Astronomers have discovered a new ocean world outside the solar system that orbits extremely close to its parent star. This means that the extrasolar planet, or “exoplanet,” is in hot water, both literally and figuratively. The exoplanet orbits the star HD 176071 located about 335 light years away, and is approximately two and a half times the size of Earth, but has about eight and a half times the mass of our planet. That tells scientists that this planet is approximately 50% water. At a distance from its star of just under 1.5 million miles (about 1.6% of the distance between Earth and the Sun) and with a year lasting just 14 hours, HD 176071 b is a rare example of a “warm water world.” That close distance to its star means that the planet experiences extreme tidal forces due to the pull of the star’s gravity. These forces are propelling the planet, designated HD 176071 b, toward a swift and inexorable fatal encounter that will eventually see it violently devoured by its host star. The exoplanet is also a rare occupant of what scientists call the “Neptunian Desert,” a term that describes the lack of Neptune-sized worlds orbiting very close to their host stars. This is thought to be because intense stellar radiation strips planets of their gaseous shells, effectively shrinking them. However, HD 176071 b appears to defy this suggestion, as this aquatic world surprisingly possesses a dynamic atmosphere that has not been stripped away. “By comparing historical data from the Kepler space telescope with new observations from the TESS (Transisting Exoplant Survey Satellite) six years later, we noticed an unexpected phase shift in the modulation of the reflected light,” principal investigator Sylvain N. Breton of the Italian National Institute of Astrophysics (INAF) said in a statement. You may like “This tells us that we are not looking at a static photograph, but rather a living and changing atmosphere, in which dense, highly reflective clouds form, evolve and move along the coldest edges of the planet.” That’s not the only extraordinary thing about HD 176071 b. The way the ocean exoplanet was initially discovered is also quite special. This ocean world does not intersect with its stellar parent. Currently, NASA’s catalog of confirmed exoplanets has more than 6,000 entries, and thousands more worlds beyond the solar system await formation. The vast majority of these were discovered when they passed from their host star and caused a small drop in the starlight that reaches us from that stellar body in what is known as the transit method for detecting exoplanets. Therefore, the transit method requires the planet to pass between its star and Earth. As you can imagine, this is a really lucky arrangement for astronomers when it comes to finding new planets. Most of the known planets in the Milky Way don’t do that. “The vast majority of planets in the universe never transit in front of their star from Earth’s perspective, making them virtually invisible,” Breton said. HD 176071 b is precisely one of those worlds, with this geometric obstacle, so Breton and his colleagues had to be clever to discover this water world. A diagram shows how the transit method for exoplanet detection works (Image credit: NASA) The team still used light from HD 176071 b’s parent star to detect the exoplanet, but instead looked for small fluctuations in brightness caused when light from this star reflected off the planet. HD 176071 b is tidally locked, meaning it has a permanent “day side” facing its star and a perpetual “night side” facing into space. The variations in brightness are because sometimes the night side of the planet is facing us and other times our instruments see its day side. “Thanks to the data set acquired with the HARPS-N instrument on the Galileo National Telescope, we have managed to overcome this geometric obstacle: 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 to planetary populations that were previously almost inaccessible.” Breton said. The discovery of HD 176071 b shows that current astronomical techniques are sophisticated enough to characterize the atmosphere and internal structure of worlds beyond the solar system. This is a big boost for the detection of worlds that do not pass between Earth and their host star. The team’s research was published Tuesday (Aug. 25) in the journal Astronomy & Astrophysics.
