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.