Scientists have discovered water vapor on a planet the size of Neptune located 124 light-years from Earth, marking the first such discovery in the atmosphere of a planet outside our solar system that is smaller than Jupiter.
Called HAT-P-11b, the exoplanet -- as alien planets are called -- is about four times the size of Earth. It was first discovered in the constellation Cyngus in January of 2009 using astronomy's most effective detection tool, a technique called the transit method in which exoplanets are seen and studied primarily through light they absorb as they pass in front of their local star. Like a planetary silhouette, an exoplanet affects the light output of the star behind it as it transits, letting astronomers deduce the planet's mass and size even though it's impossible to view it with a telescope.
Now, that very same transmit method has let astronomers peak inside the air one might breathe if they stepped foot on HAT-P-11b, yielding the discovery of water vapor on the smallest planet yet and unlocking a deeper understanding of the formation of planets and the possibility of life outside the solar system.
The researchers -- a international team consisting of astronomers from Maryland, California and Seattle as well as the United Kingdom, Chile and Switzerland -- published their findings in the journal Nature on Wednesday.
The astronomers were able to achieve the breakthrough for the first time using data collected from the Hubble Space Telescope, Spitzer Space Telescope and Kepler spacecraft. The technique, called transmission spectroscopy, can be done only as the planet transits and lets you analyze the chemical composition of an exoplanet's atmosphere as sunlight is absorbed by present atoms and molecules.
Prior to the discovery, planets of this size with warmer atmospheres contained little hydrogen, meaning they were shrouded in haze and hard-to-study clouds that made studying the atmosphere difficult. Water vapor has traditionally been found in larger, gassier giants as large or larger than Jupiter.
HAT-P-11b, however, has both trace amounts of water vapor as well as large amounts of hydrogen, which gave the astronomers the right conditions to study its atmosphere. It is still unclear what factors contribute to the formation of atmospheres that may form to contain water and possibly even life. But such discoveries are helping scientists better understand the role those compounds -- alongside factors like mass and proximity to a star -- play in dramatically affecting the multibillion-year evolution of planets.
"In the long run, if we can detect water, methane, carbon monoxide, carbon dioxide, etc., in dozens to hundreds of exoplanet atmospheres of various bulk properties, then we will be able to paint a much clearer picture of how planets form, and, likewise, how Earth formed," lead author Jonathan Fraine, a graduate student at the University of Maryland, told Space.com. "This was just one of the beginning brush strokes to painting the full picture of how planets, as well as ourselves, were formed."