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.
It
will be a very long time, if ever, before scientists who study
exoplanets -- planets orbiting other stars -- get the chance to watch
an interstellar probe coast into orbit around an exo-Jupiter, dozens
or hundreds of light-years away. But if they ever do, it's a safe bet
the scene will summon echoes of Juno.
"The
only way we're going to ever be able to understand what we see in
those extrasolar planets is by actually understanding our system, our
Jupiter itself," said David Ciardi, an astronomer with NASA's
Exoplanet Science Institute at Caltech.
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?
Not
likely, Ciardi says; such planetary migrations probably occur early
in the life of a solar system.
"In
order for migration to occur, there needs to be dusty material within
the system," he said. "Enough to produce drag. That phase
of migration is long since over for our solar system."
Jupiter
itself might already have migrated from farther out in the solar
system, although no one really knows, he said.
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, Calif. "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."
Measuring
the water is a key step in understanding how and where Jupiter
formed.
"If
Juno detects a high abundance of oxygen, it could suggest that the
planet formed farther out," Quintana said.
A
probe dropped into Jupiter by NASA's Galileo spacecraft in 1995 found
high winds and turbulence, but the expected water seemed to be
absent. Scientists think Galileo's one-shot probe just happened to
drop into a dry area of the atmosphere, but Juno will survey the
entire planet from orbit.
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.
"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.
"For
a long time, people thought Jupiter was essential to habitability
because it might have shielded Earth from the constant influx of
impacts [during the solar system's early days] which could have been
damaging to habitability," she said. "What we've found in
our simulations is that it's almost the opposite. When you add
Jupiter, the accretion times are faster and the impacts onto Earth
are far more energetic. Planets formed within about 100 million
years; the solar system was done growing by that point,"
Quintana said.
"If
you take Jupiter out, you still form Earth, but on timescales of
billions of years rather than hundreds of millions. Earth still
receives giant impacts, but they're less frequent and have lower
impact energies," she said.
Another
critical Juno measurement that could shed new light on the dark
history of planetary formation is the mission's gravity science
experiment. Changes in the frequency of radio transmissions from Juno
to NASA's Deep Space Network will help map the giant planet's
gravitational field.
Knowing
the nature of Jupiter's core could reveal how quickly the planet
formed, with implications for how Jupiter might have affected Earth's
formation.
And
the spacecraft's magnetometers could yield more insight into the deep
internal structure of Jupiter by measuring its magnetic field.
"We
don't understand a lot about Jupiter's magnetic field," Ciardi
said. "We think it's produced by metallic hydrogen in the deep
interior. Jupiter has an incredibly strong magnetic field, much
stronger than Earth's."
Mapping
Jupiter's magnetic field also might help pin down the plausibility of
proposed scenarios for alien life beyond our solar system.
Earth's
magnetic field is thought to be important to life because it acts
like a protective shield, channeling potentially harmful charged
particles and cosmic rays away from the surface.
"If
a Jupiter-like planet orbits its star at a distance where liquid
water could exist, the Jupiter-like planet itself might not have
life, but it might have moons which could potentially harbor life,"
he said.
An
exo-Jupiter's intense magnetic field could protect such life forms,
he said.
Juno's
findings will be important not only to understanding how exo-Jupiters
might influence the formation of exo-Earths, or other kinds of
habitable planets. They'll also be essential to the next generation
of space telescopes that will hunt for alien worlds. The Transiting
Exoplanet Survey Satellite will conduct a survey of nearby bright
stars for exoplanets beginning in June 2018, or earlier. The James
Webb Space Telescope, expected to launch in 2018, and WFIRST
(Wide-Field Infrared Survey Telescope), with launch anticipated in
the mid-2020s, will attempt to take direct images of giant planets
orbiting other stars.
"We're
going to be able to image planets and get spectra," or light
profiles from exoplanets that will reveal atmospheric gases, Ciardi
said. Juno's revelations about Jupiter will help scientists to make
sense of these data from distant worlds.
"Studying
our solar system is about studying exoplanets," he said. "And
studying exoplanets is about studying our solar system. They go
together."
