A Billion or More Jupiter-Like Worlds Could Be Orbiting Stars in the Milky Way
Using data from NASA’s Juno probe, astronomers hope to gain a better understanding of Jupiter and the the many Jupiter-like planets in our galaxy.
Our galaxy is home to a bewildering variety of Jupiter-like worlds: hot ones, cold ones, giant versions of our own giant, pint-sized pretenders only half as big around.
Astronomers say that in our galaxy alone, a billion or more such Jupiter-like worlds could be orbiting stars other than our sun. And we can use them to gain a better understanding of our solar system and our galactic environment, including the prospects for finding life.
It turns out the inverse is also true — we can turn our instruments and probes to our own backyard, and view Jupiter as if it were an exoplanet to learn more about those far-off worlds. The best-ever chance to do this is now, with Juno, a NASA probe the size of a basketball court, which arrived at Jupiter in July to begin a series of long, looping orbits around our solar system’s largest planet. Juno is expected to capture the most detailed images of the gas giant ever seen. And with a suite of science instruments, Juno will plumb the secrets beneath Jupiter’s roiling atmosphere.
Not all Jupiters are created equal
Juno’s detailed examination of Jupiter could provide insights into the history, and future, of our solar system. The tally of confirmed exoplanets so far includes hundreds in Jupiter’s size-range, and many more that are larger or smaller.
The so-called hot Jupiters acquired their name for a reason: They are in tight orbits around their stars that make them sizzling-hot, completing a full revolution — the planet’s entire year — in what would be a few days on Earth. And they’re charbroiled along the way.
But why does our solar system lack a “hot Jupiter?” Or is this, perhaps, the fate awaiting our own Jupiter billions of years from now — could it gradually spiral toward the sun, or might the swollen future sun expand to engulf it?
Looking back in time
If Juno’s measurements can help settle the question, they could take us a long way toward understanding Jupiter’s influence on the formation of Earth — and, by extension, the formation of other “Earths” that might be scattered among the stars.
“Juno is measuring water vapor in the Jovian atmosphere,” said Elisa Quintana, a research scientist at the NASA Ames Research Center in Moffett Field, California. “This allows the mission to measure the abundance of oxygen on Jupiter. Oxygen is thought to be correlated with the initial position from which Jupiter originated.”
If Jupiter’s formation started with large chunks of ice in its present position, then it would have taken a lot of water ice to carry in the heavier elements which we find in Jupiter. But a Jupiter that formed farther out in the solar system, then migrated inward, could have formed from much colder ice, which would carry in the observed heavier elements with a smaller amount of water. If Jupiter formed more directly from the solar nebula, without ice chunks as a starter, then it should contain less water still. Measuring the water is a key step in understanding how and where Jupiter formed.
That’s how Juno’s microwave radiometer, which will measure water vapor, could reveal Jupiter’s ancient history.
The chaotic early years
Where Jupiter formed, and when, also could answer questions about the solar system’s “giant impact phase,” a time of crashes and collisions among early planet-forming bodies that eventually led to the solar system we have today.
Our solar system was extremely accident-prone in its early history — perhaps not quite like billiard balls caroming around, but with plenty of pileups and fender-benders.
“It definitely was a violent time,” Quintana said. “There were collisions going on for tens of millions of years. For example, the idea of how the moon formed is that a proto-Earth and another body collided; the disk of debris from this collision formed the moon. And some people think Mercury, because it has such a huge iron core, was hit by something big that stripped off its mantle; it was left with a large core in proportion to its size.”
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