Cotton Candy Planets

 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.