Rick Robinson

Jun 17th 2020

Cotton-Candy Exoplanet Mystery


Exoplanet research, the study of planets orbiting distant stars, has discovered more than 4,000 confirmed exoplanets, according to Universe Today, with nearly 5,000 more potential candidates awaiting confirmation. Those numbers are astronomical, considering our own solar system boasts a mere eight major planets.

Very few exoplanets can be directly imaged, even by our most powerful telescopes. Most are discovered, and then further studied, by a combination of indirect methods, such as measuring the slight dimming of a star when a planet passes directly in front of it, blocking out some of its light. This was the method used by the Kepler space mission to discover many of these planets, and now being used by its successor, NASA’s Transiting Exoplanet Survey Satellite (TESS).

Featherweight Worlds

Many of the known exoplanets, according to Digital Trends, have characteristics similar to those of the eight major planets in our solar system.

These fall into one of three broad groups:

  1. Relatively small and rocky planets like Earth and Mars, known as terrestrial planets
  2. Much larger planets resembling Jupiter or Saturn, called gas giants because they are believed to be mostly gaseous
  3. An intermediate group called ice giants, resembling Uranus and Neptune, made up largely of plain old ice and similar frozen materials

But some exoplanet discoveries have downright weird characteristics, unlike those of any of the familiar worlds of our solar system. Perhaps the oddest mysterious planets are the ones that have the density of cotton candy, which astronomers have informally dubbed super-puff planets.

A Whole Lot of Nothing?

This whimsical-sounding name refers to the planets’ most striking characteristic. They’re big — comparable in size to Jupiter and Saturn — but relative to their size they’re extreme featherweights, only a few times more massive than Earth. The implication is that their density must be extremely low, roughly a hundred times less dense than Jupiter, which itself is about five times less dense than rocky Earth.

Three planets orbiting a sunlike star called Kepler 51, some 2,600 light-years from Earth, are prime exemplars of the cotton candy planet phenomenon. According to EarthSky, all three are similar in size to Jupiter and Saturn, but only weigh in at a few times the mass of (relatively) tiny Earth.

Astronomer Jessica Libby-Roberts, who led a research team that studied them, sums them up succinctly: “They’re very bizarre.”

Origin Stories

How can a planet so tenuous even hold itself together? The researchers, in fact, found hints that at least one of them is not holding itself together: The innermost of the three worlds, notes EarthSky, is losing material at a rate of billions of tons every second.

It happens that Kepler 51 and its planets are relatively young, only about half a billion years old (compared to some 4.5 billion for our sun and solar system). To another member of the research team, Zachory Berta-Thompson, this suggests that “we’re seeing them at a time in their development where we’ve rarely gotten the chance to observe planets. This system offers a unique laboratory for testing theories of early planet evolution.”

Lords of the Rings

A separate research team offers another possible explanation: The super-puff planets may not be as big as they seem. Instead, they may be smaller planets surrounded by enormous ring systems.

In our solar system, notes, all four of the largest planets have rings, though only Saturn’s are extensive and bright enough to be a spectacular sight. And while we are a long ways from getting Saturn-style portraits of any exoplanet, astronomers should be able to indirectly detect planetary rings — and have been a bit puzzled at not finding any.

It may be, however, that we have detected exoplanet rings, without being quite able to recognize them as such. Our methods of detecting exoplanets are operating so close to their limits of precision that we cannot readily tell whether starlight is being blotted out by a giant planet or by rings surrounding a smaller planet. No existing telescope can tell the difference.

But that will change, says with the upcoming James Webb Space Telescope. It will allow observations so precise that we’ll be able to tell planets and their rings apart. Today’s cotton candy exoplanets will be mysterious planets no longer.

No need to be disappointed, though. For every mystery solved, we can be confident that the Webb, and other new satellites and instruments, will discover a host of new mysteries for us to puzzle over.