San Diego State astronomers discovered a third planet in the Kepler-47 system, securing the system’s title as the most interesting of the binary-star worlds.
Using data from NASA’s Kepler space telescope, a team of researchers led by astronomers at San Diego State detected in April 2019, the new Neptune-to-Saturn-size planet, dubbed Kepler-47d, orbiting between two previously known planets.
With its three planets orbiting two suns, Kepler-47 is the only known multiplanet circumbinary system. Circumbinary planets are those that orbit two stars.
The planets in the Kepler-47 system were detected via the “transit method.” If the orbital plane of the planet is aligned edge-on as seen from Earth, the planet can pass in front of the host stars, leading to a measurable decrease in the observed brightness. The new planet, dubbed Kepler-47d, was not detected earlier because of weak transit signals.
As is common with circumbinary planets, the alignment of the orbital planes of the planets change with time. In this case, the middle planet’s orbit has become more aligned, leading to a stronger transit signal. The transit depth went from undetectable at the beginning of the Kepler Mission to the deepest of the three planets over the span of just four years.
The SDSU researchers were surprised by both the size and location of the new planet. Kepler-47d is the largest of the three planets in the Kepler-47 system.
“We saw a hint of a third planet back in 2012, but with only one transit we needed more data to be sure,” said SDSU astronomer Jerome Orosz. His team’s research was recently published in the Astronomical Journal. “With an additional transit, the planet’s orbital period could be determined, and we were then able to uncover more transits that were hidden in the noise in the earlier data.”
Dr. William Welsh, SDSU astronomer and the study’s co-author, said he and Dr. Orosz expected any additional planets in the Kepler-47 system to be orbiting exterior to the previously known planets. “We certainly didn’t expect it to be the largest planet in the system. This was almost shocking,” Dr. Welsh said.
With the discovery of the new planet, a much better understanding of the system is possible. For example, researchers now know the planets in this circumbinary system are very low density—less than even that of Saturn, the solar system planet with the lowest density.
This work was supported in part by grants from NASA and the National Science Foundation.