Sky & Telescope - USA (2020-01)

The Infrared Sky

22 JANUARY 2020 • SKY & TELESCOPE

throughout its orbit as it shows us different fractions of its

starlit side. This pattern, called a phase curve, shows how well

the atmosphere redistributes the energy of absorbed starlight.

When astronomers converted Spitzer’s phase curve into a

map of the temperature distribution for the Jovian-mass

exoplanet HD 189733b, the map showed that the hottest

spot on this exoplanet is not at the point where the star is

directly overhead. Rather, the hotspot is displaced by about

30 degrees in longitude, likely due to winds of thousands of

miles per hour transporting energy before it can be radi-

ated away. Spitzer has seen similar offsets on other planets,

including 55 Cancri e. In the case of the recently discovered

super-Earth LHS 3844b, by contrast, the absence of such an

offset, combined with the drastic drop in temperature from

the dayside to the nightside, shows that this exoplanet has at

most a very thin atmosphere. Although many telescopes have

measured transits, Spitzer has stood almost alone in its ability

to measure eclipses and phase curves.

The discussion above illustrates how scientists have used

Spitzer and other telescopes to derive remarkably detailed

information about exoplanets, even though they’re never

directly seen. The architectures of these systems differ from

that of our own solar system. Indeed, if our familiar eight

planets orbited a nearby star at the same distance they orbit

the Sun, they would have gone undetected by most of the

techniques used to date.

Nevertheless, there are remarkable similarities between

our own solar system and exoplan-

etary systems. Systems with multiple

planets are common. The silicate

materials found often resemble those

seen in comets, such as Hale-Bopp

and Tempel 1. Many systems show

evidence for two bands of circumstel-

lar dust, corresponding roughly to the

zodiacal dust in the inner solar system

and the Kuiper Belt farther out. In at

least one case, four giant planets orbit

in the region between these two belts, just as Jupiter, Saturn,

Uranus, and Neptune lie between the two solar system belts.

Finally, collisions between 100 km-size asteroids in systems,

inferred from transient increases in the dust orbiting the

stars, are counterparts to the violent events that shaped our

system’s inner planets.

Thus, the evolution of the universe has led in many cases

to conditions similar to those in our own system, including

conditions that might be favorable to the development of life.

The Distant Universe

Spitzer has also observed beyond the stars and exoplanets of

our own galaxy, reaching out to the billions upon billions of

galaxies in the universe. Understanding how galaxies form

and evolve has been a driving question in astrophysics for

many decades. Infrared observations have been applied to this

question in two separate domains: low and high redshifts.

These domains split at a redshift of 3, corresponding to a

lookback time of approximately 11.5 billion years.

With its enormous gain over prior missions in imaging

sensitivity, predominantly at 24 microns, and its substantial

spectroscopic capability, Spitzer has probed infrared-bright

galaxies throughout the universe’s last 11.5 billion years. For

these galaxies, any infrared emission at wavelengths longer

than 5 microns is generally the warm glow from dust heated

by young stars. This radiation is a proxy for the number

of young stars, and from this glow we can determine the

Planet light

Starlight

Duration of one orbit (17.8 hours)

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uPHASE CURVE When 55

Cancri e transits in front of its

star (A in both orbit and light

curve diagrams), the dip’s size

reveals the planet’s diameter.

When the planet moves behind

the star, its infrared glow disap-

pears (C), revealing its bright-

ness. Together, the two dips

tell astronomers the planet’s

temperature. However, the peak

of the planet’s light curve (B) is

offset from its eclipse, indicating

that the hottest point is not at

high noon. That suggests strong

winds redistribute the star’s heat

across the planet. GR

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