Two teams of scientists using NASA’s Hubble Space Telescope report they have characterized the atmospheres of a pair of planets with masses intermediate between gas giants, like Jupiter, and smaller, rockier planets, like Earth.

A survey by NASA’s Kepler space telescope mission previously showed that objects in this size range are among the most common type of planets in our Milky Way galaxy. The researchers described their work as an important milestone on the road to characterizing potentially habitable, Earth-like worlds beyond the solar system.

The two planets studied are known as GJ 436b and GJ 1214b. GJ 436b is categorized as a “warm Neptune” because it is much closer to its star than frigid Neptune is to our Sun. The planet is located 36 light-years away in the constellation Leo.

GJ 1214b is known as a “super-Earth” type planet. Super-Earths are planets with masses between that of Earth and Neptune. Because no such planet exists in our solar system, the physical nature of super-Earths is largely unknown. GJ1214b is located just 40 light-years from Earth, in the constellation Ophiuchus.

Both GJ 436b and GJ 1214b can be observed passing in front of, or transiting, their parent stars. This provides an opportunity to study these planets in more detail as starlight filters through their atmospheres.

An atmospheric study of GJ 436b based on such transit observations with Hubble over the last year is presented in one of the papers, led by Heather Knutson of the California Institute of Technology in Pasadena, Calif. The news is about what they didn’t find. The Hubble spectra were featureless and revealed no chemical fingerprints whatsoever in the planet’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.”

Evidence of high clouds

Observations similar to those obtained for GJ 436b had been previously obtained for GJ 1214b. The first spectra of this planet were also featureless and presented a similar puzzle: The planet’s atmosphere either was predominantly water vapor or hydrogen-dominated with high-altitude clouds.

A team of astronomers led by Laura Kreidberg and Jacob Bean of the University of Chicago used Hubble to obtain a deeper view of GJ 1214b that revealed what they consider definitive evidence of high clouds blanketing the planet. These clouds hide any information about the composition and behavior of the lower atmosphere and surface. The new Hubble spectra also revealed no chemical fingerprints whatsoever in the planet’s atmosphere, but the high precision of the new data enabled them to rule out cloud-free compositions of water vapor, methane, nitrogen, carbon monoxide, or carbon dioxide for the first time.

“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 that 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.”

Models of GJ 436b and GJ 1214b predict clouds that 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.

The Chicago team had to make a big effort to conclusively determine the nature of GJ 1214b’s cloudy atmosphere. Kreidberg explained, “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.

“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.”

---

Abstract of Nature paper (Laura Kreidberg et al.)

Recent surveys have revealed that planets intermediate in size between Earth and Neptune (‘super-Earths’) are among the most common planets in the Galaxy. Atmospheric studies are the next step towards developing a comprehensive understanding of this new class of object. Much effort has been focused on using transmission spectroscopy to characterize the atmosphere of the super-Earth archetype GJ 1214b, but previous observations did not have sufficient precision to distinguish between two interpretations for the atmosphere. The planet’s atmosphere could be dominated by relatively heavy molecules, such as water (for example, a 100 per cent water vapour composition), or it could contain high-altitude clouds that obscure its lower layers. Here we report a measurement of the transmission spectrum of GJ 1214b at near-infrared wavelengths that definitively resolves this ambiguity. The data, obtained with the Hubble Space Telescope, are sufficiently precise to detect absorption features from a high mean-molecular-mass atmosphere. The observed spectrum, however, is featureless. We rule out cloud-free atmospheric models with compositions dominated by water, methane, carbon monoxide, nitrogen or carbon dioxide at greater than 5σ confidence. The planet’s atmosphere must contain clouds to be consistent with the data.

Abstract of Nature paper (Heather A. Knutson et al.)

GJ 436b is a warm — approximately 800 kelvin — exoplanet that periodically eclipses its low-mass (half the mass of the Sun) host star, and is one of the few Neptune-mass planets that is amenable to detailed characterization. Previous observations have indicated that its atmosphere has a ratio of methane to carbon monoxide that is 10⁵ times smaller than predicted by models for hydrogen-dominated atmospheres at these temperatures. A recent study proposed that this unusual chemistry could be explained if the planet’s atmosphere is significantly enhanced in elements heavier than hydrogen and helium. Here we report observations of GJ 436b’s atmosphere obtained during transit. The data indicate that the planet’s transmission spectrum is featureless, ruling out cloud-free, hydrogen-dominated atmosphere models with an extremely high significance of 48σ. The measured spectrum is consistent with either a layer of high cloud located at a pressure level of approximately one millibar or with a relatively hydrogen-poor (three per cent hydrogen and helium mass fraction) atmospheric composition.