About 437 light years from Earth—more than 2 quadrillion miles—a planet about the size of Neptune orbits a bright star every four days. Astronomers at Princeton University first spotted this exoplanet in 2011 but a detailed study of its atmosphere, reported this week by NASA, reveals that it is an entirely new kind of planet. "We're seeing something that goes against how we think about how our solar system formed," says lead study author Hannah Wakeford, a postdoctoral fellow at the NASA Goddard Space Flight Center.
Joel Hartman, who led the Princeton team that found this exoplanet, called it a "puffy Neptune," because its large radius hinted at a small core supporting a gaseous globe. Its nearness to its star—compare its four-day orbit to Earth's 365-day trip around our sun—meant this exoplanet (a planet outside our solar system), named HAT-P-26b, was also extremely hot.
Wakeford and colleagues used a method called transmission spectroscopy to determine the makeup of HAT-P-26b's atmosphere. With this approach, astronomers study an exoplanet as it crosses in front of its sun. During that transit, the distant sun's light shines through the atmosphere of the exoplanet. As that light hits the gases in the planet's atmosphere, those gases emit colors. Astronomers can detect those colors as a way to identify what molecules are floating around the planet, such as water, methane and carbon dioxide.
The NASA scientists studied four transits of HAT-P-26b with the Hubble and Spitzer telescopes and found a strong color signature for water vapor. Finding water in the universe is not unusual, but that strong signal enabled astronomers to measure the amount of water vapor in the planet's atmosphere. And they found that HAT-P-26b contained less water than expected.
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That measurement was surprising because it doesn't match what astronomers know about other Neptune-mass exoplanets. Wakeford explains that water is a barometer for other molecules. In particular, the quantity of water correlates with the quantity of the so-called heavy metals; that is, all elements other than hydrogen and helium. The more water vapor there is, the richer the atmosphere is in heavy metals.
In our solar system, larger planets have less heavy metals in the atmosphere. As the size of the planet increases, the atmosphere's "metallicity" decreases. Neptune has far more heavy elements in its atmosphere compared to Jupiter's.
But HAT-P-26b breaks that patterns. Although it's roughly the size of Neptune, its atmosphere contains far fewer heavy metals than astronomers expect for a Neptune-sized world. It turns out that HAT-P-26b is not like Neptune or Uranus, a similarly sized planet, at all. In fact, it's more like Jupiter.
Among the thousands of exoplanets astronomers have found, Neptune-sized worlds are the most abundant. But only a handful orbit stars bright enough for transmission spectroscopy studies, making this observation extremely rare and valuable.
Hartman, who notes how difficult gathering measurements like this are for astronomers, calls the study "groundbreaking." The insight is important because it sheds galactic light not only on how exoplanets form but also on our own local planets. An exoplanet atmosphere that defies our expectations can provide clues about how our solar system formed and whether this particular formation is rare or common in the universe. "We've got a planet that doesn't fit what we thought," says Wakeford, "That's the beauty of the scientific process."
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