Inflated Hot Jupiters Spun Out From Stellar Mergers

Over the years, wide area transit surveys have revealed a population of inflated hot Jupiters. These planets are basically Jupiter-mass planets that have remarkably large diameters and orbit very close to their host stars. It has been proposed that inflated hot Jupiters can form when stars in tightly-bound binary systems merger. The magnetic activity of stars in a tight binary system can act as a “brake” and cause the binary system to gradually lose angular momentum. This process brings the 2 stars closer to each other, further accelerating the rate of angular momentum loss. Eventually the 2 stars merge. The merging process can launch a substantial amount of material into orbit which settles into a disk around the newly formed star. Such a disk of material is known as an excretion disk.

Material in the excretion disk quickly coalescences to form one or more Jupiter-mass planets around the star. These planets, where newly formed, are known as inflated hot Jupiters. The reason for the term “inflated” is because these planets are still hot, and it will take time for them to cool and contract. Also, they are called “hot Jupiters” due to the intense stellar irradiation on their daysides as these planets orbit very close to their host star. An excretion disk has a much lower angular momentum compared to a normal protoplanetary disk. As a result, planets that formed out of an excretion disk will remain in close-in orbits around the star. Since a newly formed Jupiter-mass planet cools and contracts with time, measuring the degree of inflation of a hot Jupiter can shed light on how long ago did the merger event that led to the formation of the planet took place.

Hot Jupiters orbit very close to their host stars, typically around 0.05 AU. At such a distance, a hot Jupiter can raise strong tides on its host star. These tides cause the host star to spin-up by taking angular momentum away from the planet’s orbit. The outcome is that the planet’s orbit shrinks. If the planet is massive enough, it can spin-up its host star to co-rotate with its orbit around the star (i.e. the star’s spin period become the same as the planet’s orbital period), and the planet can delay or even avoid spiralling into its star. If the planet is not massive enough, it will continue to lose angular momentum, eventually spiralling into its star and get tidally destructed.

Reference:
Martin et al. (2011), “A binary merger origin for inflated hot Jupiter planets”, arXiv:1102.3336 [astro-ph.SR]