Distant world hosts water

Outside our solar system, this planet is the tiniest shown to have water

Artist’s idea of what the “water-rich” gas planet might look like. The orange glow of its sun (upper right in this picture) might give a reddish hue to the planet’s atmosphere. 

David A. Aguilar, CFA

The smallest, coolest exoplanet known to host water is roughly the size of Neptune. Until now, astronomers had found water only on distant worlds that are about the size of Jupiter. The newfound water source — known as HAT-P-11b — is just a bit more than four times the diameter of Earth.

And this distant planet’s temperature? It’s a very mild 605° Celsius (plus or minus 50°), or 1,121° Fahrenheit (plus or minus 90°). That’s hot enough not only to vaporize water but also to melt lead. So this would hardly be an inviting vacation destination.

Jonathan Fraine is an astronomer at the University of Maryland in College Park. He and his colleagues discovered the distant planet’s water after a year and a half of observations with the Hubble, Spitzer and Kepler space telescopes. The researchers shared their news Sept. 25 in the journal Nature.

Gases such as water vapor leave their mark in a planet’s atmosphere. They absorb particular colors (or wavelengths) of light. When HAT-P-11b comes between Earth and its star, the planet’s atmosphere filters out some of the starlight. The astronomers noticed the disappearance of some of that star’s infrared light each time the planet passed between it and Earth. The planet’s host star is an orange dwarf. It resides about 122 light-years away in the constellation Cygnus.

The new analysis showed the planet has a relatively clear atmosphere, rich in hydrogen. All of that hydrogen is consistent with theories of planet formation. They hold that giant planets made from gas initially formed around a rocky or icy core. That core would quickly have attracted an atmosphere by pulling hydrogen out of the gaseous disk encircling an infant star.

The new data “reveal the crystal-clear signature of water-vapour,” says Eliza M.R. Kempton. She’s an astronomer at Grinnell College in Iowa. “From the strength of the absorption,” she says, it appears “the planet’s atmosphere has a composition not dissimilar to those of the giant planets of our solar system.” It consists mostly of hydrogen, with trace amounts of heavier elements, she writes in a report published in the same issue of Nature. Those heavier elements include oxygen in the form of water vapor.

Power Words

astronomy    The area of science that deals with celestial objects, space and the physical universe as a whole. People who work in this field are called astronomers.

constellation  Patterns formed by prominent stars that lie close to each other in the night sky. Modern astronomers divide the sky into 88 constellations, 12 of which (known as the zodiac) lie along the sun’s path through the sky over the course of a year. Cancri, the original Greek name for the constellation Cancer, is one of those 12 zodiac constellations.

element  (in chemistry)Each of more than one hundred substances for which the smallest unit of each is a single atom. Examples include hydrogen, oxygen, carbon, lithium and uranium.

exoplanet  A planet that orbits a star outside the solar system.

extraterrestrial  Anything of or from regions beyond Earth.

hydrogen  The lightest element in the universe. As a gas, it is colorless, odorless and highly flammable. It’s an integral part of many fuels, fats and chemicals that make up living tissues.

infrared light  A type of electromagnetic radiation invisible to the human eye. The name incorporates a Latin term and means “below red.” Infrared light has wavelengths longer than those visible to humans. Other invisible wavelengths include X rays, radio waves and microwaves. It tends to record a hit signature of an object or environment.

Jupiter  (in astronomy) The solar system’s largest planet, it has the shortest day length (10 hours). A gas giant, its low density indicates that this planet is composed of light elements, such as hydrogen and helium. This planet also releases more heat than it receives from the sun as gravity compresses its mass (and slowly shrinks the planet).

light-year  The distance light travels in a year, about 9.48 trillion kilometers (almost 6  trillion miles). To get some idea of this length, imagine a rope long enough to wrap around the Earth. It would be a little over 40,000 kilometers (24,900 miles) long. Lay it out straight. Now lay another 236 million more that are the same length, end-to-end, right after the first. The total distance they now span would equal one light-year.

planet  A celestial object that orbits a star, is big enough for gravity to have squashed it into a roundish ball and it must have cleared other objects out of the way in its orbital neighborhood. To accomplish the third feat, it must be big enough to pull neighboring objects into the planet itself or to sling-shot them around the planet and off into outer space. Based on that definition, the International Astronomical Union has ruled that our solar system now consists of eight planets: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune (and not Pluto).

Neptune  The furthest planet from the sun in our solar system. It is the fourth largest planet in the solar system.

star  Thebasic building block from which galaxies are made. Stars develop when gravity compacts clouds of gas. When they become dense enough to sustain nuclear-fusion reactions, stars will emit light and sometimes other forms of electromagnetic radiation. The sun is our closest star.

telescope    Usually a light-collecting instrument that makes distant objects appear nearer through the use of lenses or a combination of curved mirrors and lenses. Some, however, use a network of antennas to collect radio emissions (energy from a different portion of the electromagnetic spectrum).

water vapor  Water in its gaseous state, capable of being suspended in the air.

wavelength  The distance between one peak and the next in a series of waves, or the distance between one trough and the next. Visible light — which, like all electromagnetic radiation, travels in waves — includes wavelengths between about 380 nanometers (violet) and about 740 nanometers (red). Radiation with wavelengths shorter than visible light includes gamma rays, X-rays and ultraviolet light. Longer-wavelength radiation includes infrared light, microwaves and radio waves.

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