Kepler-289 c is a newly discovered low-mass, low-density planet in orbit around a Sun-like star. Its orbit lies between two other known planets: Kepler-289 b (inner planet) and Kepler-289 d (outer planet). Perturbations from its inner and outer planetary siblings cause the 66 day orbit of Kepler-289 c to vary by as much as ~5 hours. For comparison, the orbital period of Earth varies by only one second or so.
The non-periodic orbit of Kepler-289 c allows it to elude automated computer algorithms that search through the light curve data collected by NASA’s planet-hunting Kepler space telescope. In fact, the discovery of Kepler-289 c was made possible by enlisting the help of citizen scientists who visually scan through the light curve data collected by Kepler as part of the Planet Hunters program. Computers can’t find the unanticipated, but humans can.
Kepler-289 b, the inner planet, has an orbital period of 35 days, 7.3 times Earth’s mass, 2.15 times Earth’s radius and receives about 25 times more insolation than Earth gets from the Sun. Kepler-289 d, the outer planet, has an orbital period of 126 days, 132 times Earth’s mass, 11.6 times Earth’s radius and receives about 4.4 times more insolation than Earth gets from the Sun.
The middle planet, Kepler-289 c, has 4 times Earth’s mass and 2.7 times Earth’s radius, giving it a density of 1.2 g/cm³. Such a density is remarkably low for a planet of its mass. This requires about half the planet’s mass to be in the form of a substantial gaseous envelope of hydrogen and helium. Kepler-289 c joins a growing list of low-mass, low-density planets.
Kepler-289 b, c and d, are all too hot to be habitable. An interesting characteristic of the planetary system is that the three planets could be residing in a 1:2:4 Laplace resonance since the outer planet (Kepler-289 d) has an orbital period about twice as long as that of the middle planet (Kepler-289 c) and the middle planet’s orbital period is about twice as long as that of the inner planet (Kepler-289 b).
Schmitt et al. (2014), “Planet Hunters VII. Discovery of a New Low-Mass, Low-Density Planet (PH3 c) Orbiting Kepler-289 with Mass Measurements of Two Additional Planets (PH3 b and d)”, arXiv:1410.8114 [astro-ph.EP]