Seeing the Formation of Planets


Planets begin as dense knots in clouds of
dust swirling around a young star. But how do they go from something like this, to something
like this? With the James Webb Space Telescope astronomers
will be able to study how planets come to be and how they change as they get older. After centuries of searching, astronomers
are finding exoplanets just about everywhere. Ranging from giant planets with masses much
greater than Jupiter’s to worlds only a few times more massive than Earth. But where do the planets we know best fit
into the menagerie of worlds astronomers are finding? How did our solar system come to
be the way it is? Why is Earth a balmy water rich world and are there other worlds like
it elsewhere in the galaxy? These are the kind of questions astronomers
will address with Webb. For planets that pass directly in front of their stars, Webb will
search for chemical fingerprints, identifying atmospheric gases like water vapor, carbon
dioxide, and methane that absorb specific wavelengths of the star’s light. Webb will
also study the dusty disks where new planets form to reveal how the chemical compositions
of younger and older disks change with time, and identifying how these changes are reflected
in the planets we find. Such studies will be revolutionary in their
own right. And by applying Webb’s capabilities closer to home, astronomers will better understand
planetary systems. For example, how do our asteroids, comets,
and other small bodies like Pluto relate to the objects that create dusty disks around
other stars? The Webb telescope will determine the physical and chemical properties of these
bodies with unprecedented sensitivity in wavelengths unavailable to telescopes on the ground. By learning more about the small bodies in
our solar system, scientists will be able to address questions about the solar system’s
past, and compare it to other planetary systems we find in similar phases of construction. For example, did Earth’s oceans arrive by
impacts with small icy bodies? If so, is the same process happening elsewhere and can we
find those locations?
Webb also will study the outer planets and their moons. Of particular interest is Titan,
the largest moon of Saturn, now being explored by NASA’s Cassini spacecraft. Titan is as
big as the planet mercury, possesses an atmosphere half again as thick as Earth’s, and a frigid
surface with lakes of liquid hydrocarbons. Webb will map Titan’s chemical makeup with
six times Cassini’s resolution and monitor the moon’s seasonal changes over a decade
or more. Next stop Uranus. When Voyager 2 returned
this image in 1986, the planet’s south pole was facing the sun and few clouds could be
seen. But as Uranus neared its equinox in 2007, bright clouds suddenly materialized.
So far scientists are at a loss to explain this profound seasonal change. During Voyager’s visit, the northern hemispheres
of Uranus’s big moons were all in shadow. But when Webb begins service, the moons’ northern
halves will face the sun and give astronomers abundant new real estate to explore. Three years later, in 1989, Voyager 2 passed
Neptune and imaged its strange dark spot. Over the following years, astronomers have
seen the dark spot disappear, and then reappear. Voyager easily picked out clouds despite Neptune’s
greater distance from the sun. Why is weather on Neptune and Uranus so different? Neptune’s big moon Triton is unusual too.
Nitrogen-spewing volcanoes and other geological forces reshaped this frozen surface in ways
we’re just beginning to understand. Comets, asteroids, the outer planets and their
moons, and beyond them, the icy bodies of the Kuiper belt: these objects provide us
with the closest and most detailed look at how our own solar system evolved. The James Webb Space Telescope makes it possible
to take that understanding a step further, to probe the makeup of nearby planetary systems
at comparable distances from their stars. Webb will allow astronomers to directly compare
the chemical and physical properties of our outer solar system with similar zones around
nearby stars.

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