its host star, or it could be the remnant of multiple collisions between super-Earths. The planet's existence in the Neptunian desert, where intense stellar radiation typically strips atmospheres, supports the stripped-gas-giant scenario. Future observations with the James Webb Space Telescope, including transmission spectroscopy and secondary eclipse spectroscopy, may distinguish between a hydrogen-dominated or water-dominated atmosphere and help constrain its interior composition. The planet's deep interior pressure is estimated at about 5,000 gigapascals, enough to make most elements behave as metals.

Did You Know?

• TOI-1853 b has a density higher than steel and nearly twice that of Earth.
• The planet is 73 times Earth's mass but only 90% of Neptune's radius.
• Two proposed origins are a stripped gas giant or a collision remnant of super-Earths.

From Candidate to Confirmed: The Road to TOI-1853 b

The story of TOI-1853 b began in early 2020, when the Transiting Exoplanet Survey Satellite flagged a periodic dimming of the orange dwarf star TOI-1853 in the constellation Boötes. At that stage the object carried only a provisional label—TOI-1853.01—and its planetary nature was still unverified. The credit for converting that candidate into a confirmed world belongs to a team led by Luca Naponiello, who spent nearly four years assembling the evidence required for a definitive answer. Their campaign included catching additional transits in May and June 2020, searching for any distant stellar companions with high-resolution imaging in 2020 and again in early 2021, and—most critically—measuring the tiny gravitational wobble the planet induces on its host star through Doppler spectroscopy over a stretch from February 2021 to August 2022. Once every line of evidence converged, Naponiello and collaborators published their confirmation in the journal Nature in August 2023, officially retiring the ".01" suffix and introducing a new, extraordinarily dense Neptune-sized planet to the scientific community.

A World Denser Than Steel: Physical Properties

TOI-1853 b packs 73.2 Earth masses into a sphere only 3.46 times Earth's radius—roughly 89.5 percent of Neptune's size and about 77 percent of Saturn's mass. The result is a bulk density of approximately 9.74 grams per cubic centimeter, a figure that exceeds the density of solid steel and sits nearly double that of our own planet. Surface gravity on this world would press down at about 60 metres per second squared, more than six times what we experience here. In a 2026 study, Maxwell Kroft and colleagues placed TOI-1853 b into a newly emphasized class of dense, Neptune-mass worlds they call mega-Earths, defined by radii between 2.1 and 5 Earth radii and densities above 5.5 g/cm³. As of March 2026, only thirteen such planets had been confirmed, and TOI-1853 b stands as the most massive member of the group. Its density tells us the interior must be overwhelmingly composed of heavy elements—rock, metal, and possibly high-pressure ice—rather than the light hydrogen and helium that dominate typical gas giants.

Two Possible Birth Stories: Composition and Formation

Because no direct probe can reach TOI-1853 b, scientists infer its makeup from density and orbital context. Naponiello and colleagues outlined two leading internal models. In the first, the planet is almost entirely rock and metal—99 percent by mass—with just a whisper of a hydrogen-helium atmosphere. In the second, roughly half the planet is rock and metal and the other half is water locked as high-pressure ice or supercritical fluid, again capped by a trace gaseous envelope. Both scenarios assume a differentiated interior with an iron core beneath a silicate mantle, and both predict interior pressures around 5,000 gigapascals, fifty times Earth's core-mantle boundary, where most elements would behave as metals. The water-rich picture gains extra plausibility if TOI-1853 b was forged in a series of violent collisions between super-Earths. Alternatively, it may have once been a fuller gas giant whose envelope was stripped away by its host star's radiation. Either way, the planet's extreme density poses a serious challenge to the standard pebble-accretion model of giant-planet formation.

Burning in the Neptunian Desert: Orbit and Fate

TOI-1853 b circles its orange dwarf host at a semi-major axis of just 0.0213 AU—about 3.19 million kilometres—completing one orbit in a mere 1.24 days. That proximity roasts the planet to an equilibrium temperature near 1,480 kelvin, or roughly 1,210 degrees Celsius. Its orbital period places it squarely inside the so-called Neptunian desert, a zone where periods shorter than about 3.2 days host very few Neptune-sized worlds, likely because intense stellar radiation erodes their atmospheres and shrinks their radii. The orbit is nearly circular, with eccentricity below 0.03, and tilted 84.7 degrees relative to the sky plane, which is what allows the planet to transit its star from our vantage point; each transit lasts about 1.19 hours. Tidal coupling between planet and star is expected to slowly drain orbital energy, but models suggest TOI-1853 b will endure for at least four billion more years before it finally spirals into its host star.

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