Although our solar system doesn't contain gaseous planets with thick atmospheres enriched with helium, observations by NASA's Spitzer Space Telescope suggests the galaxy may be strewn with these planetary oddities.
"We don't have any planets like this in our own solar system," said Renyu Hu, of NASA's Jet Propulsion Laboratory in Pasadena, Calif., in a news release. "But we think planets with helium atmospheres could be common around other stars." Hu is lead author of a new study published in the Astrophysical Journal.
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While hunting for a specific class of exoplanet, known as "warm-Neptunes," Hu's team realized that many of these worlds could be baked by their host stars so much that the majority of the hydrogen in their atmospheres would boil off, leaving an enriched helium atmosphere behind.
Many warm-Neptunes studied are roughly the same size or smaller than our solar system's blue-hued, hydrogen-rich "ice giant" of the same name. Neptune and Uranus are often referred to as ice giants due to their higher proportion of ices - such as water, ammonia and methane.
But warm-Neptunes, owing to their close proximity to their host stars, likely have very different compositions, chiefly the lack of hydrogen.
"Hydrogen is four times lighter than helium, so it would slowly disappear from the planets' atmospheres, causing them to become more concentrated with helium over time," said Hu. "The process would be gradual, taking up to 10 billion years to complete."
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Hu's team was led to this finding through the study of GJ 436b, a warm Neptune located over 33 light-years away, with Spitzer. When studying the thermal emissions from GJ 436b (also known as Gliese 436b), the researchers noticed something striking - the exoplanet's atmosphere had a distinct lack of methane.
Our own Neptune contains methane, a molecule that absorbs red light, creating the ice giant's beautiful rich blue hue we all know and love. But GJ 436b contains little to no methane, and yet is apparently rich in carbon. As methane contains one carbon atom linked with four hydrogen atoms (CH4), methane should be present - unless the world is missing a key component: hydrogen.
Instead of linking up with hydrogen, the carbon atoms are linking with oxygen, creating an enriched carbon monoxide (CO) and carbon dioxide (CO2) atmosphere, indicators that this class of exoplanet is abundant in helium, the next most abundant gas in the galaxy after hydrogen. Sure enough, Spitzer has deduced a chemical signature for carbon monoxide in GJ 436b's atmosphere, showing that there is a lack of hydrogen and likely an abundance of helium.
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Like the artist's impression above, warm Neptunes, robbed of their opportunity to create methane in their atmospheres, are likely pale and washed-out, very different to Neptune's "healthy" azure hue.
Although astronomers cannot currently directly look for helium in these exoplanets, they hope that NASA's upcoming James Webb Space Telescope may one day achieve this goal, and they hope to employ the Hubble Space Telescope to survey more of these warm Neptunes to seek out CO and CO2.
"Any planet one can imagine probably exists, out there, somewhere, as long as it fits within the laws of physics and chemistry," said co-author Sara Seager of the Massachusetts Institute of Technology in Cambridge and JPL. "Planets are so incredibly diverse in their masses, sizes and orbits that we expect this to extend to exoplanet atmospheres."