Kepler 18 - the packed system

Credit: McDonald observatory
The graphic shows the orbits of Kepler-18b, c, and d around their star compared to Mercury's orbit. The lower graphic shows the relative size of planets to their star and Earth to our Sun.

Kepler Space Telescope hunting for exoplanets found three planets orbiting a star, Kepler-18, which is 10% larger than our Sun but has only 97% of solar mass. There are still planets yet to discovered in this system. What is special for this system is that the planets are in resonant orbiting path.

Those three planets are called Kepler-18b, c, and d. They orbit their star at very close range, even closer than Mercury which is the closest planet in our solar system. Kepler-18b uses only 3.5 days to orbit around the star. It is approximately 6.9 times Earth's mass and two times larger than our Earth. Thus, it is called "super-earth"

Kepler-18c uses 7.6 days to orbit the star. It is 17 times Earth's mass and 5.5 times larger. Kepler-18d has a period of 14.9 days. It is 16 times Earth's mass and 7 times larger. From these information, Kepler-18c and d are categorized as Neptune-like with low density.

Kepler-18c orbits the star twice as Kepler-18d just finish its orbit. However, both planets do not transit the star with exact period. One planet will come early when another planet will come late, then they both come at their time, then vice-versa. It is like Kepler-18c and Kepler-18d are "dancing" together. They have effects on each other. When they come close to each other, they will exchange energy and push/pull each other.

The activity of Kepler-18c and Kepler-18d suggests that both of them should be in the same system. However, the characteristic of Kepler-18b as a super-earth is more complicated. The researchers use the technique planet validation, which they examined the probability that an object could be something other than a planet, like background star. At first, the researchers use high-resolution images of the space around Kepler-18 star to see if the transit signal could have been stimulated by background objects. The result turns out to be that Kepler-18b has a probability of being a planet 700 times more than being a background body.

From:

space.com: 3 weird alien planets found around Sun-like star

sciencedaily.com: Kepler spacecraft discovers new multi-planet solar system