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Giant Exoplanet, Three Times The Mass Of Jupiter, Discovered In Dynamic Planetary System

clockApr 30 2020, 19:39 UTC

The newly discovered exoplanet Kepler-88 d has an orbit outside of the other planets in the system.W. M. Keck Observatory/Adam Makarenko

Just over 1,200 light-years away lies the Kepler-88 planetary system. Two of the sun-like star’s orbiting planets are already well-known. In fact, the relationship between Kepler-88 b and Kepler-88 c’s orbital periods and its dynamic impact has ultimately led to the system being dubbed “the King of transit variations” – more on that later.

But to add another dimension to Kepler-88’s existence, a team of astronomers led by the University of Hawai’i Institute for Astronomy (UH IfA) have discovered a new planet three times the mass of Jupiter. Planet Kepler-88 d was found to elliptically orbit its star every four years, based on six years of observations using the High-Resolution Echelle Spectrometer (HIRES) instrument on the 10-meter (33-foot) Keck I telescope, located on the summit of dormant volcano Maunakea, Hawaii.


Kepler-88 d now holds the title for the system’s most massive planet, beating previous record-holder Kepler-88 c, which “only” weighs in at around one Jupiter mass. The researchers believe this new planet’s mass may have proved influential in the system’s past.

Kepler-88 Planetary System from Keck Observatory on Vimeo. W. M. Keck Observatory/Adam Makarenko

“The high mass and eccentricity of Kepler-88 d indicate that it has likely been an important dynamical component in this planetary system’s history,” the authors wrote in a paper announcing their findings, published in The Astronomical Journal.

However, as previously mentioned, Kepler-88’s other planets have already garnered interest from the scientific community. As Kepler-88 c’s orbital period is twice that of its much smaller sub-Neptune sized companion Kepler-88 b (22 days and 11 days respectively), the planets have struck up a bizarre dynamic known as mean motion resonance. This is somewhat akin to a parent pushing a child on a swing, after every two laps that planet b makes, planet c gives it a “boost” in an energetically efficient process.


Whilst a parent keeps its child steady on the swing after a push, planet c’s comparatively large mass throws the inner planet off course, dramatically changing its orbital timings. As Kepler-88 b transits its star (as seen from Earth), these variations can be quantified by tracking the times at which the transits occur. Observations showed that the planet’s orbit was impacted by up to half a day due to Kepler-88 c’s periodic gravitational influence. This is one of the largest timing variations to be discovered, hence the system’s nickname “King of Transit Variations.”

Kepler-88 d’s existence only furthers the intrigue of how larger gas giants in planetary systems influence their surroundings.

“At three times the mass of Jupiter, Kepler-88 d has likely been even more influential in the history of the Kepler-88 system than the so-called King, Kepler-88 c, which is only one Jupiter mass,” Dr Lauren Weiss, a postdoctoral fellow at UH IfA and lead author on the discovery team, said in a statement. “So maybe Kepler-88 d is the new supreme monarch of this planetary empire – the empress.”

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