A Neptune-size planet beyond the solar system has telltale traces of water vapor in its atmosphere, making it the smallest exoplanet known to have the wet stuff yet, scientists say.
Several massive Jupiter-size giants have had the components of their atmosphere examined, but until now, the atmospheres of smaller planets have proved more elusive. In this new study, scientists discovered traces of water on the alien planet HAT-P-11b, which orbits a star 124 light-years from Earth in the constellation Cygnus.
“Water is the most cosmically abundant molecule that we can directly observe in exoplanets, and we expect it to be prevalent in the upper atmospheres of planets at these temperatures,” lead author Jonathan Fraine said in an email interview. Fraine, a graduate student at the University of Maryland, worked with a team lead by Drake Deming, also of the University of Maryland.
“Detecting it is both a confirmation of our theories and revealing for the bulk of the spectrum that we can observe,” Fraine told Space.com.
Detecting Alien Planet Atmospheres
As a planet passes, or transits, between Earth and its sun, it blocks light from the star. The dip in light is how many exoplanets are first found. But these transits also allow astronomers to study the atmospheres of exoplanets. By observing the spectrum of light that passes through an exoplanet’s atmosphere, scientists can determine what it is made up of.
For HAT-P-11b, a planet roughly four times the radius of Earth, that makeup is 90 percent hydrogen, with traces of water vapor. The Neptune-size planet orbits its sun every five days, at a distance that is only one-twentieth of the Earth-sun distance (which is 93 million miles, or 150 million kilometers). As a result, the temperature climbs higher on HAT P-11b than it does on gas giants in the solar system, reaching a sizzling 1,120 degrees Fahrenheit (605 degrees Celsius).
Scientists have been studying the atmospheres of Jupiter-like planets for years, but smaller planets produce a smaller signal that is more challenging to observe. For the new study, researchers examined the atmospheres of four other smaller exoplanets — two roughly the size of Neptune and two smaller super-Earths — but the results were disappointingly featureless.
“We do indeed have the technology — the resolution — to observe Neptune-size exoplanets, and even super-Earths,” Fraine said.
But the chemical compositions of the other four planets were blocked by a familiar phenomenon — clouds.
“We’ve just been seeing a whole lot of nothing,” Eliza Kempton, of Grinnell College in Iowa. Kempton models planetary atmospheres but was not involved in the research.
Kempton added that the flat, featureless signals observed for the other planets were attributed to clouds or hazes in the upper atmosphere. The high clouds blocked light from the star, keeping it from penetrating through to the observers’ side of the planet and leaving scientists unable to characterize the chemicals in the atmosphere.
“It’s not crazy to think that there should be clouds in these exoplanet atmospheres, because we see clouds in all the planetary atmospheres in our solar system,” Kempton said.
Although the hot, Neptune-size planet lives in a different environment from the icy giants in the solar system, it is similar to one of the four smaller planets whose atmosphere had already been studied. Those planets are known as GJ436b, GJ1214b, HD97658b and GJ3470b.