Why do star clusters have hotter Jupiters than average? A group of astronomers has proposed a new solution. This solution involves a lot of stellar neighbour swapping.
Astronomers were surprised to discover an entirely new class of object when they first began studying exoplanets. These are hot Jupiters. Hot Jupiters are Jupiter-sized planets that orbit their stars closer to the sun than Mercury does. This proximity significantly raises their temperatures. This causes them to swell, extend, and rain iron.
Hot Jupiters are uncommon. Only about 1% of the stars with planets have one. But the percentage is much higher in star clusters. Hot Jupiters appear to be several times more common than average in star clusters.
A group of astronomers have developed an analysis that they believe can explain why star clusters have hotter Jupiters. The key point is that stars in a cluster are much closer together than average. Because they are so close, stars are bound to interact with their neighbours in some way. They can even lift one of their neighbours into orbit in some cases. Even though stars begin as solitary objects, they can evolve into binary systems.
The astronomers discovered that if this capture occurs early enough in the planet formation process, it can result in a hot Jupiter. If a Jupiter-sized planet forms around a star and that star captures a neighbour, the gravitational influence of that neighbour can begin to stretch the planet’s orbit.
The planet’s orbit becomes increasingly elliptical as it stretches. The orbit’s farthest point becomes more distant from the star. The closest point becomes closer. If the planet gets too close to the star, it can be captured in a brand-new orbit. Then the normal giant planet transforms into a hot Jupiter.
The astronomers discovered that this occurs in approximately 2% of the stars in a cluster. In order to create a hot Jupiter, certain conditions must be met. Such as the orbit getting close, but not too close.
Astronomers discovered that if giant planets form around 10% of stars, there will be enough interactions in a star cluster in about a billion years. It will trigger the migration that turns a giant planet into a hot Jupiter. These interactions are rare outside of star clusters. This helps to explain why hot Jupiters are more common within them.
The paper is now available on the arXiv preprint server.
More information: Daohai Li et al, Making hot Jupiters in stellar clusters: the importance of binary exchange, arXiv (2022). DOI: 10.48550/arxiv.2211.16015