Is Jupiter a failed star? And what would happen if it really became one?
Not infrequently, especially on the Web, one still finds astronomy articles claiming that
the planet Jupiter should be considered a "failed star," that is, a celestial
body whose mass "almost" allowed it to trigger nuclear fusion reactions, but failed to do so.
What is true about this statement? Let's go find out together!
In all likelihood, this is a belief born in the late 1970s, when it was discovered that Jupiter
radiates about one and a half times the amount of heat it receives from the Sun into space. This may
have led to the suggestion that at one time the planet may have produced heat by nuclear fusion;
heat that would then have been partially conserved by thermal inertia.
But if this were indeed the case, how is it that Saturn,
which has a mass more than three times less than that of Jupiter, emits as much as two and a half
times the heat it receives from the Sun? Indeed, it is impossible that a celestial
body with a certain mass emits more heat than another with a higher mass.
And in fact, the explanation for the phenomenon is another. Jupiter's internal structure consists
of several layers: below the gaseous atmosphere, we find a layer of liquid hydrogen and helium,
one of metallic hydrogen, and a solid core whose composition is still uncertain. In the
first layer, helium, being heavier, tends to descend, pushing up hydrogen. This results in
a release of gravitational energy, and thus heat. In the case of Saturn, since it is farther from
the Sun and therefore colder, the "stirring" due to thermal motions is less, and therefore the
helium sinks more easily. Hence the greater intensity of heat emitted by the planet.
In short, Jupiter is a very normal planet. "Failed stars" are quite another matter...
But are we really sure? Is the difference between
a massive gas planet and the smallest star we know, really that big and unbridgeable?
Although there is a minimum boundary, below which we speak of a planet, and a maximum boundary,
beyond which we speak of a star, in reality, these boundaries are still being defined,
and understanding the true nature of a celestial object is by no means simple
However... we know that the smallest known star in our galaxy is called EBLM
J0555-57Ab and is a Red Dwarf belonging to a triple system located about 630 light-years
from Earth in the constellation Pictor. Given its small mass (less than half the
solar mass), such a star cannot develop high pressures inside, and so nuclear reactions
triggered in the core produce only a modest amount of energy. Precisely because of the
extreme smallness of their combustion, these stars have extraordinarily long lives: periods
on the order of hundreds of billions of years are calculated for the smallest. Since the age of the
universe is around 13 billion years, this means that to date no red dwarf has yet gone extinct!
Since the surface area of these stars is very small, the radiation losses of the
remaining energy are extremely slow, and so, even once all the hydrogen is used up,
a red dwarf will continue to emit an increasingly dim light, and slowly
disappear from view altogether, transformed by now into an icy, invisible black dwarf.
In short ... EBLM is a really small star, with just enough mass, according
to today's models of stellar evolution, to trigger hydrogen fusion in its core.
Its diameter is 120,000 km... not even one-tenth that of the Sun and even smaller than Jupiter's
equatorial diameter, which is 143,000 km! Yet it shines, and emits light!
So, you immediately wonder... why Jupiter,
which is even bigger, has remained a planet and has not become a star?
The short answer is this: Jupiter is larger in size, but it does not have enough mass
to trigger the process of nuclear fusion and the transformation of hydrogen into helium inside it.
For this to happen, in fact, an object must be massive enough and hot enough for individual
atoms to overcome repulsive forces and compact. And if this tiny Red Dwarf, succeeds in igniting
its nuclear furnace, it is because despite its small size it possesses a mass that is
about 85 times that of Jupiter: just the minimum value for a globe of gas
to get to compress its core so much that it heats it up to the point of nuclear fusion!
"Hey, guys, just a moment before we continue... BE sure to join the Insane Curiosity Channel...
Click on the bell, you will help us to make products of ever-higher quality!"
But wait a minute... how is it 85 times more massive than Jupiter while being smaller? And
here the answer can only be immediate: it succeeds because of the incredible value of its density,
which is 188 grams per cubic centimeter! And, do you want to know why we call this
value "incredible"? Well, it will suffice only to think that the density of our Earth, which is also
the highest in the solar system, is only 5.5 grams per cubic centimeter, while Jupiter's is really
very low...: only 1.3 grams per cubic centimeter! All this means only one thing... Jupiter could
only become a star if its mass suddenly became 85 times greater than the mass it possesses now!
Is there any chance that this could happen? In the real world, absolutely not,
but in the science-fiction world of the great Arthur Clarke, as we shall
see in a moment, it did happen for real! There is, however, a decidedly more viable
route for Jupiter to become a star anyway… that of trying to become a Brown Dwarf. namely, a
substellar object whose mass is insufficient to be able to trigger the nuclear burning of hydrogen,
but sufficient to trigger that of deuterium, which is a lighter isotope of hydrogen.
