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Water vapor in atmosphere of exoplanet four times Earth’s size

The exoplanet HAT P-11b – about Neptune’s size – has an atmosphere that’s cloudless at high altitudes. That’s how astronomers could identify water vapor there.

Because no clouds blocked the view, scientists were able to observe water vapor on a Neptune-sized planet for the first time. The smaller the planet, the more difficult it is to observe its atmosphere, and other small planets have been obscured by clouds. The upper atmosphere of HAT-P-11b appears nearly cloud-free, as shown in this artist's depiction.  Image via NASA/JPL/Caltech

Because no clouds blocked the view, scientists were able to observe water vapor on a Neptune-sized planet for the first time. The smaller the planet, the more difficult it is to observe its atmosphere, and other small planets have been obscured by clouds. The upper atmosphere of HAT-P-11b appears nearly cloud-free, as shown in this artist’s depiction. Image via NASA/JPL/Caltech

Here on Earth, water = life. That’s why astronomers are excited about a finding of water vapor in the atmosphere of a planet only about four times bigger than Earth. The planet is called HAT P-11b. It’s some 124 light-years – nearly 729 trillion miles – away, in the direction of our constellation Cygnus the Swan. We now know more than 1,800 planets orbiting stars other than our sun, but astronomers say this is the smallest exoplanet in whose atmosphere they’ve been able to identify some chemical components. The journal Nature will publish their findings on September 25, 2014.

University of Maryland astronomers said they used a “quirk” of light that happens when a planet transits, or passes in front of, its host star. They wrote in a press release:

Material in the planet’s atmosphere absorbs some of the star’s light, and that makes the planet appear bigger. By plotting changes in the exoplanet’s size, and relating them to the wavelength of electromagnetic radiation that the telescope observes, astronomers get a graph that shows how much of the star’s radiation the planet’s atmosphere is absorbing. The shape of that graph, called a transmission spectrum, can reveal what chemicals are present in the atmosphere.

In recent years, astronomers have found water vapor in the atmospheres of some larger exoplanets, for example, the Jupiter-sized worlds Tau Boötis b and HD 189733b – 51 and 63 light-years away, respectively. That’s because, these astronomers say:

The bigger the planet, the more obvious are the changes in the planet’s size during its transit across its host star.

But HAT P-11b (discovered by the Hungarian-made Automated Telescope – or HAT – network) is only about four times Earth’s radius and 26 times Earth’s mass. Of the planets in our solar system, it’s closest in size to Neptune. These astronomers were able to find water vapor for such a small planet via observations with two NASA telescopes – the Hubble Space Telescope, which measures visible and near-infrared light, and the Spitzer Space Telescope, which records only infrared light — between July 2011 and December 2012. The team compared those data to observations of HAT-P-11b’s portion of the sky by NASA’s Kepler Space Telescope.

HAT P-11b is much closer than Earth or Neptune to its host star. That means it’s much hotter, about 878 kelvins, or 1,120 degrees Fahrenheit. These astronomers say this distant world probably has a rocky core, wrapped in a thick, gaseous envelope of about 90 percent hydrogen. They say its atmosphere is cloudless at high altitude.

The cloudless upper atmosphere of HAT P-11b is what enabled astronomers to find the evidence of water vapor here. A similar detection on other small planets has been obscured by clouds.

There are several reasons these astronomers are so excited about this discovery. On our world, water is a precondition for life, although the presence of water vapor – or even surface water – on an alien world would not necessarily mean that life exists there.

Astronomers are curious people, and they also want to know how our own solar system – and distant solar systems – formed. Our ideas about the formation of solar systems came mostly from observing our own sun and planets. They say that finding water vapor on a distant world like HAT P-11b is:

… a key piece of the puzzle … consistent with astronomers’ main ideas on the formation of planets.

Read more about HAT P-11b via University of Maryland

In this artist's depiction, the Neptune-sized planet HAT-P-11b crosses in front of its star in the constellation Cygnus. Astronomers observe such crossings, or transits, to glean information about the atmospheres of distant planets. The blue part of the planet's rim is due to scattered light. The orange band in front of the star shows the region where water vapor was detected.  Image via NASA/JPL-Caltech

In this artist’s depiction, the Neptune-sized planet HAT-P-11b crosses in front of its star in the constellation Cygnus. Astronomers observe such crossings, or transits, to glean information about the atmospheres of distant planets. The blue part of the planet’s rim is due to scattered light. The orange band in front of the star shows the region where water vapor was detected. Image via NASA/JPL-Caltech

Bottom line: The exoplanet HAT P-11b – about the size of Neptune – has an atmosphere that is cloudless at high altitude, but appears to have water vapor.

Deborah Byrd

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