Multi-planet systems are extremely rare, and none with more than three planets have ever been found, until now.
Credit: NASA/Kepler mission/Wendy Stenzel
SYDNEY: A six-planet system has been discovered by NASA's space-telescope Kepler, and is the first known transiting planetary system beyond our own that has more than three planets orbiting a sun.
The planets are much smaller than many that have been discovered outside of our Solar System, which are often referred to as ‘hot Jupiters’ – a class of large planets that have a close proximity to their parent stars.
“These smaller planets may be somewhat like small Neptunes,” said Jonathan Fortney of the Department of Astronomy and Astrophysics at the University of California, Santa Cruz, co-author of the study published in Nature.
A dip in brightness reveals planets
The Kepler space-telescope finds planets by observing the ‘dip’ in a star’s brightness that occurs when a planet passes between its sun and Earth. By observing multiple passes or ‘transits’, scientists can determine qualities about the planets such as their radius and year-length.
Kepler has been searching for planets since 2009, and in that time it has discovered about 1,200 planet candidates, and nine confirmed planets. However multi-planet systems have remained elusive, and none with more than three planets have ever been found.
Using the Kepler telescope, Fortney and colleagues have found five confirmed planets transiting the star known as Kepler-11, with a sixth planet-candidate likely to be confirmed as a planet with further observations. The system is 2000 light years from Earth.
Flatter than an LP record
The fact that there are multiple planets has allowed the planets’ masses to be calculated, something that cannot be done when only one planet is present. “The masses were derived using a very recent technique of watching the planets gravitationally interact, which changes the time between a given planet’s transits,” said Fortney. “The gravity of the planets change each other’s orbits.”
Another remarkable feature of the system is its flatness. “For Kepler, we must see planets that are well aligned – all in the same plane,” explained Fortney. “This system is as flat as an LP record. Flatter than a CD, and flatter than our Solar System.”
The inner five planets have year-lengths between 10 and 47 Earth days, making them all closer to their sun than the planet Mercury in to ours. The sixth planet orbits a little more sedately, taking just over 118 Earth days to complete its year.
