One is a rocky planet 1.5 times the size of Earth. The other is a gaseous world nearly four times Earth’s size. Together they form a spectacular system in which two planets orbit closer to each other than any yet discovered.
"We’ve never known of planets like this," said Yale University astronomer Sarbani Basu, a member of the research team that analyzed the system. "If you were on the smaller planet looking up, the larger planet would seem more than twice the size of Earth’s full moon. It would be jaw-dropping."
Basu’s research focused on determining the properties of the planets’ host star -- work that was essential for discerning the characteristics of the orbiting planets.
The 46-member, international team, led by astronomers at Harvard and the University of Washington, report their discovery June 21 in Science Express, the early release version of the journal Science.
"These two worlds are having close encounters," said Josh Carter, lead author of the paper and a Hubble Fellow at the Harvard-Smithsonian Center for Astrophysics.
Located about 1200 light years away, the two-planet system -- now called Kepler-36 -- orbits a star similar to Earth’s sun, but bigger and older.
The larger outer planet, Kepler-36c, is a hot, gaseous, Neptune-like planet. The smaller inner planet, Kepler-36b, is rocky and subject to quakes and volcanic eruptions caused by the interplay of the planets’ gravitational forces on each other.
Like our sun, Kepler-36 pulsates constantly. Data on its quakes enabled the team to determine its size, weight, and age (all greater than those of our sun). Knowing the star’s radius and mass enabled the calculation of the sizes and masses of the planets. From this information, astronomers could determine the planets’ densities and characteristics: the smaller planet is denser than the Earth and hence must be rocky; the larger planet is much less dense, in fact less dense than water, suggesting it is gaseous.