Sunday, March 20, 2016

Searching for Water on Alien Worlds

Astronomers detected water vapor in the atmosphere of a Neptune-sized extrasolar planet known as HAT-P-11b. Credit: NASA/JPL-Caltech

Water is crucial for life as we know it and could be also essential for alien life forms on other worlds. In their search for extraterrestrials, scientists are “following the water” hoping that it would unveil traces of microorganisms on the surfaces of habitable exoplanets. Even water vapor in the atmosphere could yield important information helpful in this permanent search. Although we haven’t found any alien life forms so far, we more often detect water vapor in the atmospheres of giant extrasolar worlds. So does it mean that water is common in exoplanet atmospheres?

“So far we have mainly studied large gas giant planets, and the answer for those appears to be yes. The planets are quite hot, so the water is in the gaseous form in the atmospheres,” David Sing of the University of Exeter, UK, told Astrowatch.net.

Sing is an astrophysics researcher studying transiting exoplanets. His research involves taking advantage of transit events to gather and study detailed information about the planet, such as composition, temperature and atmospheric structure.

He noted that from past studies we could tell water was in the planets, but some planets appeared to have much less water than others and it wasn’t clear as to why. Now we know that it is clouds covering the water that is there, and water is likely common on all the hot gas giant planets.

“Water is common on the gas giant exoplanet atmospheres that we have studies so far. These planets are usually hot (over 800 K) to often very hot (1,000-3,000 K), so water is in gas phase. That water is present on most of the exoplanets is not surprising, what would be surprising is not to find water. This is because water is made of hydrogen and oxygen, the most abundant species in the universe,” Jean-Michel Desert of the University of Colorado Boulder, told Astrowatch.net.

Desert together with Sing recently studied the diversity of exoplanet atmospheres. They tried to find out why some of Jupiter-sized planets seem to have less water than expected—a long-standing mystery. The researchers made the largest ever study of hot Jupiters, exploring and comparing ten such planets in a bid to understand their atmospheres. Their study revealed that, while apparently cloud-free exoplanets showed strong signs of water, the atmospheres of those hot Jupiters with faint water signals also contained clouds and haze—both of which are known to hide water from view.

According to Desert, currently the best method to detect water in exoplanet atmospheres is to do spectroscopy and look for spectral signature that are exhibited when water is in gas phase. These signatures correspond to strong absorption due to rotational or vibrational modes of the agitated water molecules present in the exoplanet atmosphere.

“For example, one of the strong spectral band signature of water covers the wavelength bandpass between one and two microns - with a characteristic spectral shape. This same strong absorption is present in the Earth atmosphere, since there are also water molecules in our atmospheres, and precludes astronomical observations between one and two microns. Thus, to observe in this bandpass, we need to go above the Earth atmosphere, in space, and the Hubble space telescope is a great instrument to do so,” Desert explained.

When exoplanets orbit close to their parent stars, they are very hot (about 2,000 K) and turn fast around their host stars, so we can detect molecular spectral signature by looking at the spectral position of the absorption lines at their expected position, taking into account the planet high orbital velocity to separate the exoplanets to the host stars spectral lines. Since there are millions of absorptions lines for water over the near-Infrared bandpass, this is done statistically.

Sing and Desert noted that future space telescopes like the James Webb Space Telescope (JWST) and the Transiting Exoplanet Survey Satellite (TESS) should be very helpful in the search for water and biosignatures on alien planets, especially smaller ones.

“JWST and TESS should be very helpful in searching for water in much smaller even potentially rocky exoplanets. Finding organic compounds and traces of life will be extremely difficult for JWST, as it was never designed for that purpose, but arguably it will be the telescope where these searches will begin,” Sing said.

Desert concluded that TESS will be dedicated to finding planets transiting bright stars so that we can study their atmospheres with JWST. So water, organic species, and biosignatures will be looked for in exoplanet atmospheres with this facility within the coming five years.

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