Sky & Telescope - May 2020 - 40

To Make a Planet

poor in easily vaporized compounds such as H2O and CO,
as those volatiles should have condensed into ice and thus
been locked into planet cores along with the solids. Therefore,
we can use the chemical signatures in exoplanet atmospheres to infer where a planet formed, and thus whether it
migrated into its current position. For example, analysis of
the HR 8799 system, which hosts four super-Jupiter planets,
revealed differences in the ratio of carbon to oxygen in the
planets' atmospheres, consistent with the outer two planets
having formed outside the water snow line and the inner two
planets having formed inside of it.

When Do Planets Start to Form?

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

ONE-STOP SHOP The MWC 758 system shows not only a large
(and slightly off-center) gap but also a faint spiral arm and two
bright clumps within its disk.

planet cores occurs even earlier than 1 million years. If planets
assemble at the same time as the star and disk, then we would
have to fundamentally change our models of planet formation.
To chase down the answers to these mysteries, astronomers will soon add to their arsenal the next generation of
30-meter telescopes, which will give us an unprecedented
ability to probe the structure and motions in young protoplanetary disks (S&T: Dec. 2018, p. 14). ALMA recently
detected the ﬁrst signatures of planet-building material
ﬂowing into a gap in a protoplanetary disk, where a young
planet is thought to be orbiting. The observation provides an
exciting preview of what is yet to come. We'll combine with
these ground-based instruments the upcoming space-based
missions like the James Webb Space Telescope (JWST), set to
launch in 2021. Its sensitivity at infrared wavelengths will
enable the ﬁrst demographic chemistry studies of planetforming environments. Together, these facilities promise to
enable us to place our solar system within its larger galactic
context, rather than the other way around.

¢ MEGAN ANSDELL is a Research Fellow at the Flatiron Institute's Center for Computational Astrophysics and Center for
Computational Mathematics. Her research focuses on observing planet formation and applying machine learning to astrophysical problems.

NR AO / AUI / NSF

A big question that astronomers are now reviving based on
these new observations is how quickly planets form. Traditionally, we estimated how long it takes a disk to dissipate and
used that as a proxy. The reasoning is simple: Material must
still exist around the star in order to make planets.
Astronomers can fairly easily determine whether or not
there is any dusty material orbiting a star by searching for
extra infrared emission. Dust grains warmed by their proximity to the star glow brightly at these wavelengths. Large-scale
infrared surveys conducted by the Spitzer spacecraft in the
2000s conﬁrmed that nearly all stars host protoplanetary
disks when they are very young, roughly 1 million years old.
But only 20% of stars still host disks by 5 million years of
age, and by 10 million years all but 5% of stars have lost their
disks. Therefore, astronomers have typically concluded that
planets must form within the ﬁrst 5 million years or so.
This timeframe set what we thought was a very strict limit,
because our theoretical models struggled to make planets so
quickly. But the infrared measurements from Spitzer didn't
tell us much about the structure and amount of material
around young stars, which can provide important clues to the
planet formation process.
The recent results from ALMA and SPHERE now suggest
that planet formation may occur even more rapidly than we
thought based on Spitzer data. One of the starkest examples
is the iconic HL Tau disk (see page 38), which was the ﬁrst
to be imaged with the full resolving power of ALMA. The HL
Tau system is just 1 million years old; it is still deeply embedded in its natal cloud, which is dumping material onto the
forming star and disk. However, ALMA can see through this
cloud and has revealed a disk with many concentric gaps and
rings. If forming planets carved these gaps, then they would
indicate that planets come together when the star and disk
themselves are still forming, on the order of hundreds of
thousands of years rather than millions of years.
Evidence from our own solar system also supports such
rapid planet formation. The age of Jupiter's core, inferred from
dating different meteorite populations, indicates that it had
already assembled by 1 million years after the start of the solar
system. Scientists also think that on the order of 10 Earth
masses of ice, rock, and metals went into forming Jupiter's
early core, implying that the growth of planetesimals and

Sky & Telescope - May 2020

Table of Contents for the Digital Edition of Sky & Telescope - May 2020