



Astronomers working on NASA’s Kepler Mission have discovered two planets of very different composition and densities, locked in extremely close orbits to each other around their parent star.

One of the planets is a rocky ‘super-Earth’ about 1.5 times the size of our planet and 4.5 times the mass. The other is a ‘Neptune-like’ gaseous planet 3.7 times the size of Earth and eight times the mass. Following their very similar orbits, the planets get 30 times closer than any pair of planets in our solar system, at a distance where each planet would appear considerably larger in each other’s sky than the Moon appears from the Earth.

The star system that these planets were discovered in is called Kepler-36, and is located around 1,200 light years from Earth. It’s a perfect example of extrasolar planets breaking radically with the patterns of our solar system. In ours, and it was thought everywhere, rocky planets orbit close to the sun and gas giants orbit much farther away.

“The planetary system reported in this paper is another example of an ‘extreme’ planetary system that will serve as a stimulus to theories of planet migration and orbital rearrangement,” researchers wrote in the paper.





“Small, rocky planets should form in the hot part of the solar system, close to their host star – like Mercury, Venus and Earth in our Solar System. Bigger, less dense planets – Jupiter, Uranus – can only form farther away from their host, where it is cool enough for volatile material like water ice, and methane ice to collect. In some cases, these large planets can migrate close in after they form, during the last stages of planet formation, but in so doing they should eject or destroy the low-mass inner planets,” Steve Kawaler, an Iowa State University professor of astronomy and physics is quoted as saying. Along with other researchers he created a description of the host star, measuring changes in the star’s brightness to get an exact idea of it’s size, mass, and age.

“Here, we have a pair of planets in nearby orbits but with very different densities. How they both got there and survived is a mystery.”

The discovery was possible because of NASA’s Kepler Mission. Kepler is a spacecraft launched in 2009 to find Earth-like planets that could potentially host life as we know it. The spacecraft uses a photometer to measure changes in the brightness of alien stars. These tiny variations in the brightness of a star are an indication of a possible orbiting planet. And also, with further observation, lead to knowledge of a planet’s size, orbit, and composition.



Data from that photometer is also used by the Kepler Asteroseismic Investigation to study star oscillations (changes in brightness), which give insight into a star’s internal structure. The research is led by a four-member steering committee: Kawaler, Chair Ron Gilliland of the Space Telescope Science Institute based in Baltimore, Jorgen Christensen-Dalsgaard and Hans Kjeldsen, both of Aarhus University in Aarhus, Denmark.

The Kepler spacecraft was essential to the discovery of “this puzzling pair of planets,” according to Kawaler.

“The seismic signal is very small, and only Kepler has the sensitivity and persistence to reveal it,” Kawaler said. “Also, the transit signal from the planets crossing in front of the star is very small, and only visible with Kepler’s level of sensitivity.”

Source: Iowa State University

Image Credits: Harvard-Smithsonian Center for Astrophysics/David Aguilar, NASA