I am new to this community and not quite sure if this is an appropriate question to be asked here. Anyways its just my curiosity. As far as I could know about WASP-18b is that, it orbits very close to its sun and takes less than 24 hours to complete one orbit. However this was not the case always, as the planet actually was formed in a cold part of the space and later got tug into of its sun's gravitational pull.

Given this is the case, I am curious to know how it was formed in the first place? If I am not wrong planets are formed when the debris orbiting in the gravitational pull of a star stick together overtime gets in to the form of the planetary bodies. So, if WASP-18b was not always so close to its sun and it was formed far away in cold space, then could there be a possibility that, this exoplanet was formed in an orbit of another star and overtime it got away from its original orbit and trapped in the tug of its current sun? (Its my imagination! just bare with it :))

Also could any one point me to good documentations from where I can learn more.


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    $\begingroup$ It is rare for a star to capture a passing planet. Space and velocity differences are too big for the observed number of hot Jupiters to form that way. I think the main idea today is that hot Jupiters form far out and then interact with another gas giant leading to one of the two planets becoming a hot Jupiter and the other being thrown out to become a vagabond starless planet. But we've got updated experts here, let's hear the real answer. $\endgroup$
    – LocalFluff
    Jun 6, 2015 at 10:12

1 Answer 1


@LocalFluff's comment is more-or-less complete. The process of "swapping" planets between stars could happen during the early lives of stars when most of them are born in relatively dense clusters. You can find some more details and references in my answer to this question, but the outcome is that a captured planet would usually be in a very wide orbit.

Approximately 1% of stars have a "hot Jupiter", but even in dense cluster environments it is thought that only a few percent of stars can capture a planet, and because almost all of these end up in wide orbits there are just too many hot Jupiters to be explained in this way.

There are two classes of theory to explain hot Jupiters. One is the dyanmical explanation offered by LocalFluff. This is itself divided into several different mechanisms: "Secular chaos", (e.g. Wu & Lithwick 2011 - this paper also briefly reviews some of the other mechanisms) which is a gradual change in the planetary system that then suddenly drives a transition to a new configuration. Planet-planet scattering where interaction between planets causes one orbit to become highly eccentric and then tidal circularisation results in a hot Jupiter in a close orbit (e.g.Ford & Rasio 2008). The Kozai mechanism is the interaction beteen two planets that results in an interchange between the inclination and eccentricity of the inner planet. This excites an increase in eccentricity that, when combined with tidal circularisation, can also result in a close-in planet.

The other class of mechanism involve migration through interaction with a disk. This must necessarily happen early in the star's life whilst surrounded by a disk of circumstellar material. Basically viscous drag in the disk itself causes the planet to move inwards (or sometimes outwards). There are three possible subtypes of disk migration and they are briefly defined here.

It is probable that all of these mechanisms play some role in forming hot Jupiters and one of the goals of exoplanetary study is to figure out which may be more important. Typical parameters of interest are the distribution of planetary orbits and eccentricities and whether there is evidence of previous interactions in the form of misalignments between the rotation axis of the star and the orbital axis of the planet.


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