I'm watching a BBC special on the formation of solar systems, and they're covering the topic of "hot Jupiters" that exist near their host stars. The prevailing theory, according to this 2015 special, is that the planets form farther out, then migrate in. This addresses the issue of their gas being driven off by the host star.

Isn't also possible that these planets formed shortly before the ignition of the host star? The mass that will form the star is already mostly collected, and could hold bodies in orbit, but the heat has yet to begin, and wouldn't drive off gas.

  • $\begingroup$ If you consider stellar nurseries, all sizes of planets and stars can form without necessarily being part of a fusion critical mass. non fusion dwarfs and large planets perhaps form in stellar nurseries more often than stars themselves. We can perhaps compare stellar nurseries to other forms of vapor to solid crystallization and compare to fluid dynamics of splashing, where smaller entities are more common than larger ones. so planets are similar to snowflakes, different physical and chemical conditions in the cloud encourage different snowflakes. $\endgroup$ Jan 1, 2017 at 15:42
  • $\begingroup$ Some dwarf stars are the same diameter as jupiter but 15 -50 times heavier. whats considerable about jupiter is that is is very light it gives evidence of centrifugal sorting relative to the sun. $\endgroup$ Jan 1, 2017 at 15:57

1 Answer 1


Hot Jupiters can and do form before the hot star "ignites", as you put it.

The conventional core-accretion theory, by which gas-giant planets form, requires them to accrete from a disk of gas and dust for about 10 million years. Coincidentally (or perhaps not a coincidence?), the empirically determined lifetimes of circumstellar gas and dust disks around young stars is also about 10 million years.

Meanwhile, the pre-main-sequence lifetimes of newly-born stars (which I define here as the length of time it takes to contract from a large ball of gas down to a much more compact configuration, with a central temperature high enough to initiate hydrogen fusion), range from about 10 million years for stars a little more massive than the Sun to 100 million years or more for stars that are less massive.

I think, though, that the information that you are missing is that there really is no period of time where the "heat has yet to begin" whilst the protostar is surrounded by a gas/dust disk. A protostar is actually more luminous when it is younger and becomes fainter as it contracts towards its main sequence configuration. The energy that powers this luminosity is derived from gravitational potential energy. The inner portions of the gas/dust disk are always too hot to form hot Jupiters in situ and so all current models focus on how hot Jupiters can be "moved" from where they were formed to where they are found now.

  • $\begingroup$ Interesting, thank you. Oops, "ignite," sorry, speaking chemically of nuclear processes. :) $\endgroup$ Jan 1, 2017 at 15:29
  • $\begingroup$ @DonBranson I was not criticising the chemistry analogy - all astrophysicists talk in terms of "nuclear burning", which does indeed "ignite" in a reasonably sudden way. The problem is more in the concept that the star suddenly becomes brighter as a result, whereas as I explain, the truth is actually the opposite; the "ignition" actually just stops the star from becoming fainter! $\endgroup$
    – ProfRob
    Jan 1, 2017 at 15:35

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