Jupiter: King of the planets or a failed star?

Jupiter: King of the planets or a failed star?


The brilliant planet Jupiter made its closest approach to the planet Earth in April this year and remains a fantastic sight to behold especially in telescopes above 4” aperture.

Roman observers named Jupiter after the patron deity of the Roman state following Greek mythology, which associated it with the supreme god, Zeus.

Astronomers soon recognized Jupiter as the largest planet in the solar system long before any spacecraft provided detailed exploration. The planet’s massive size 88,846 miles(142,984Kms) at the equator holds 2.5 times the mass of all the other planets combined. This makes Jupiter the most dominant body in the solar system after the Sun. The planet’s volume is so great that 1,321 Earths could fit inside of it.

Jupiter is a magnificent example of a gas giant planet. It has no solid surface and is composed of a small rocky core enveloped in a shell of metallic hydrogen, which is surrounded by liquid hydrogen, which in turn is surrounded by a blanket of hydrogen gas. By count of atoms, the atmosphere is 90% hydrogen and 10% helium, this is compositionally more similar to brown dwarfs and small stars.

So, it’s not difficult see why many astronomers wondered whether it should be considered a king-sized planet or a failed star.

As the solar system condensed out of interstellar gas and dust, Jupiter acquired most of the matter that was not ejected into interstellar space and did not fall inward to form the Sun. But although Jupiter is large as planets go, it would need to be about 75 times its current mass to ignite nuclear fusion in its core and become a star. Had it had this requisite mass it would have become a star in visible light and we would today inhabit a binary or double star system, with two suns in our sky; something which is very common throughout our Milky Way galaxy.

We of course would undoubtedly think this would be a natural and beautiful experience but you could end up with a lifetime of sleepless nights: more likely life might never have evolved on Earth because the temperatures would have been to high and the atmospheric characteristics all wrong. Life may have existed Jim, “but not as we know it” (Star Trek).

Now astronomers have found other stars orbited by planets with masses far greater than Jupiter’s.

What about sub-stellar brown dwarfs?

Our largest planet doesn’t come close to these “almost stars”. Astronomers define brown dwarfs as bodies with at least 13 times Jupiter’s mass. At this point, a hydrogen isotope called deuterium can undergo fusion early in a brown dwarf’s life.

So, the reality is that while Jupiter is a planetary giant, its mass falls far short of the mark to consider it a failed star.

I’m off to the mountains of southern Spain to observe Jupiter under dark skies and at a higher altitude, With my trusty 8” Newtonian reflector and my 4.25” apochromatic triplet refractor I’m looking to see the N & South Equatorial belts, the N & S Temperate zones, the Great Red Spot (a great anticyclonic storm bigger than the Earth), maybe a White Oval and the Galilean satellites, Io, Europa, Ganymede and Callisto, mmmh and some nice  tapas and Rioja!

Adios Amigos

El Beadlo FRAS

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