Liquid water is essential for the existence of life as we know it, so it is always exciting to find traces of H2O in any form on an exoplanet. Using the Hubble's Wide Field Camera 3, astronomers discovered a Neptune-size exoplanet with water vapor in the atmosphere, though that does not mean it's necessarily habitable. Though it is quite small relative to other exoplanets that have been extensively studied, the planet’s clear skies aided its discovery. The research was led by Jonathan Fraine of the University of Maryland and the paper was published in Nature.
The planet in question, HAT-P-11b, sits about 120 light years away in the constellation Cygnus. It orbits its orange dwarf star every 5 days. This incredible proximity has allowed astronomers to obtain information as it transits. Astronomers have previously determined that the planet had a thick atmosphere surrounding the mantle and rocky core.
"When astronomers go observing at night with telescopes, they say 'clear skies' to mean good luck," Fraine said in a press release. "In this case, we found clear skies on a distant planet. That's lucky for us because it means clouds didn't block our view of water molecules.”
Using a technique known as transmission spectroscopy, Fraine’s team has been able to analyze the composition of the thick atmosphere. As the planet transits the star, light gets filtered through the atmosphere. Each element has a different wavelength of light it absorbs, so analyzing the light that gets filtered through the atmosphere is very telling about the composition. They discovered that hydrogen gas and water vapor are plentiful in HAT-P-11b’s atmosphere, in addition to other molecules that have not been identified and released yet.
HAT-P-11b is now the smallest exoplanet to have specific molecules within the atmosphere identified.
"We set out to look at the atmosphere of HAT-P-11b without knowing if its weather would be cloudy or not," co-author Nikku Madhusudhan added. "By using transmission spectroscopy, we could use Hubble to detect water vapor in the planet. This told us that the planet didn't have thick clouds blocking the view and is a very hopeful sign that we can find and analyze more cloudless, smaller, planets in the future. It is groundbreaking!”
There was a possibility that the water vapor they were picking up on was from cool spots in the star. The team verified that it was, in fact, coming from HAT-P-11b by comparing data collected over several years by Kepler, and infrared images from Spitzer. Ultimately, all three telescopes confirmed the water was from the planet.
Information about the atmospheric composition can provide clues about planetary formation, and will be useful when studying super-Earths and exoplanets closer to our planet’s size. It currently isn’t useful for finding water on the surface and determining habitability, but it’s an encouraging start to understanding more about the conditions of exoplanets.
"We are working our way down the line, from hot Jupiters to exo-Neptunes," explained Drake Deming, another co-author of the study. "We want to expand our knowledge to a diverse range of exoplanets.”
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