### Is Jupiter a failed star?

• The elemental make-up of Jupiter is about entirely hydrogen and helium, along with a very small fraction of the atmosphere being made up of compounds such as ammonia, sulfur, methane, and water vapor. These elements are dominant in stars, so how likely is it that Jupiter was a companion to our sun, but failed to ignite?

Also: stars are also mostly made of H and He. Other elements are not dominant in terms of how I would define that word.

Popular culture: I don't know why, but this opening, from Star Trek: TNG, immediately came to mind. https://youtu.be/_-js38rP_-8?t=16 . I don't know what the artists there are trying to suggest, but it depicts a different, glowing Saturn than the usual. Vaguely starlke.

It's widely reported that Carl Sagan popularized this description, in _Cosmos_, but did not originate it. Before _Cosmos_ "a star that failed to materialize" was used in criticism of Nikolai Aleksandrovich Kozyrev's 1977 hot model for the interior of Jupiter, although M. Kozyrev never claimed such that I can see. The most widely known popular culture reference is surely _2010_ by Arthur C. Clarke. All from the late 1970s and early 1980s.

You can also find the headline question asked and answered in Terence Dickinson's 1993 book _From the Big Bang to Planet X: The 50 Most-asked Questions about the Universe-- and Their Answers_. It's even the title of Joel N. Shurkin's 1979 book _Jupiter-The Star That Failed_.

Pedantically, we are all companions to our sun and failed to ignite. *"how likely"* = mass required to ignite? = "13 Jupiter-masses." (Don't blame him; he didn't *fail* to do anything.) How big is that? 7.69% as big as it needs to be to do so.

I feel like the answer to this question depends a lot on what Jupiter's original life goals were :)

• No.

Besides the 13 Jupiter-masses required to ignite deuterium burning, and make Jupiter into a Brown Dwarf, there is a clear difference between the formation pathways of Brown Dwarves and Gas Giants.

Gas Giants are planets, that form via processes in their parent protoplanetary disc. Contrasting this, Brown Dwarves form via direct fragmentaion of the parent giant molecular cloud, possibly as binary fragments of a more massive companion star. The distinction between those two processes is supported by the finding and characterisation of the Brown Dwarf desert in i.e. Grether & Lineweaver (2006).

The 'Brown Dwarf' desert is a sharp break in the number distributions between low-mass stars and high-mass planets. This is commonly interpreted as those two classes of object being distinct in origin. Furthermore we understand distinct theoretical pathways to form each of those object classes, as mentioned above.

Note that if you're talking about the other elements, or as astronomers would call them, metals, then there is data on those as well: The sun, and any star possesses a certain amount of metals, see i.e. Asplund et al. (2009). For example the ratio of Oxygen to Hydrogen in the solar atmosphere is about $$n(O)/n(H)\sim 5\times 10^{-4}$$, which is the most abundant 'other' element in the sun.

The values of heavier elements found in the gaseous planets are elevated compared to the solar values. This is thought to represent a contribution of solid material that planets need to form according to current theories. If the giant planets would have formed through direct collapse from the same cloud material as the sun did, the abundances of metals in the giant planets would not be elevated compared to the solar values.

For the distinct formation part, here's a paper that suggests one could distinguish planets and brown dwarfs by their orbits' eccentricities: https://iopscience.iop.org/article/10.3847/1538-3881/ab5b11#ajab5b11s6

But *is* there evidence of planets large enough to start fusion? Even if there are not many, it does not seem impossible *prima facie.* They would be distinct from "normal" companion brown dwarfs, as I have learned just now, in orbit and composition -- simply planets that got a little too large. Somehow a fascinating image.

@Peter-ReinstateMonica: In the sample used by Grether & Lineweaver (2006) there seem to be 2 planets above the deuterium burning limit. I would agree with you, there is no physics prohibiting planets to be so massive!

It all of course depends on how you define the term failed star. In general, a star should be able to generate heat by fusing atoms together, and it requires about 13 times the mass of Jupiter for conditions to be adequate for sustained deuterium fusion, and about 63 times the mass of Jupiter for fusion of lithium to take place. All other nuclei require even more heat/pressure, so these two define the lower bound for what could be reasonably called a star. In my opinion, Jupiter is quite far from these measures, even if you factor in any mass loss since the formation of solar system.