skyandtelescope.com • JANUARY 2020 21begins within a few million years of the disks’ formation.
Spitzer has also seen the very stuff that life as we know it
depends on being absorbed into forming planetary systems.
Spectra of face-on protoplanetary disks show us warm gas
rich in water vapor within the central few astronomical units
around the protostar. At the same time, looking edge-on
through a cold disk we see absorption due to silicate dust,
as well as the telltale signatures of frozen water and other
ices that have condensed on the cold surfaces of the silicate
grains. These icy grains might one day participate in the for-
mation of habitable worlds.Exoplanets
The study of exoplanets is one of the most exciting areas
of contemporary astrophysical research. Astronomers have
detected only a few dozen exoplanets directly, because it is
very diffi cult to see the light from a planet in the glare of the
nearby host star. But exoplanets are so common that many lie
in orbits seen edge on, passing fi rst in front of, then behind
their stars from our perspective. This geometry gives Spitzer
multiple ways to learn about alien worlds.
One of the most famous examples of this work is the Trap-
pist-1 planetary system. Following up on ground-based obser-
vations that hinted at a peculiar system, a 20-day Spitzer
campaign caught seven Earth-size planets transiting across
the face of the faint red star Trappist-1 in 2016. Three of these
exoplanets may lie in the star’s habitable zone, where liquid
water could exist stably on their surfaces.
Spitzer’s precise timing of these worlds’ transits enabled
astronomers to determine that gravitational tugs exchanged
by the planets changed the exact moment when each planet
crossed in front of the star. The altered transit times in turn
revealed the exoplanets’ masses. As the planets’ radii are
known from how much starlight they block as they transit,we thus also know the worlds’ densities. This makes Trap-
pist-1 perhaps the best characterized planetary system outside
of the solar system.
Astronomers can also use Spitzer to study planets’ heat
signatures. If a planet glows brightly enough in the infrared,
then when it passes behind its star Spitzer will detect a tiny
drop in the system’s emission, because the light of the planet
is no longer seen. The depth of this eclipse tells us how much
infrared radiation the planet emits. When combined with the
planet’s size, this measurement indicates the planet’s tem-
perature. Spitzer has measured planets as hot as 3000K and
as cool as 700K, but it cannot reach down to Earth’s tempera-
ture, which is about 300K.
Transiting systems can also tell us about exoplanet atmo-
spheres. Spitzer’s measurements can be combined with obser-
vations at shorter wavelengths to study the composition of an
exoplanet’s atmosphere and even to diagnose the presence of
clouds or hazes. Spitzer eclipse measurements in fi ve infrared
bands between 3.6 and 16 microns show that the exoplanet
GJ 436b, for example, has a much higher fraction of heavy
elements in its gaseous atmosphere than does its host star. GJ
436b is about the size of Neptune, which, interestingly, shows
a similar enhancement in heavy elements relative to the Sun.
In addition, we can study another aspect of a planet’s
atmosphere by observing the change in its brightnessWavelength (microns)B
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