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Meet what may be the largest carnival delights known to science: the 鈥渟uper-puff鈥� worlds of the Kepler 51 star system.听
As their confectionary name suggests, these planets are as lightweight as cotton candy鈥攍iterally. The fluffy globes are the lowest density exoplanets ever discovered beyond Earth鈥檚 solar system. And Jessica Libby-Roberts, a graduate student in the Department of Astrophysical and Planetary Sciences (APS) at the University of Colorado Boulder, wants to learn more about them.听
鈥淭hey鈥檙e very bizarre,鈥� said Libby-Roberts, who led听an effort to take the closest look yet at the planets.听
In a , she and her colleagues set out to uncover the components that make up the exoplanets鈥� atmospheres鈥攁nd to provide new clues around how such low-density planets may have formed in the first place.听
Top: An artist's depiction of the Kepler 51 star system; bottom: Kepler 51's three planets compared to the size of planets from our solar system. (Credits:听NASA/ESA/STScI)
鈥淭his is an extreme example of what鈥檚 so cool about exoplanets in general,鈥� said Zachory Berta-Thompson, an assistant APS professor and a coauthor of the new research. 鈥淭hey give us an opportunity to study worlds that are very different than ours, but they also place the planets in our own solar system into a larger context.鈥�
Candy shell
Berta-Thompson explained that super-puff planets are relatively rare in the galaxy鈥攆ewer than 15 have been recorded so far.听
Kepler 51鈥檚 trio, which was first described in 2014, take planetary puffiness to new levels. Their discovery was 鈥渟traight-up contrary to what we teach in undergraduate classrooms,鈥� Berta-Thompson said.
To get a closer look, he and Libby-Roberts used the Hubble Space Telescope to zoom in on the Kepler 51 star system. It鈥檚 located about 2,400 lightyears, or thousands of trillions of miles, from Earth and is a relative youngster at 500 million years old.听
The team鈥檚 first question: Were these planets for real?
The researchers developed new estimates for the trio鈥檚 masses and densities. And, sure enough, all three planets had a density less than 0.1 grams per cubic centimeter of volume鈥攁lmost identical to the sweet pink treats you can buy at any fairground, Libby-Roberts said.听
鈥淲e knew they were low density,鈥� she said. 鈥淏ut when you picture a Jupiter-sized ball of cotton candy鈥攖hat鈥檚 really low density.鈥�
The researchers also wanted to look deeper, using Hubble to peer into the planets鈥� atmospheres. That鈥檚 when they ran into trouble: The atmospheres of the super-puffs weren鈥檛 transparent at all. Instead, they appeared to be shrouded by a high-altitude layer of something opaque.听
鈥淚t definitely sent us scrambling to come up with what could be going on here,鈥� Libby-Roberts said. 鈥淲e expected to find water, but we couldn鈥檛 observe the signatures of any molecule.鈥�
Using computer simulations and other tools, the group theorized that the Kepler 51 planets are mostly hydrogen and helium by mass鈥攍ightweight gases that give these worlds their puffiness. That hydrogen and helium, however, also seems to be covered up by a thick haze made up of methane.听
In that sense, the exoplanets could resemble Saturn鈥檚 moon Titan, which is surrounded by a similarly carbon-rich smog.听
鈥淚f you hit methane with ultraviolet light, it will form a haze,鈥� Libby-Roberts said. 鈥淚t鈥檚 Titan in a nutshell.鈥�
Shrinking volume
The researchers made another observation, too鈥攐ne that indicated that these fluffy planets might not be so strange after all.听
Both Kepler 51 planets seemed to be shedding gas at a rapid pace. The innermost of the three worlds, for example, dumps an estimated tens of billions of tons of material into space every second. The group calculated that if that trend continued, the planets could shrink considerably over the next billion years, losing their cotton candy-like puffiness.
In the end, they might wind up looking more like a common class of exoplanets called 鈥渕ini-Neptunes.鈥�
鈥淧eople have been really struggling to find out why this system looks so different than every other system,鈥� Libby-Roberts said. 鈥淲e鈥檙e trying to show that, actually, it does look like some of these other systems.鈥�
Berta-Thompson agreed: 鈥淎 good bit of their weirdness is coming from the fact that we鈥檙e seeing them at a time in their development where we鈥檝e rarely gotten the chance to observe planets.鈥�
Other coauthors on the new study include researchers from the University of Amsterdam, Princeton University, the University of Texas at Austin, NASA Goddard Space Flight Center, California Institute of Technology, University of Chicago, University of California, Santa Cruz, Arizona State University and Netflix.
