An exoplanet is a planet that is the offspring of a distant star, and resides outside our own Solar System. Some of these alien worlds resemble the planets inhabiting our Sun’s family, while others are so different that they are true “oddballs”–unlike anything astronomers have every observed in our Solar System. In the process of hunting for distant alien worlds beyond our Star, astronomers have come to the unavoidable conclusion that planets can be composed of almost anything. In December 2019, a team of astronomers announced their discovery of a completely new class of planet unlike anything ever seen before. These “puffed up” oddballs are so bloated that they are almost the same size as Jupiter, but only 1/100th its mass.
Mercury Venus, Earth, and Mars are the terrestrial planets inhabiting our own Solar System. In dramatic contrast, the most massive of the quartet of outer planets, Jupiter and Saturn, are both classified as gas giants. Uranus and Neptune, the two outermost of the enormous planets, are diffferent in composition from the gas giants, and are classified as ice giants.
The quartet of terrestrial planets, like our own Earth, are solid worlds that are primarily made up of silicate rocks or metals. All four worlds bask in the warm and well-lit inner region of our Solar System, and are relatively close to our Sun. They are situated between our roiling, broiling Star, and the Main Asteroid Belt that is located between Mars and Jupiter.
The enormous duo of gas giants, Jupiter and Saturn, are gas-laden worlds mostly made up of hydrogen and helium. Gas giants are sometimes referred to as “failed stars”. This is because they contain the same basic elements as a star.
In the 1990s, astronomers came to the realization that Uranus and Neptune are really a distinct class of planet, unlike their two much larger gaseous siblings. This beautiful bluish duo are both classified as ice giants. Ice giants are mainly composed of elements heavier than hydrogen and helium–which are the two lightest atomic elements. These two distant worlds are composed of heavier atomic elements such as oxygen, carbon, sulfur, and nitrogen.
“Cotton candy” alien worlds are currently referred to as Super Puffs. These puffy planets might represent a short-lived transitory phase in planet evolution. Because this phase is brief, it could explain why astronomers don’t see anything like them in our Solar System. It has been proposed that Super Puffs may have been born much farther from their stars, and then migrated inward towards the heat and warmth of their stellar parents. At this point, their low-density hydrogen and helium atmospheres fly off into the space between planets. In the future, much smaller planets might be left behind to tell their story.
Our Star’s Familiar Planets
As of December 1, 2019, there are 4,135 validated exoplanets inhabiting 3,073 systems, with 673 sporting more than one solitary planet. Some of these planets bear a close resemblance to those in our Star’s familiar family of major planets, while others are so exotic that astronomers never dreamed that such worlds could exist–until they were discovered.
The three classes of major planet in our own Solar System are distinct from one another. The quartet of inner terrestrial planets all display a solid surface, which makes them appear very different from the quartet of outer gaseous planets–both the two gas-giants and the two ice giants. The four larger outer planets contain some combination of hydrogen, helium, and water existing in an assortment of physical states.
All of our Solar System’s terrestrial planets sport the same basic type of structure. This means that all four small, rocky planets have a central metallic core, composed mostly of iron with a surrounding silicate mantle. Earth’s Moon is similar to the four major inner planets, but it has a much smaller iron core.
During the early years of our Solar System, when it was first in the process of forming, there were likely many more terrestrial planets. However, most of these ancient terrestrial planetesimals are thought to have collided and merged with one another–or were unceremoniously evicted from our Solar system altogether by the four existing terrestrial planets.
The two heavily gas-blanketed banded behemoths, Jupiter and Saturn, are almost entirely made up up hydrogen and helium, with heavier atomic elements amounting to 3 to 13 percent of the mass. The two gas-giant denizens of the outer Solar system are believed to be made up of an outer layer of molecular hydrogen surrounding a layer of metallic hydrogen. The enormous duo are also thought to have molten rocky cores. The outermost region of their hydrogen atmosphere is composed of numerous layers of visible clouds that are primarily made up of water and ammonia. The layer of metallic hydrogen accounts for the bulk of each of the two planets, and is referred to as “metallic” because the very large pressure causes hydrogen to morph into an electrical conductor. The giant duo’s cores are believed to consist of heavier elements at such extremely high temperatures that their properties are not well understood.
Uranus and Neptune are the two outermost giant planets, and they are primarily composed of elements that are heavier than hydrogen and helium. In astrophysics and planetary science the term “ices” refers to volatile chemical compounds with freezing points above about 100 K, such as water, methane, or ammonia, with freezing points of 273K, 91K, and 195K, respectively.
The constituent solids sported by the two ice giants were probably already solids when they were incorporated into the duo during their formation, either directly in the form of ices or trapped in water ice. Currently, very little of the water in Uranus and Neptune remains in the form of ice. Instead, water mostly exists as a supercritical fluid at the temperatures and pressures within the duo.
The ice giants are made up of only aboaut 20% hydrogen and helium in mass, in dramatic contrast to our solar system’s gas-giants, Jupiter and Saturn, which are both more than 90% hydrogen and helium in mass.
The Strange Case Of The “Super Puff” Planets
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.
It will be possible for astronomers to finally be able to determine the atmospheric composition of the duo of puffy planets with NASA’s upcoming James Webb Space Telescope (JWST). JWST will have a sensitivity to longer infrared wavelengths of light, and may be able to pierce through the cloud layers. Future observations with this telescope could shed new light on what these puffy oddballs are actually composed of, thus solving an intriguing mystery.