Two new puffy hot Jupiter exoplanets discovered

Gas giants in close orbits around their stars tend to swell toward the end of the stars' lives.
By Laurel Kornfeld | Feb 08, 2018
Two puffy, inflated hot Jupiter exoplanets have been discovered, both of which are orbiting stars approaching the ends of their lives.

Both planets were found by NASA's Kepler extended planet hunting mission, known as K2, and each one is larger than computer models predict it should be.

EPIC 229426032 b is about 63 percent bigger than Jupiter and has 1.36 Jupiter masses. It orbits its parent star, a 2.55-billion-year-old F6V spectral type 40 percent larger and 30 percent more massive than our Sun, once every 2.18 days at a distance of 0.036 AU (astronomical units, with one AU equal to the average Earth-Sun distance or 93 million miles).

The star is located about 1,500 light years from Earth. Based on the planet's low density, scientists believe it is inflated in size.

EPIC 246067459 b, which also has a low density, has about 0.86 Jupiter masses and a radius of 1.3 Jupiter radii. It orbits its star every 3.2 days. Approximately 5.6 billion years old, that star, a G2V spectral type, is 60 percent larger and 20 percent more massive than the Sun. The planet orbits at a distance of 0.046 AU from the star.

This system is approximately 1,480 light years from Earth, and its planet also has an inflated size.

"For EPIC 246067459 b, we find its radius to be consistent with the models of Fortney et al. (2007) for a hydrogen and helium dominated planet with no core," a team of researchers led by Maritza Soto of the University of Chile in Santiago, who studied both planets wrote in a paper published in the journal Earth and Planetary Astrophysics.

They propose the planet has a high metal content in either its core or envelope in spite of its inflated size, which makes it look like a world composed largely of hydrogen and helium.

Astronomers know planets puff up when their parent stars approach the ends of their lives, but they are uncertain as to why this happens. Planets' inflation at this stage could be caused by energy deposited by the host star or by something inhibiting the cooling process the planets would normally undergo at this stage.

Hot Jupiters, gas giants that orbit their stars in periods of 10 days or less, often appear inflated. Further study of these worlds will help scientists better understand the mechanisms that cause this inflation along with the physics of these planets' atmospheres and the processes by which they evolve.

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