Discovery Offers New Insights into Formation of Retrograde Exoplanets

An international team of astronomers using the Subaru Telescope has detected a stellar companion and a Jupiter-sized exoplanet orbiting the star GSC 03547-01402 that has been known to harbor a giant retrograde planet. The discovery offers new insights into how retrograde planets may form and endure.

This is an image of HAT-P-7 and its companion star HAT-P-7B, left, taken with the Subaru Telescope (NAOJ)

GSC 03547-01402, also known as HAT-P-7 and Kepler-2, is a yellow white dwarf star located 1,044 light years from Earth in the constellation Cygnus.

In 2008, Japanese astronomers led by Dr Norio Narita of National Astronomical Observatory of Japan discovered a retrograde extrasolar planet, named HAT-P-7b, circling the star.

Although retrograde planets, which have orbits that run counter to the spin of their central stars, are absent in our Solar System, they occur in other planetary systems in the Universe. However, scientists don’t know how such retrograde planets formed.

Since the initial discovery of HAT-P-7b, Dr Narita has pursued his quest to explain its origin. He and his colleagues used the High Contrast Instrument of the Subaru Telescope to take high contrast images of the HAT-P-7 system.

The astronomers detected a common proper motion stellar companion to HAT-P-7, named HAT-P-7B. They also confirmed a long-term radial velocity trend for the parent star. This indicated the existence of another giant planet, HAT-P-7c, orbiting between the orbits of HAT-P-7b (the retrograde planet) and HAT-P-7B (the stellar companion). The findings appear in the Publications of the Astronomical Society of Japan (full paper).

This artist’s impression of the HAT-P-7 system shows the retrograde planet HAT-P-7b, nearest to the parent star, another giant planet in the foreground, and a companion star, upper right (NAOJ)

The team believes that the existence of the companion star and the newly confirmed outer planet HAT-P-7c are likely to play an important role in forming and maintaining the retrograde orbit of the inner planet via the Kozai mechanism – a long-term process during which a more massive object has an effect on the orbit of another. In the case of HAT-P-7b, the astronomers proposed so-called ‘sequential Kozai migration’ as an explanation of this retrograde planet.

They suggest that the companion star first affected the orbit of the newly confirmed outer planet HAT-P-7c 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 HAT-P-7b through the Kozai mechanism, so that it became retrograde. This sequential orbital evolution of the planet is one of the scenarios that could explain the origin of retrograde planets.

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Bibliographic information: Narita N et al. 2012. A Common Proper Motion Stellar Companion to HAT-P-7. Publ. Astron. Soc. Japan, vol. 64, L7