The mysterious super puff planets are sometimes referred to as “cotton candy planets” because they sport the density of cotton candy. New data acquired from NASA’s Hubble Space Telescope (HST) have provided the first valuable clues to the chemistry of a duo of these puffy planets, which both reside in the Kepler 51 system. This particular exoplanet system actually contains a trio of super puffs in orbit around a youthful Sun-like star. The system itself was discovered by NASA’s planet-hunting Kepler Space Telescope in 2012. However, it was not until 2014 that the extremely low density of these “cotton candy” exotic worlds was determined–much to the amazement of many planetary scientists.
The recent HST observations enabled a team of astronomers to more precisely determine the size and mass estimates for these planets–independently validating their extremely low-density “puffy” character. Even though these strange “cotton candy” worlds are no more than several times our own planet’s mass, their hydrogen and helium atmospheres are so bloated that they are almost the size of our own Solar System’s banded behemoth Jupiter. Although the super puffs are almost Jovian in size, they are approximately a hundred times lighter in terms of mass.
How and why the atmospheres of these exotic super puffs expanded outward is unknown. However, their inflated atmospheres have rendered them especially fascinating targets for further atmospheric studies. Using HST, the team of astronomers went on the hunt for further clues.They were especially interested in searching for water in the atmospheres of the planets, dubbed Kepler 51 b and 51 d. HST observed the planets when they transited (passed in front of) the glaring face of their parent-star. The scientists were aiming to spot the infrared color of their sunsets–thus determining the quantity of light absorbed by the atmosphere in infrared light. This type of observation enables planetary scientists to search for the tattle-tale signs of the planet’s chemical constituents–such as water.
The HST astronomers were surprised to find that the spectra of both planets did not show any tattle-tale chemical signatures. The scientists attributed this result to clouds of particles floating high in their atmospheres. “This was completely unexpected. We had planned on observing large water absorption features, but they just weren’t there. We were clouded out,” commented Dr. Jessica Libby-Roberts in a December 2019 Hubble Observatory Press Release. Dr. Libby-Roberts is of the University of Colorado at Boulder.
Unlike Earth’s own water clouds, the clouds of the “cotton candy” planets may be made up of salt crystals or photochemical hazes, similar to those found on Saturn’s largest moon, Titan. Titan’s surface is blanketed by a thick golden-orange hydrocarbon smog.
The clouds belonging to both Kepler 51 b and 51 d stack up against other low-mass, gaseous planets situated beyond our Solar System. When comparing the flat spectra of the “cotton candy” planets against the spectra of other planets, the astronomers were able to devise a hypothesis proposing that cloud and haze formation are linked to the temperature of a planet–the cooler a planet is, the cloudier it becomes.
The astronomers also investigated the possibility that these planets were not really super puffs at all. The gravitational pull among planets causes slight changes to develop in their orbital periods. As a result of these timing effects planetary masses can be determined. By combining the variations in the timing of when a planet floats in front of the fiery face of its parent-star (transiting) with those transits observed by the Kepler Space Telescope, the scientists were better able to constrain the planetary masses and dynamics of the system. Their results proved to be in agreement with earlier measured ones for Kepler 51 b. However, they found that Kepler 51 d was slightly less massive (or the planet was even more puffy) than previously determined.
Finally, the team came to the conclusion that the low densities of these planets are in part the result of the young age of the system, which is a mere 500 million years old.. By comparison, our own Sun was born 4.6 billion years ago. Models indicate that these “cotton candy” planets formed outside of what is termed a star’s snow line. A star’s snow line is a region of possible orbits where icy materials can survive. The planets of this youthful system ultimately migrated inward towards their stellar parent, in a way that has been compared to a “string of railroad cars.”
With the planets now much closer to their star, their low density atmospheres should evaporate into space within the next few billion years. Using planetary evolution models, the team of astronomers demonstrated that Kepler 51 b–the planet closest to its star–will, in a billion years or so, look very much like a smaller and hotter version of our own Solar System’s Neptune. This particular type of exoplanet is fairly common throughout our Milky Way Galaxy. However, it appears that Kepler 51 d, which is farther from its parent-star, will continue to be a low-density oddball world–even though it will both shrink and lose some small amount of its puffy atmosphere. “This system offers a unique laboratory for testing theories of early planet evolution,” commented Dr. Zach Berta-Thompson in the December 2019 Hubble Observatory Press Release. Dr. Berta-Thompson is also of the University of Colorado at Boulder.