Astronomers believe that, typically, a brown dwarf has a mass between 13 and 80 times
the mass of Jupiter, but what the precise boundaries are is still somewhat debated.
Brown dwarfs: they may appear as planets but form as stars, that is, they collapse
directly from an entire nebula of molecular hydrogen, rather than from the accumulation
of material in the disk around a star. The minimum mass limit to be able to speak
of a brown dwarf is therefore 13 Jovian masses: upon reaching which deuterium
burning begins. Past 65 Jovian masses, on the other hand, lithium fusion also takes place.
Deuterium and lithium, however, are two elements found in small quantities within a brown dwarf,
so energy production is extremely limited in time and the fate of these celestial bodies
is slow and gradual cooling. We know of brown dwarfs with surface temperatures
around 2,000 °C and others, evidently older, that barely reaches 200 °C.
In short, if Jupiter wanted to settle for becoming a brown dwarf, it would have to
increase its mass by "only" 13 times. And albeit to a very modest extent, it would begin to emit
light and heat, and slowly manage to warm the planetary environment in its vicinity.
Could it be done? Once again, the answer is: NO! Indeed, where could we find 12
other globes of matter as massive as Jupiter? However, as we mentioned a moment ago, Arthur
Clarke left us something to write about this... Do you remember these words?
"ALL THESE WORLDS ARE YOURS, EXCEPT EUROPA. ATTEMPT NO LANDING THERE.
USE THEM TOGETHER. USE THEM IN PEACE." In the novel 2010: Odyssey Two (from which
the film 2010: The Year We Make Contact was later based) Jupiter actually transforms into a star,
albeit with the help of billions of alien monoliths that increase the
planet's density beyond the critical point. In this way, the former gaseous planet begins
to radiate energy, and it is thanks to this scientific miracle that Jupiter's
four satellites suddenly become worlds ready for colonization by humans...
Of course, this is just good science fiction -- not least because increasing Jupiter's
mass could have some unpleasant downsides on the solar system.
Let's try to figure out which ones, imagining that we have somehow managed to increase Jupiter's mass
by 85 times, and start the nuclear reaction Meanwhile, it is necessary to say that
doing so would not - as one might think - also result in a disproportionate increase in size.
As a result of the compression of materials toward the center, the diameter
would in fact increase by only 20 percent. We would thus obtain a red dwarf 170,000 km in
diameter and 300 times less luminous than the Sun; and since Jupiter is 4 times farther away from us
than the Sun, we would get an increase in energy here on Earth equal to only one five-thousandth
of what we receive from the Sun. Practically, we would experience no change in temperature.
Instead, there would be definite consequences for the brilliance of the star,
which would appear in our night skies 80 times brighter than the full Moon.
Which means that the night would be erased for long periods of the year.
And this would have enormous repercussions on the nocturnal habits of higher animals,
deprived of the ability to hunt or hide properly. Phytoplankton would suddenly be
able to grow without interruption, changing the balance in the marine ecosystem. Stars
would only be seen very few times a year, and astronomical observation
conducted from the land would cease, as would the nighttime migration of certain bird species.
But all this would be nothing compared to the catastrophic gravitational changes
that would result from Jupiter's sudden transformation into a star!
The greatest danger, in the immediacy, would not come, however, from the changes in the orbits
traveled by the planets, which would be felt only over several centuries, but in the chaos that the
enormous mass of the red dwarf would bring to the fragile balance of the Asteroid Belt.
In fact, the Asteroid Belt is currently governed and held together by the very
gravitational influence of Jupiter. If Jupiter's mass were to be increased almost a hundredfold,
the billions of objects that make up the Belt, currently orbiting the Sun like
peaceful herds of bison, would be transformed into a chaotic horde of crazed animals.
Many would be drawn toward the outer solar system and would impact the surviving gas giants-Saturn,
Uranus and Neptune-destroying and scattering all their moons.
Others would be directed toward the inner solar system, where they would bombard
and again reduce the rocky planets, including our Earth, to formless, nearly liquid masses.
All the larger objects in the solar system would be devastated by the blows of this
cosmic billiard gone mad, and over time the remaining planets would either be ejected
or diverted into much wider orbits capable of circling the common barycenter of the two stars.
All this would occur, of course, if Jupiter's change between planet and star were instantaneous.
It would be different if the solar system had already formed a binary star system.
In that case, all planetary formation and the evolution of life would have
taken different binaries. And we probably would not be here talking about it now.
Jupiter is therefore not a "failed star." It never even came close to becoming a star, even
if it was a small brown dwarf-it was not meant to be one, and it never had the slightest chance.
All the better for us! Don't you agree?