WWW.ASTRONOMY.COM 21
the MHD wind hypothesis. The
best evidence so far for an MHD
wind is a 2021 study in The
Astrophysical Journal of an active
young star, where astronomers
have measured how high the
wind appears to sustain itself
as it f lows away from the disk.
Measuring how powerful these
winds are — and therefore how
much angular momentum they
carry away — will be the next
major task in testing the theory.
Disk worlds
At the same time a star is form-
ing, so are the planets that will
orbit it. Both star formation and
planet formation happen within
disks via accretion. As gas and
dust swirls around the star, delin-
eations begin to appear in the
disk. Astronomers saw this for
the first time in 2014 in a young
star called HL Tauri. This obser-
vation, made using ALMA, was a
major advancement in our under-
standing of how planets form.
In a nascent planetary system,
the congregation of matter
depends on lots of different
factors. Turbulence, magnetic
fields, and the play of viscosity
between gas and dust may cause a
kind of traffic jam that eventually
congeals to form protoplanets.
Closer in, most of the gas gets
consumed by the star, leaving
rocky material and heavy metals,
which form terrestrial planets.
But there is a long-standing
question about how rocky bodies
accrete, involving a concept
called the bouncing barrier.
Electrostatic forces cause small
grains to stick together and larger
planetesimals are attracted to
each other by gravity. But how
does a particle become a planet?
Models show that objects in that
middle range between tiny and
massive just tend to bounce off
each other. So how do amassing
objects overcome this barrier to
growth?
One theory is that the particles
experience a drag force as they
move through gas in the disk.
“There’s a strong interaction
between solid particles and the
gas in the disk,” says James Stone,
an astrophysicist at PrincetonUniversity. “This causes clumps
to form, which over time can pro-
duce larger and larger objects.”
So far, this protoplanetary
evolution mechanism, called
streaming instability, seems to be
a promising way to grow things
from centimeter to kilometer
sizes. The most intriguing part
of the theory is that the gas is the
crucial component: Without it,
dust in the disk couldn’t coalesce
to form planetesimals. But there
is not yet direct evidence.The accretion
disk that
surrounds the
elliptical galaxy M87
feeds its central
supermassive black
hole (SMBH) and
relativistic jet in this
artist’s concept. ESO/M.
KORNMESSERThe ghostly
halo we see in
the groundbreaking
image of an SMBH
from the Event
Horizon Telescope
is the hot, glowing
plasma in the
accretion disk
surrounding it.
EVENT HORIZON TELESCOPE
COLLABORATION