Using data from NASA's Kepler Space Telescope, a team of researchers, led by astronomers at San Diego State University, detected the new planet the size of Neptune to Saturn orbiting between two previously known planets.
With its three planets orbiting two suns, the Kepler-47 is the only known multi-planet circumbinar system. Circular planets are those that orbit two stars.
The planets of the Kepler-47 system were detected by the "transit method". If the planet's orbital plane is aligned above, seen from Earth, the planet can pass in front of host stars, leading to a measurable decrease in observed brightness. The new planet, dubbed Kepler-47d, was not detected earlier due to weak traffic signals.
As is common with circling planets, the alignment of the orbital planes of the planets changes with time. In this case, the orbit of the middle planet became more aligned, leading to a stronger traffic signal. The depth of the transit went from undetectable at the beginning of the Kepler mission to the deepest of the three planets over just four years.
SDSU researchers were surprised by 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 suggestion for a third planet in 2012, but with just one traffic we needed more data," said SDSU astronomer Jerome Orosz, the study's lead author. "With additional traffic, the planet's orbital period could be determined, and then we could find more transits that were hidden in the noise in the previous data."
William Welsh, an SDSU astronomer and co-author of the study, said he and Orosz expected any additional planets in the Kepler-47 system to be in orbit outside previously known planets. "We certainly did not expect it to be the largest planet in the system. That was almost shocking," Welsh said. His research was recently published in the Astronomical Journal.
With the discovery of the new planet, a much better understanding of the system is possible. For example, researchers now know that the planets of this circumbinarian system have very low density – smaller than that of Saturn, the lowest-density solar system in the solar system.
Although low density is not so uncommon for hot-Jupiter type exoplanets, it is rare for moderate-temperature planets. The equilibrium temperature of Kepler-47d is approximately 50 o F (10 o C), while that of Kepler-47c is 26 o F (32 o C). The most intimate planet, which is the smallest known circling planet, is a much warmer 336 F (169 o C).
The inner, middle, and outer planets are 3.1, 7.0, and 4.7 times the size of the Earth, and take 49, 187, and 303 days, respectively, to orbit around their suns. The stars orbit in just 7.45 days; one star is similar to the Sun, while the other is one-third the mass of the Sun. The entire system is compact and would fit inside the Earth's orbit. It is approximately 3340 light-years away in the direction of the constellation Cygnus.
"This work is based on one of Kepler's most interesting discoveries: densely packed and low-density planetary systems are extremely common in our galaxy," said Jonathan Fortney, an astronomer at the University of California, Santa Cruz, who was not part of the study. . "Kepler47 shows that whatever process these planets make-an outcome that did not happen in our solar system-is common to single-star and circumbinary planetary systems."
This work was supported in part by grants from NASA and the National Science Foundation.
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