January 25, 2013
Two Giant Planets, Companion Star Discovered In Known Planetary System
Lee Rannals for redOrbit.com — Your Universe Online
Astronomers discovered that the HAT-P-7 planetary system includes at least two giant planets and one companion star.The discovery of a previously unknown companion, HAT-P-7B, to the central star, HAT-P-7, as well as confirmation of another giant planet, HAT-P-7c, orbiting outside of the retrograde planet HAT-P-7b offers up new insights into how such planets may form.
Astronomers used the Subaru Telescope to discover the first evidence of a retrograde orbit of an extrasolar planet back in 2008. Since this discovery, they pursued a question to try and explain HAT-P-7b's origin, which sits about 1,040 light years away from Earth in the constellation Cygnus.
The astronomers took high contrast images of the system with the High Contrast Instrument for the Subaru Next Generation Adaptive Optics (HiCIAO) to develop a more complete picture of it.
They first discovered two companion candidates around the system in 2009, and measured their proper motion over a three-year period until 2012. They confirmed that one of the two candidates is a common proper motion stellar companion to HAT-P-7, known as HAT-P-7B.
The team confirmed a long-term radial velocity trend for the system, indicating that the existence of another giant planet, HAT-P-7c, was orbiting between the orbits of HAT-P-7b and HAT-P-7B.
They believe that the existence of the companion star, and the newly confirmed outer planet are likely to play an important role in forming and maintaining the retrograde orbit of the inner planet via the Kozai mechanism.
Norio Narita of National Astronomical Observatory of Japan (NAOJ), and his colleagues, suggests that the companion star first affected the orbit of the newly confirmed outer planet through the Kozai mechanism, causing it to tilt.
When the orbit of that planet inclined enough, HAT-P7c altered the orbit of the inner planet through the Kozai mechanism, so that it became retrograde. This orbital evolution of the planet is one of the scenarios that could explain the origin of retrograde/tilted/eccentric planets.
The team demonstrated the importance of conducting high-contrast direct imaging observations for known planetary systems to check for the presence of outer faint companions. The findings provide important clues for understanding the origin of a variety of planetary systems.