Astronomers Find Rare 'Hot Water World' Exoplanet Destined to Be Swallowed by Its Star

Astronomers have discovered a new ocean world outside the solar system that orbits so close to its star that it is destined to eventually be swallowed by it. The exoplanet, designated HD 176071 b, orbits a star cataloged as HD (Henry Draper Catalogue) 176071, roughly 335 light-years away, and is about two and a half times the size of Earth but roughly eight and a half times its mass, indicating the planet is about 50% water.

HD 176071 b sits just under 1.5 million miles from its star, about 1.6% of the distance between Earth and the sun, and completes an orbit in just 14 hours. That closeness subjects it to extreme tidal forces that are driving the planet toward an eventual violent end inside its star. It is also a rare occupant of the "Neptunian Desert," a region where few Neptune-sized worlds are found close to their stars because intense stellar radiation is thought to strip away their gaseous layers, yet HD 176071 b has kept a dynamic atmosphere.

"Comparing historical data from the Kepler space telescope with new TESS observations six years later, we noticed an unexpected phase shift in the modulation of the reflected light," said lead researcher Sylvain N. Breton of the Italian National Institute for Astrophysics (INAF). "This tells us that we are not looking at a static photograph, but a living and changing atmosphere, in which dense, highly reflective clouds form, evolve, and move along the planet's coldest edges."

The discovery method was unusual. NASA's catalog of confirmed exoplanets holds more than 6,000 entries, most found by the transit method, which requires a planet to pass directly between its star and Earth, dimming the starlight. HD 176071 b does not transit its star from Earth's perspective. Instead, Breton's team used the HARPS-N instrument on the Galileo National Telescope to detect tiny fluctuations in the star's light caused by light reflecting off the tidally locked planet's permanent dayside and nightside as it orbits.

"By measuring with extreme precision how the light reflected by the planet varies throughout its orbital journey, we can not only detect these elusive worlds, but also begin to characterize their structure and atmosphere, opening a fundamental window onto previously almost inaccessible planetary populations," Breton said. The research was published Tuesday, August 25, in the journal Astronomy & Astrophysics.