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In my physics lessons, my teachers have always been keen to tell my class that Jupiter is considered a 'failed star' by scientists. Is this true?

In my own effort I wondered if maybe this could just be being regurgitated from an outdated physics syllabus that still considers the Solar System to have nine planets. From that thought onward, through my research on the Internet, I haven't found people referring to Jupiter as such and people always call it a planet rather than a brown dwarf.

Furthermore, it's my understanding that brown dwarfs possess more mass than Jupiter suggesting to me that Jupiter possesses too little mass for fusion to even be plausible.

So am I correct in thinking that Jupiter is 'only' a planet, or are my physics teachers correct in saying it is a failed star (and if so, why)?

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Could you please focus on one specific question since the formation of Jupiter and brown dwarfs are remotely related it would take some considerable effort to give a full answer to your question. – Tigran Khanzadyan Jul 17 '11 at 18:48
OK, is Jupiter considered a failed star by scientists and if so, why? – alexjohnj Jul 17 '11 at 19:01
I consider myself to be a failed star. If only I'd been born with another billion or so yottagrams of hydrogen! sigh – Keith Thompson Apr 14 '12 at 22:15
Related: – Qmechanic May 9 '12 at 19:05
up vote 35 down vote accepted

The answer kind of depends on how old you are. At a very introductory level, say, maybe middle school or younger, it's "okay" to refer to Jupiter as a failed star to get the idea across that a gas giant planet is sort of similar to a star in composition. But around middle school and above (where "middle school" refers to around 6-8 grade, or age ~12-14), I think you can get into enough detail in science class where this is fairly inaccurate.

If you ignore that the solar system is dominated by the Sun and just focus on mass, Jupiter is roughly 80x lighter than the lightest star that undergoes fusion. So it would need to have accumulated 80 times what it already has in order to be a "real star." No Solar System formation model indicates this was remotely possible, which is why I personally don't like to think of it as a "failed star."

Below 80 MJ (where MJ is short for "Jupiter masses"), objects are considered to be brown dwarf stars -- the "real" "failed stars." Brown dwarfs do not have enough mass to fuse hydrogen into helium and produce energy that way, but they do still produce their own heat and glow in the infrared because of that. Their heat is generated by gravitational contraction.

And Jupiter also produces heat through both gravitational contraction and differentiation (heavy elements sinking, light elements rising).

Astronomers are not very good at drawing boundaries these days, mostly because when these terms were created, we didn't know of a continuum of objects. There were gas giant planets, like Jupiter and Saturn, and there were brown dwarf stars, and there were full-fledged stars. The line between brown dwarf and gas giant - to my knowledge - has not been drawn. Personally, and I think I remember reading somewhere, the general consensus is that around 10-20 MJ is the boundary between a gas giant planet and brown dwarf, but I think it's fairly arbitrary, much like what's a planet vs. minor planet, Kuiper belt object (KBO) or asteroid.

So during Solar System formation, was there a chance Jupiter could have been a star and it failed ("failed star!") because the mean Sun gobbled up all the mass? Not really, at least not in our solar system. But for getting the very basic concept across of going from a gas giant planet to a star, calling Jupiter a "failed star" can be a useful analogy.

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As a note on where the brown dwarf / gas giant boundary exists, consensus seems to be the deuterium burning limit, usually taken to be about 13 MJ. Here's a recent paper where the the deuterium burning limit is calculated. It's found to depend on a few parameters (notably the metal content of the object), which leads leaves a range of a few Jupiter masses. – Warrick Jul 18 '11 at 7:31
Thanks, Warrick, That number was coming to mind, but I couldn't remember where I heard it. What throws it off, though, is that people are reporting exoplanets with a minimum mass of up to 20 Jupiter masses. So, yeah ... – Stuart Robbins Jul 19 '11 at 19:03
You were spot on. I just thought I'd provide a recent reference. The distinction needs a rethink, but the ideas people suggest, mostly related to how the system forms, are things we can't observe. There'll probably be some IAU criteria in a few years... – Warrick Jul 20 '11 at 6:59
I've seen composition used to draw the line between a brown dwarf and a Super Jupiter before as well. A brown dwarf would have the same mix of elements as the stars it formed with; while a Super Jupiter would have a different mix. As more data on exo-planets is gathered this could end up failing due to the discovery of a continuum between between Jovian type objects with elemental compositions significantly different than their host star and those that have identical compositions. – Dan Neely Apr 16 '12 at 13:58
Great stuff - also great points about children of different ages - I demonstrate in my daughter's school's science room and there are quite a few planet geeks there who ask this kind of question, so thanks. – WetSavannaAnimal aka Rod Vance Dec 1 '13 at 7:26

it might be more accurate to say that our solar system is to some extent a failed binary star system - as there are stars out there with exoplanets in the 'brown dwarf range' of mass - if our star system had enough mass initially to result in such an arrangement, there probably wouldn't be an earthlike planet around for us to exist on and see it up close, but there could have been enough mass in total for very large a exoplanet to ignite.

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protected by Qmechanic May 8 '14 at 17:01

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