A research team led by University of Hawaii astronomers has found two remarkably similar gas giants orbiting very close to their parent red-giant stars.
The gas giant K2-132b expands as its host star evolves into a red giant. The energy from the host star is transferred from the planet’s surface to its deep interior, causing turbulence and deep mixing in the planetary atmosphere. The planet orbits its star every 9 days and is located about 2,000 light years away from us in the constellation Virgo. Upper left: schematic of the K2-132 system on the main sequence. Lower left: schematic of the K2-132 system now. The host star has become redder and larger, irradiating the planet more and thus causing it to expand. Image credit: Karen Teramura, University of Hawaii Institute for Astronomy.
Gas giant planets are primarily made out of hydrogen and helium, and are at least four times the diameter of Earth.
Gaseous giants that orbit scorchingly close to their host stars are known as ‘hot Jupiters.’ These planets have masses similar to Jupiter and Saturn, but tend to be much larger — some are puffed up to sizes even larger than the smallest stars.
The unusually large sizes of these planets are likely related to heat flowing in and out of their atmospheres, and several theories have been developed to explain this process.
“However, since we don’t have millions of years to see how a particular planetary system evolves, planet inflation theories have been difficult to prove or disprove,” said team member Samuel Grunblatt, a graduate student at the University of Hawaii.
To solve this issue, Grunblatt and his colleagues from Australia, Canada, Denmark and the United States searched through data collected by NASA’s K2 Mission to hunt for hot Jupiters orbiting red giants.
These stars, which are in the late stages of their lives, become themselves significantly larger over their companion planet’s lifetime.
Following a theory put forth by Dr. Eric Lopez of NASA’s Goddard Space Flight Center, hot Jupiters orbiting red giant stars should be highly inflated if direct energy input from the host star is the dominant process inflating planets.
Grunblatt and co-authors have now revealed two planets, K2-132b and K2-97b, each orbiting their host star with a period of 9 days.
Using stellar oscillations to precisely calculate the radii of both the stars and planets, the astronomers found that the planets are 30% larger than Jupiter.
Observations using the W. M. Keck Observatory on Maunakea also showed that, despite their large sizes, K2-132b and K2-97b were only half as massive as Jupiter.
Remarkably, the two planets are near twins in terms of their orbital periods, radii, and masses.
Using models to track the evolution of K2-132b and K2-97b and their host stars over time, the scientists calculated the planets’ efficiency at absorbing heat from the star and transferring it to their deep interiors, causing the whole planet to expand in size and decrease in density.
The findings, published in the Astronomical Journal, show that these planets likely needed the increased radiation from the red giant star to inflate, but the amount of radiation absorbed was also lower than expected.
“It is risky to attempt to reach strong conclusions with only two examples,” the authors noted.
“But these results begin to rule out some explanations of planet inflation, and are consistent with a scenario where planets are directly inflated by the heat from their host stars.”
“The mounting scientific evidence seems to suggest that stellar radiation alone can directly alter the size and density of a planet,” they said.
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Samuel K. Grunblatt et al. 2017. Seeing Double with K2: Testing Re-inflation with Two Remarkably Similar Planets around Red Giant Branch Stars. AJ 154, 254; doi: 10.3847/1538-3881/aa932d
