Hot Neptune-Sized Planet’s Primitive Atmosphere Contains Water
published during a full moon.

Among the most common planets seen in the Milky Way over the past 25 years are Neptune-size worlds, ones about 15 times Earth’s mass. Examples of such planets in our solar system — Uranus and, of course, Neptune — are icy giants. Now scientists have found a very different kind of Neptune-size world, a searing-hot planet named HAT-P-26b that has a strong signature of water vapor in its skies. These new findings suggest that it and some other alien Neptunes were born in different ways than their cousins in the solar system.

Distant Neptune-size planets often hug their stars much closer than Neptune and Uranus does the sun. This extra heat should turn chemicals that are otherwise frozen in Neptune and Uranus to vapor, potentially leading these “exo-Neptunes” to differ greatly in composition in comparison to their solar system counterparts. However, little is known about the makeup of such alien Neptunes, since their distances from Earth and relatively diminutive sizes renders them difficult to analyze.

To learn more about alien Neptunes, scientists examined the exoplanet HAT-P-26b, which astronomers discovered in 2010. The planet orbits the orange dwarf star GSC 0320-01027 located about 437 light years from Earth. The star is about 80 percent the sun’s mass and diameter, nearly 90 percent as hot as the sun, and at about 9 billion years in age, this star is nearly twice as old as the 4.6-billion-year-old sun.

Primitive Atmosphere

HAT-P-26b is the size of Neptune. Credit: NASA

HAT-P-26b clings its star about 20 times closer than Earth does the Sun or about eight times closer than Mercury does the sun. “The star would take up a huge amount of sky as viewed from this planet,” said study lead author Hannah Wakeford, an astrophysicist at NASA Goddard Space Flight Center in Greenbelt, Maryland.

This exoplanet has a mass comparable to Neptune and Uranus but is 65 percent wider. This disparity likely stems from how its atmosphere is swelling up because of heat — its scorching proximity to its star leaves the exoplanet cooking at roughly 1,320 degrees F (715 degrees C). The puffy nature of HAT-P-26b’s atmosphere helped scientists analyze starlight filtering through it, uncovering details that shed light on its composition.

Using four recent observations from NASA’s Hubble Space Telescope and two previous observations from NASA’s Spitzer Space Telescope, scientists now find strong, clear signs of water in HAT-P-26b’s atmosphere. “Gaining this much information about a Neptune-size world is very rare,” Wakeford said.

Primitive Atmosphere

The atmosphere of the distant “warm Neptune” HAT-P-26b, illustrated here, is unexpectedly primitive, composed primarily of hydrogen and helium. By combining observations from NASA’s Hubble and Spitzer space telescopes, researchers determined that, unlike Neptune and Uranus, the exoplanet has relatively low metallicity, an indication of the how rich the planet is in all elements heavier than hydrogen and helium. Credits: NASA/GSFC

Although HAT-P-26b possesses water vapor in its atmosphere, that does not mean it has white clouds of water in its skies. The exoplanet is much too hot for that, Wakeford said. Instead, “the clouds we expect to see in this atmosphere are composed of disodium sulfide,”

The data the researchers collected helped them pinpoint HAT-P-26b’s amount of “metals” — what astronomers call elements heavier than hydrogen or helium. Their findings suggested that this alien planet may have formed in a significantly different way than Neptune or Uranus.

In our solar system, the smaller the planet, the more metals one expects to see in its atmosphere, Wakeford said. For instance, Jupiter is 215 times the mass of Earth, and “has a heavy element abundance in its atmosphere about four times that of the sun,” she said. In contrast, “with Neptune, which is about 17 times the mass of Earth, it has about 100 to 200 times more heavy elements in its atmosphere than the sun does.”

Primitive Atmosphere

An artist’s impression of the planet WASP-142b, a gas giant twice as massive as Jupiter that follows the trend of having low metallicities. Credit: David A. Hardy.

“We think this trend occurs because of how the planets formed in a disk around the sun,” Wakeford said. “Temperatures decrease with increasing distance from the sun. At a certain distance, ices start to form — water freezes out, then carbon dioxide, methane, ammonia. These all contain heavy elements. We think Neptune and Uranus formed beyond these ‘ice lines’ and were bombarded with icy debris, whereas Jupiter and Saturn likely formed in a warmer part of the disk and encountered less icy debris.”

However, although HAT-P-26b is roughly the same mass as Neptune, it bucks this trend. “It’s much more like Jupiter, with a heavy metal abundance between four and five times that of the sun,” Wakeford said. These findings suggest that, like Jupiter and Saturn, HAT-P-26b formed relatively close to its star, “and was not bombarded with heavy elements during formation, resulting in a primordial atmosphere more akin to a star’s atmosphere,” Wakeford said.

All in all, “these findings are really important in our understanding of how our solar system fits into the global picture of planets in our galaxy,” Wakeford said. “We’re learning more about the diversity of how systems form and evolve over time.”

The scientists detailed their findings in the May 12 issue of the journal Science.