K2 finds small gas giant exoplanets orbiting bright stars

Brightness of the stars and planets' orbital positions make them ideal targets for atmospheric studies.
By Laurel Kornfeld | Sep 28, 2018
K2, NASA's exoplanet-hunting space telescope now in its extended mission, has discovered two small gas giant planets, each orbiting bright stars larger and more massive than the Sun.

A research team led by Liang Yu of the Massachusetts Institute of Technology (MIT) discovered the two gas giants while studying K2 data. Their findings were subsequently confirmed by several ground-based telescopes, based on photometric and spectroscopic observations.

"We report the discovery of two planets transiting the bright stars HD 89345 in K2 Campaign 14 and HD 286123 in K2 Campaign 13," in a paper published in the journal Earth and Planetary Astrophysics.

HD 89345, a 5.3-billion-year-old star of the spectral type G5V-G6V, is located 413 light years away and is 66 percent larger and 22 percent more massive than the Sun, with an effective temperature of 5,609 degrees Kelvin.

The planet orbiting it, HD 89345 b, is in a close orbit just 0.11 AU (astronomical units, with one AU equal to the average Earth-Sun distance or 93 million miles) from the star, which it circles every 11.8 days.

With 0.1 Jupiter masses and a radius of 0.61 Jupiter radii, this low-density gas giant is viewed by astronomers as a warm "sub-Saturn" planet

HD 286123 is a more massive star located 434 light years away. It is 6.5-billion years old and of the spectral type F9V-G0V, about eight percent more massive than the Sun with 1.25 solar radii. This star's effective temperature is 5,855 degrees Kelvin.

While its planet, HD 286123 b, has a radius of 1.08 Jupiter radii, it is nearly 60 percent less massive than Jupiter. It is also in a close orbit around its parent star, located about 0.1 AU from the star, with an orbital period of 11.2 days. Scientists consider it a warm, low-mass, low-density Jupiter-type planet.

Given the ages of their parent stars and their close orbits, both exoplanets may be ideal for studying gas giant evolution and migration within stellar systems.

"Both planets have spent their entire lifetimes near the proposed stellar irradiation threshold at which giant planets become inflated, and neither shows any sign of radius inflation. They probe the regime where inflation begins to become noticeable and are valuable in constraining planet inflation models," the researchers wrote.

Because both orbit bright stars, they make good targets for study of their atmospheres via transit spectroscopy, the scientists noted.

K2, the re-purposed Kepler mission begun after two of the telescope's reaction wheels failed in 2013, has discovered more than 300 confirmed exoplanets.

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