“Toffee Planets” Hint at Earth’s Cosmic Rarity

Robin George Andrews | Scientific American
Monday, April 8, 2019

Potential for super Earths with a malleable interior is a new concept that could change the way we evaluate new exoplanets.

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New research, led by Paul Byrne, a planetary scientist at North Carolina State University, presented at the 50th Lunar and Planetary Science Conference proposes that some super Earth exoplanets with strong gravitational pull may have stretchy, malleable, flowing rock in their interior up to nearly the surface. The authors justifiably have dubbed them "toffee planets." These features mean that the planets may be bereft of plate tectonics, oceans, and of complex life. "The science is already starting to stick in experts’ minds," writes Robin George Andrews for Scientific American. "'It’s a fascinating concept,' says Sara Seager, a professor of astrophysics and planetary science at the Massachusetts Institute of Technology. In exoplanetary science, 'you rarely see anything new like this. The fact that they came up with something new, that, in itself, is impressive.'”

Controversial Confectionary

The most contentious aspect of this thought-provoking study is not actually about any of the science. It is about the name for these possible exoplanets.

The team is composed of researchers hailing from the U.K. and Ireland. “To us,” Byrne says, “‘toffee’ means something soft and chewy.” Scientists from North America tend not to see the word that way, considering toffee to be a hard, crunchy treat. Byrne suggests “taffy” or “fudge” might be better, and a Twitter poll he ran put “squidgy” on par with “toffee” as participants’ preferred nomenclature.

Things have become even more complicated with the discovery by NASA’s Transiting Exoplanet Survey Satellite (TESS) of a brand-new confirmed rocky super Earth, HD 213885b. Byrne’s calculations suggest that this newfound world might be a toffee planet, with a brittle layer just more than two miles thick. The problem is that the radiation from HD 213885b’s parent star is akin to that of 55 Cancri e, another known rocky super Earth whose dayside is entirely molten.

“If HD 213885b is similarly hot, then any lack of rigidity at the surface won’t be from relatively higher surface gravity so much as the floor being lava,” Byrne says. It’s not quite a toffee planet, then, but something very close.

Maybe, he suggests, “fondue planets” are a thing, too.

Story Image: A speculative view from Kepler-36b, a potential “toffee planet” that may possess stretchy, almost taffy-like rock at or near its surface. (Credit: Center for Astrophysics, Harvard & Smithsonian and David Aguilar)