TL;DR: Astronomers found a non-transiting hot water world on a 14-hour orbit that is about half water by composition and is being pulled toward eventual engulfment by its star.

A team led by Italy's National Institute for Astrophysics has identified an extreme ocean-world candidate around a Sun-like pulsating star in Cygnus, using reflected light rather than a transit, with the result reported on Aug. 25 in Astronomy & Astrophysics.

According to Space.com, the planet, called HD 176071 b, circles roughly 335 light-years away and packs about two and a half times Earth's width with roughly eight and a half times Earth's mass. According to Media INAF, the host is catalogued as KIC 9139163 and the companion has about 7.3 Earth masses and a radius 2.4 times Earth's. Both accounts describe the same ultra-short orbit of only 14 hours and infer that water makes up about 50% of the planet.

A 14-hour year on the edge of its star

The candidate completes one full revolution in just 14 hours at an extremely tight separation, placing it far closer to its sun than any planet in our solar system sits to the Sun.

According to Space.com, the separation is just under 1.5 million miles, or about 1.6% of the Earth-Sun distance. That proximity leaves the world tidally locked, with one hemisphere in permanent daylight facing the star and the opposite hemisphere in perpetual night.

The host is described by Media INAF as a solar-type pulsating star in the constellation Cygnus. Space.com identifies the same system by the designation HD 176071. The researchers involved came from Italy, the United States, Spain, France, Austria and Japan under INAF leadership, according to Media INAF.

Why scientists call it a hot water world

Mass and radius together point to a density unlike rock or gas alone. The researchers interpret the combination as consistent with a body composed of about 50% water, a true boiling ocean world rather than a rocky super-Earth or a hydrogen-rich gas giant.

According to Space.com, the inferred makeup makes HD 176071 b a rare example of a hot water world. According to Media INAF, the density matches a world made fully half of water.

That label does not mean liquid oceans like Earth's. At such a small orbital distance, intense stellar heating would keep water in hot, high-pressure or vapor states, while the interior could hold exotic dense phases. The key measurement is bulk composition, not surface habitability.

Doomed by tides

Extreme closeness also controls the planet's future. Gravity from the star raises powerful tides on the planet, and those tidal interactions are reshaping its orbit.

According to Space.com, extreme tidal forces are driving rapid and inexorable orbital evolution toward a violent fatal encounter with the host star. According to Media INAF, the same tidal forces are guiding the planet, over time, to be swallowed by its own star.

The reports describe the outcome as a projection, not an event already observed. No destruction has been seen; engulfment is the expected long-term fate as orbital energy is lost through tides.

Found without a transit

Most of the more than 6,000 confirmed exoplanets in NASA's catalog were found by the transit method, in which a planet passes directly between Earth and its star and produces a small dip in starlight, according to Space.com. That geometry requires a fortunate alignment.

According to Media INAF, most of the more than six thousand known exoplanets were identified through transits. Geometry therefore implies that non-transiting planets are intrinsically far more numerous in the universe, yet harder to locate.

This candidate never crosses the disk of its star as seen from Earth. Because of its orbital tilt relative to our line of sight, it produces none of the classic mini-eclipses used in traditional searches, according to Media INAF.

How reflected light revealed an invisible world

Instead of looking for a shadow, the team tracked phase curves, the tiny periodic changes in total system brightness as the planet shows different faces during its tight orbit.

Because the planet is tidally locked, telescopes alternately receive more reflected starlight when its illuminated dayside turns toward us and less when its dark nightside faces us. Measuring that modulation with extreme precision allowed detection and initial characterization despite the missing transit.

Lead researcher Sylvain N. Breton of INAF said the vast majority of planets never transit from Earth's perspective, making them effectively invisible to the usual approach, according to Space.com. Breton said high-precision tracking of reflected light through the orbital journey can both find such elusive worlds and begin to reveal structure and atmosphere.

Ground-based spectroscopy was central to that proof. The team used HARPS-N, the high-resolution spectrograph on INAF's Galileo National Telescope in the Canary Islands, to measure stellar radial velocities, according to Media INAF. Those velocity data placed a tight limit on the companion's mass and ruled out a stellar companion or brown dwarf, confirming planetary nature.

A living, shifting atmosphere

The surprise was not only the planet's existence, but evidence that its sky changes. The team compared historical photometry from NASA's Kepler space telescope with newer observations from TESS taken six years later.

According to Space.com, that comparison revealed an unexpected phase shift in the modulation of reflected light. Breton said the signal points away from a static picture toward a living and changing atmosphere in which dense, highly reflective clouds form, evolve and move along the planet's coldest edges.

According to Media INAF, the same six-year Kepler-to-TESS comparison showed the unexpected shift, indicating highly reflective dense clouds that develop and travel near the cooler margins. The finding suggests the envelope has not been completely removed despite brutal irradiation.

A survivor in the Neptunian desert

Close-in Neptune-size planets are scarce, a pattern astronomers call the Neptunian desert. The usual explanation is that intense stellar radiation strips gaseous layers and shrinks such worlds.

This object sits inside that sparse region, yet retains a strikingly dynamic atmosphere, according to both Space.com and Media INAF. Its persistence challenges simple stripping pictures and suggests water-rich envelopes, cloud reflectivity or ongoing replenishment may help some desert residents endure.

The result therefore matters beyond one system. It shows that modern astrophysics can characterize atmosphere and internal structure even without a favorable transit geometry, according to Media INAF. The method opens a window onto planetary populations that were almost inaccessible.

Why the technique matters for U.S. missions

Kepler and TESS supplied the space-based light curves behind the discovery. Kepler provided the historical baseline, while TESS, the Transiting Exoplanet Survey Satellite, supplied follow-up photometry six years later, according to Space.com.

HARPS-N then supplied the mass constraint from the ground. Breton said the HARPS-N dataset overcame the geometric obstacle and opened a fundamental window onto previously almost inaccessible populations, according to Space.com.

European work will extend the approach. Media INAF reports that ESA's Plato mission, with launch planned for 2027, is expected to catalog many systems analogous to KIC 9139163. That future sample could improve statistics on composition and occurrence for Milky Way planets, especially short-period water-rich worlds.

For now, HD 176071 b — the KIC 9139163 candidate — remains an object to watch. It is a non-transiting world found through its changing shine, a half-water body on a half-day year, and a planet whose tides point toward ultimate absorption by its star.