Hubble finds cloudy weather on distant super-worlds

Artist's conception of the exoplanet GJ 1214b, which is known as a "super-Earth" type planet because it is slightly more than six Earth masses. Spectroscopic observations with NASA's Hubble Space Telescope provide evidence of high clouds blanketing the planet. Image Credit: NASA, ESA

Jim Sharkey

Researchers using NASA’s Hubble Space Telescope to study the atmospheres of two of the most common type of exoplanets in the Milky Way galaxy have discovered that they both may be covered in thick clouds. The two planets are in the middle range in size, between small rocky planets similar to Earth and gas giants like Saturn and Jupiter. Understanding the atmospheres of these kinds of exoplanets is an important milestone towards characterizing potentially habitable Earth-like worlds beyond the solar system. The Researchers published their findings in two separate papers in the Jan. 2 issue of the Journal Nature.

The planets studied were GJ 436b, a world classified as a “Hot Neptune” because it orbits its star much closer than the similarly sized but icy Neptune , and GJ1214b, a world called a “Super-Earth” because of it is slightly over six times the mass of the Earth. GJ 436b is located 36 light years from Earth in the constellation Leo and GJ 1214b is in the constellation Ophiuchus, about 40 light years from Earth. Both planets can be observed transiting their parent stars. This allows astronomers to study these planets in more detail as starlight filters through their atmospheres. This technique has been used by other researchers to find water vapor in the atmospheres of five exoplanets.

The group of scientists studying the atmosphere of GJ 436b was led by Heather Knutson of the California Institute of Technology in Pasadena, Calif. The team found that spectra observed by Hubble were totally featureless and lacking evidence of chemical fingerprints in GJ 436b’s atmosphere.

“Either this planet has a high cloud layer obscuring the view, or it has a cloud-free atmosphere that is deficient in hydrogen, which would make it very unlike Neptune,” said Knutson. “Instead of hydrogen, it could have relatively large amounts of heavier molecules such as water vapor, carbon monoxide, and carbon dioxide, which would compress the atmosphere and make it hard for us to detect any chemical signatures.”

Similar result had been found previously with GJ 1214b. The spectra for this were also featureless. Astronomers led by Laura Kreidber and Justin Bean of the University of Chicago used Hubble to take a closer look at GL1214. The team used 96 hours of telescope time on over a period of eleven months.

“We really pushed the limits of what is possible with Hubble to make this measurement — our work devoted more Hubble time to a single exoplanet than ever before. This advance lays the foundation for characterizing other Earths with similar techniques.” Added Bean, “I think it’s very exciting that we can use a telescope like Hubble that was never designed with this in mind, do these kinds of observations with such exquisite precision, and really nail down some aspect of a super-Earth atmosphere.”

Kreidber’s team found what they consider conclusive evidence of high clouds blanketing the planet and hiding its lower atmosphere and surface. While the new spectra were also absent of chemical fingerprints in GJ 1214b’s atmosphere they were precise enough to rule out cloud free compositions of water vapor, methane, nitrogen and carbon monoxide or carbon dioxide. It is possible that the clouds could be made out of potassium chloride or zinc sulfide at the scorching temperatures of several hundred degrees Fahrenheit predicted to be found in these atmospheres.

“You would expect very different kinds of clouds to form on these planets than you would find, say, on Earth,” said Kreidberg.

“Both planets are telling us something about the diversity of planet types that occur outside of our own solar system; in this case we are discovering we may not know them as well as we thought,” said Knutson. “We’d really like to determine the size at which these planets transition from looking like mini-gas giants to something more like a water world or a rocky, scaled-up version of the Earth. Both of these observations are fundamentally trying to answer that question.”

While the cloud layers blanketing CJ436b and CJ1214b currently obscure the lower atmospheres of the two worlds, things may become easier for astronomers when the Jame Webb Space Telescope is launched later this decade.

“Looking forward, the James Webb Space Telescope will be transformative. The new capabilities of this telescope will allow us to peer through the clouds on GJ 1214b and similar exoplanets,” Kreidberg said.

This article was written using information from a NASA press release, the Hubble Space Telescope’s website and UChicago news.

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