As we discover more exoplanets, many are more alien than we can possibly imagine. But occasionally, there’s an air of familiarity about a few of the discoveries. In new research showcased in two papers published in the journal Nature, the atmospheres of two exoplanets are described. Specifically, their cloud cover was observed, revealing that even exotic new worlds have grey days.
The two research papers focus on two exoplanets in our galactic neighborhood. One, a super-Earth, is called GJ 1214b and located ‘only’ 42 light-years away. The second world, a Neptune-sized exoplanet, is called GJ 436b and located 33 light-years away.
GJ 1214b is the smallest planet so far to have its atmosphere probed, but don’t get excited for cloud-filled alien vistas over a habitable landscape. This world, which is nearly three times the size (and 6.5 times the mass) of Earth, orbits its red dwarf star every 38 hours. Although red dwarf stars generate less energy than stars like our sun, the exoplanet’s racetrack orbit is so compact that its surface temperature is very toasty, ensuring a dense, hot atmosphere.
Previous observations in 2010 detected no atmospheric signature on GJ 1214b, even though astronomers know that it does possess one. As the world orbited in front of its star from our perspective (an event known as a “transit”), starlight appeared to not pass through any atmosphere as far as researchers could tell. That meant there were two possible conclusions: 1) GJ 1214b has an atmosphere thick with clouds that block the starlight from passing through the atmosphere or, 2) something more exotic is going on — possibly a thin, dense layer of heavy molecules (such as water) encapsulates the world; a layer that is too thin to be detected by instrumentation.
But now, using the Hubble Space Telescope, researchers have been able to discern what’s really going on: the exoplanet’s atmosphere is thick with clouds, blocking the starlight from view.
After studying 15 transits of GJ 1214b, infrared Hubble observations should have detected the spectroscopic signature of atmospheric water molecules if water was present, but it did not. Although water could still be present in the atmosphere, “there also have to be clouds” enshrouding the upper atmosphere, said lead researcher Laura Kreidberg, of the University of Chicago in Illinois.
As noted by Nature News, these clouds would be unlike anything we experience on Earth. Due to the hot, dense nature of GJ 1214b’s atmosphere, the clouds are likely rich with zinc sulphide or potassium chloride. These chemicals would be able to condense and form microscopic droplets, thus forming the cloud cover.
“It’s the first time that we’ve been able to characterize the atmosphere of an exoplanet smaller than Neptune,” said Kreidberg.
A similar technique, using Hubble data, was carried out in the GJ 436b study. Once again, spectroscopic data reveal no chemical signature of an atmosphere, an outcome that is completely unexpected from observations of a Neptune-class planet. The most likely explanation is that a thick layer of high-altitude cloud is blanketing the exoplanet, blocking the starlight from passing through the atmosphere, preventing a spectroscopic measurement from being made.
“We always knew the clouds must be there for some planets, but now we have a wave of results telling us that clouds are actually very common,” said planetary astronomer Heather Knutson, of the California Institute of Technology (Caltech) in Pasadena, Calif., and lead author of the GJ 436b study.