By Joshua Winn, Princeton University
Jupiter is massive enough to retain hydrogen and helium. In fact, hydrogen and helium are the most common elements in the universe so much so that the Sun, and all stars, are basically giant spheres of hydrogen and helium. So, then, why aren’t all planets like Jupiter? Why is Jupiter, alone, so massive? Read on to find out.
Formation of Planets and Stars
Jupiter alone is so massive because of the way in which planets are formed. Whereas stars are formed when a cloud of gas collapses under its own gravity, into a ball of gas dense and hot enough to ignite nuclear fusion reactions, planets form out of the stuff left over after star formation. A new star is surrounded by a rotating disk of hydrogen and helium gas; that’s the material that had too much angular momentum to fall directly onto the star. Hence, it spends millions of years swirling around until friction can drain away its energy, and transfer its angular momentum outward, causing the gas to fall onto the star.
Once upon a time, astronomers thought planets might come from clumps of gas within the disk that collapse under their own gravity, like stars do, but that doesn’t work. The disk is too hot, and its gravity is too weak to clump and collapse. Now, we think planets start from the microscopic flecks of heavier elements that are mixed in with the gas. Dust grains. They’re tiny—maybe only a micron in size—but that’s still a lot bigger than those individual atoms and molecules.
As a consequence, over time, gravity causes the dust to settle down into a thin layer; thin and dense enough that the dust grains start colliding and stick to each other. They grow to the sizes of pebbles, rocks, boulders, and eventually, after millions of years, all the way up to planets. Though the details of this process—which is called planetesimal formation—are still hazy, what’s clear is that the hydrogen and helium don’t participate. They’re too lightweight; their high thermal speeds prevent them from condensing.
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This raises an interesting question: Why aren’t all planets rocky? And, where did Jupiter come from? To begin with, if a rocky planet gets big enough, its escape velocity will start to exceed 10 times the thermal velocity of the gas. That’s what allows the planet to start attracting and retaining gas from the huge reservoir of hydrogen and helium all around it.
Let’s try and understand that quantitatively. We might be hazy on the details of planetesimal growth, but in the solar system, clearly this process had no trouble making objects as large as Venus and the Earth even though those planets can’t hold on to hydrogen. They’re too hot, and their escape velocities are too low. So, how much farther from the Sun would we need to move them, so that they could retain hydrogen?
We’ll take our equation for the minimum mass of molecules that can be retained which is proportional to temperature. Combining that with the equation for the equilibrium temperature, in terms of orbital distance, we can plug in the mass of hydrogen—that’s a 2 on the left side—and solve for a, the orbital distance.
This comes out to be 3.4 AU which is right in between the orbits of Mars and Jupiter; that is, it’s right at the dividing line in the solar system between the inner rocky planets, and the outer gas giant planets. So, it hangs together. A rocky planet needs to be far away from the Sun to be cold enough to start attracting hydrogen and having a chance at becoming a gas giant such as Jupiter.
Planet Formation: A Complicated Process
Now, as always in planetary science, there are some caveats. Planet formation doesn’t just stop here and is far more complicated. As a rocky planet attracts more gas, there comes a point at which the atmosphere is so massive that it starts affecting the planet’s escape velocity, which our calculation ignored. Furthermore, to make a giant planet, we need the rocky planet to attract the gas quickly enough, within a few million years, before the gaseous disk disappears. When we do all that more carefully, we find that a more realistic starting point for a giant planet is a core of 5 or 10 Earth masses, not 1 Earth mass.
And that turns out to be a difficult threshold to reach. What helps, though, is that beyond about 3 AU, there’s a more solid material in the disk because it’s cold enough for common molecules, like water, methane, and ammonia, to exist as frozen ice particles rather than as vapor. In astro-jargon, we say that a planetary system has a ‘snow line’, beyond which the abundance of solid material rises sharply. That gives us more ‘snow’ to pack onto a growing proto-planet, and makes it easier to reach that threshold mass for accreting the surrounding gas, and becoming a giant planet.
Common Questions about Understanding Planet Formation
Whereas stars are formed when a cloud of gas collapses under its own gravity, into a ball of gas dense and hot enough to ignite nuclear fusion reactions, planets form out of the stuff left over after star formation.
We think planets start from the microscopic flecks of heavier elements that are mixed in with the gas. Dust grains. They’re tiny—maybe only a micron in size—but that’s still a lot bigger than those individual atoms and molecules.
As a rocky planet attracts more gas, there comes a point at which the atmosphere is so massive that it starts affecting the planet’s escape velocity.