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Weird Worlds? Yes — by the Trillion

Illustration that compares sizes of various categories of exoplanets
The several thousand planets so far confirmed to be in orbit around other stars – exoplanets – fall into four broad categories: large, gas giants, Neptune-like worlds, "super Earths" bigger than Earth but smaller than Neptune, and terrestrial planets in Earth's size range. Within these categories, however, scientists find even more variety. Among the gas giants, for instance, are "hot Jupiters," infernal worlds with tight, star-hugging orbits.

A trip down the list of exoplanets found so far is a wild ride. These planets beyond our solar system, whether orbiting other stars or floating freely between them, can make the planets closer to home look tame by comparison. “Hot Jupiters” are star-hugging, infernal worlds. “Super Earths” are super mysterious. Frozen planets, gas giants that make Jupiter look puny, or small, rocky planets in Earth’s size range but in tight orbits around red dwarf stars – the catalog keeps growing, and soon, that growth will become exponential.

The more than 3,900 exoplanets confirmed so far are really a tiny sampling of what could amount to trillions in our galaxy. And they likely will be joined by tens of thousands more that are expected to be discovered by NASA’s TESS space telescope (the Transiting Exoplanet Survey Satellite).

Astronomers who analyze data from the last exoplanet survey, by NASA’s Kepler space telescope, can already paint a demographic portrait of what TESS will likely find.

According to NASA’s Exoplanet Archive, of the 3,924 exoplanets confirmed so far, 1,665 can be classed as “Neptune-like” – gaseous worlds around the size of Neptune. The rest of the breakdown:

  • 1,213 earn the title of gas giant, like Jupiter or Saturn.
  • 878 are classified as super Earths, a reference only to their size – larger than Earth and smaller than Neptune – but not suggesting they are necessarily similar to our home planet. The true nature of these planets remains shrouded in uncertainty because we have nothing like them in our own solar system – and yet, they are among the most common planet types found so far in the galaxy.
  • 156 of the confirmed exoplanets are considered terrestrial, that is, rocky planets about the size of Earth; further investigation will determine whether some of them possess atmospheres, oceans, or other signs of habitability.
  • 12 are simply classed as “unknown.” In other words, their presence has been detected by one of several indirect methods, but we know little else about them.

More variety is hidden within these broad categories. Hot Jupiters, for instance, were among the first planet types found – gas giants like Jupiter, yes, but orbiting so close to their stars that their temperatures soar into the thousands of degrees (Fahrenheit or Celsius). These large planets make such tight orbits that they cause a pronounced “wobble” in their stars, their gravity tugging them first this way, then that. That made them easier to detect in the early days of planet hunting.

Or consider the rogue planets: worlds hurtling alone through the galaxy, with no companion star. Many of these worlds could have been ejected from their original solar system, amid the gravitational jostling during the early phases of formation. The final “kick” could have come from another planet or even from the star itself.

The galaxy also seems to be home to a great many oddly sized planets, including those super Earths. Are they super-sized, rocky worlds, like scaled-up versions of planets in Earth’s size range? Or are they low-density worlds with puffy atmospheres? Further investigation is needed.

As if that weren’t enough, scientists also have noted what seems to be a strange gap in planet sizes. It’s been dubbed the Fulton gap, after Benjamin Fulton, lead author on a paper describing it. The Kepler data show that planets of a certain size-range are rare – those between 1.5 and 2 times the size of Earth. It’s possible that this represents a critical size in planet formation: Planets that reach this size quickly attract a thick atmosphere of hydrogen and helium gas and balloon up into gaseous planets, while planets smaller than this limit are not large enough to hold such an atmosphere and remain primarily rocky. On the other hand, the smaller planets that orbit close to their stars could be the cores of Neptune-like worlds that had their atmospheres stripped away.

Explaining the Fulton gap will require a far better understanding of how solar systems form.

As is often the case in science, the more we learn about the kaleidoscope of exoplanets, the more questions they provoke – and the more mysterious our universe becomes.

Pat Brennan is a science writer for NASA's Exoplanet Exploration Program. He joined JPL in 2015 after a 30-year career as a newspaper journalist.