"Twin" hot Jupiters may explain growth process of the largest gas giants

Stellar radiation could be the primary influence on planets' sizes and densities.
By Laurel Kornfeld | Nov 29, 2017
Two giant exoplanets found in data returned by NASA's K2 extended planet-hunting mission may help scientists better understand the process by which gas giants grow to extremely large sizes.

Both are "hot Jupiters," gaseous planets composed largely of hydrogen and helium, that orbit extremely close to their parent stars.

K2-132b and K2-97b orbit separate red giant stars but are nearly "twins" in that they have similar orbital periods, radii, and masses.

Red giants are Sun-like stars that have run out of hydrogen in their cores. They have inflated atmospheres, are extremely luminous, and have relatively cool temperatures of 5,000 degrees Kelvin or lower.

The largest gas giant planets can puff up to sizes bigger than those of the smallest stars. Their diameters are at minimum four times that of Earth, and their masses are similar to those of Jupiter and Saturn.

Since hot Jupiters were discovered, scientists have been puzzled as to how they become so large.

In a new study, University of Hawaii Institute for Astronomy graduate student Samuel Grunblatt looked through K2 data for hot Jupiters in close orbits around red giant stars, based on the theory that the sizes of these planets is related to the flow of heat in and out of their atmospheres.

Because growth of gas giants occurs over millions of years, scientists are unable to test theories that could potentially explain that growth, Grunblatt said.

He based his study on a theory by Eric Lopez of NASA's Goddard Space Flight Center, which proposed that hot Jupiters orbiting red giant stars become inflated by energy they receive from the stars.

Both K2-132b and K2-97b orbit their stars every nine Earth days and are approximately 30 percent larger than Jupiter.

By observing the planets with the W.M. Keck Observatory in Mauna Kea, Hawaii, Grunblatt found each to have approximately half Jupiter's mass in spite of their large sizes.

The researchers then used computer models to trace the lifetime evolution of both the parent stars and their giant planets and to calculate how efficiently the planets absorbed heat from their stars and conveyed that heat into their deep interiors.

Although the models confirmed the planets required radiation from the red giants to become larger, they also revealed the planets absorbed less stellar radiation than predicted.

Grunblatt's team plans to search for more hot Jupiters orbiting red stars and will conduct more detailed measurements of these two worlds using NASA's Spitzer Space Telescope.

"Studying how stellar evolution affects planets is a new frontier, both in other solar systems as well as our own. With a better idea of how planets respond to these changes, we can start to determine how the Sun's evolution will affect the atmosphere, oceans, and life here on Earth," Grunblatt emphasized.

Findings of the study have been published in the November 27 issue of The Astronomical Journal.

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