Sky & Telescope - May 2020 - 36

To Make a Planet

By the end of the 1900s, the generally accepted theory
was that the dust grains orbiting in these disks would collide, stick together, and build up into rocky bodies known
as planetesimals, eventually growing into planet cores. The
planet cores that grew quickly enough and were sufﬁciently
massive to grab gas from what remained of the surrounding disk would become giant planets like Jupiter and Saturn;
those with less mass would remain as smaller, rocky worlds
like Earth.
In this picture, the split between the small inner and giant
outer planets in our solar system could be naturally explained
by the fact that the outer regions of protoplanetary disks
should contain more planet-building material, and also that
this material should more easily coagulate. That's because
these outer regions are much colder due to their larger
distances from their host stars, and so their dust grains are
icy and thus more likely to stick together when they collide.
(Think of how easy it is to pack snow into a ball compared
to dry dirt or pebbles.) The extra stickiness accelerates the
growth of dust grains into large planetary cores that can
quickly accrete the surrounding gas to become giant planets.
Although we have since uncovered some roadblocks in this
theory's speciﬁcs, the overarching picture holds: Planets are
assembled in disks of gas and dust surrounding young stars
that form as a natural consequence of the star formation process. Indeed, we see these protoplanetary disks around nearly
all young stars in nearby star-forming regions, just as nearly
all mature stars in our galaxy likely host at least one exoplanet.
However, as our observations of protoplanetary disks
improve, we are ﬁnding that, as with exoplanets, the disks
exhibit a surprising diversity that has challenged key parts of
our traditional theories of planet formation.

We see these protoplanetary disks
around nearly all young stars in nearby
star-forming regions, just as nearly all
mature stars in our galaxy likely host
at least one exoplanet.
A Better View
In recent years two facilities in particular, the Atacama Large
Millimeter/submillimeter Array (ALMA) and the SpectroPolarimetric High-contrast Exoplanet Research (SPHERE)
instrument, have been providing the most detailed views
of protoplanetary disks that we have ever seen. These two
facilities take very different observations, but the data sets
complement each other to improve our understanding of
planet formation.
ALMA is what we call an interferometer, consisting of
more than 60 antennas spread up to 16 km across the Atacama Desert in Chile. These individual dishes work together
to act as a single, much larger telescope to obtain sensitive,
high-resolution images at radio wavelengths. In contrast,
SPHERE is an extreme adaptive optics system used with one
of the four 8.2-m Very Large Telescope (VLT) mirrors located
on Chile's Cerro Paranal. This instrument produces high-resolution and high-contrast images at optical and near-infrared
wavelengths.
SPHERE is tuned to detect the starlight that scatters off
small, micron-size dust grains near the surfaces of protoplanetary disks. These dust grains are comparable in size to smoke
particles, and like smoke they ﬂoat easily, interspersed in the
disk's gas. This behavior makes them good tracers of where the

q DISK COMPOSITION In the center of a protoplanetary disk, starlight sublimates dust, leaving a gas-filled hole around the star. This gas accretes
onto the star. Farther out, the dust coagulates and settles toward the disk midplane. Beyond the snow lines, ices make grains stickier, fostering giant
planet growth. Different wavelengths probe different parts of the disk. Distances are approximate.

Near-infrared
Giant planet formation?

Scattered visible/
near-infrared
Mid-infrared

Dust settling

Gas only
Snow line
Grain growth
100

1
Distance (a.u.)

M AY 2 0 2 0 * S K Y & T E L E S C O P E

Magnetospheric
accretion flows
(ultraviolet)

G REGG DINDER M A N / S&T

Submillimeter and millimeter

Ices

Sky & Telescope - May 2020

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