A Hot Water World Is Spiraling Toward Its Star

Astronomers have found a water-rich exoplanet in one of the most dangerous planetary neighborhoods known: less than 1.5 million miles from its star. Named HD 176071 b, the planet orbits the star HD 176071, located about 335 light-years from Earth. It is approximately 2.5 times wider than Earth but has roughly 8.5 times Earth’s mass. Based on those measurements, scientists estimate that about half of the planet could be water. The world’s year lasts only 14 hours. Its tight orbit exposes it to powerful gravitational forces from its star, which are gradually pushing it toward an eventual encounter in which the star will consume the planet. HD 176071 b is also unusual because it occupies the so-called Neptunian Desert, a region close to stars where planets about the size of Neptune are rarely found. One explanation for this scarcity is that intense stellar radiation strips away their gaseous outer layers. However, the newly identified planet appears to retain a dynamic atmosphere despite its hostile surroundings. According to lead researcher Sylvain N. Breton of the Italian National Institute for Astrophysics, the team compared older observations from NASA’s Kepler space telescope with data collected by the Transiting Exoplanet Survey Satellite six years later. The comparison revealed a shift in the pattern of reflected light, suggesting that the atmosphere is changing rather than remaining fixed. Breton said the observations point to dense, reflective clouds forming, changing and moving around the planet’s colder regions. The planet was not found using the more familiar transit method. HD 176071 b does not pass between its star and Earth, so it does not produce the characteristic dip in starlight used to identify most known exoplanets. Instead, researchers examined subtle changes in the star’s brightness as light reflected from the planet reached Earth. Because HD 176071 b is tidally locked, one side permanently faces the star while the other remains in darkness. The observed brightness changes as different portions of those two hemispheres turn toward Earth. The study, published in Astronomy & Astrophysics, demonstrates that reflected light can help astronomers detect and examine planets that never transit their stars.