Sky & Telescope - October 2022 - 29

GREGG DINDERMAN / S&T, SOURCE: S. R. KANE ET AL. /
JOURNAL OF GEOPHYSICAL RESEARCH: PLANETS 2021
(such as HD 80606b, discovered in 2001,
which changes its distance from its star by
a factor of 30 over the course of its year),
and planets orbiting in planes dramatically
different from their star's rotation
(such as HAT-P-11b, characterized in
2010, on a nearly polar orbit).
These early exoplanet discoveries
Earth
Mercury
Venus
EXTREME ORBIT The Jupiter-size exoplanet
HD 80606b follows an elongated path around its
star, taking it from 0.03 astronomical unit out to
0.89 a.u. (Solar system orbits shown for reference.)
to the star, it is hard to push them onto
very elliptical orbits thereafter.
In the third scenario, gas giants
revealed that not all planetary systems
share our solar system's key characteristics.
With new missions and more sensitive
instruments, we learned that inner
planetary systems can also be home to
medium and small planets, some of which
are packed very close together.
The diversity of planetary systems is teaching us that our
origins theory is incomplete. We need a new blueprint that
lays out why we find something like our solar system orbiting
one star, a misaligned hot Jupiter on an elongated orbit
around another, and a system of five tightly spaced planets
close in to a third. Investigating giant planets that occupy the
inner zone of solar systems is an important step in drawing
this blueprint: They are likely the extreme outcomes of physical
processes that are at work in many planetary systems,
and they are also most at odds with our origins theory. If we
understand them, then we'll be much closer to understanding
the full diversity of planetary systems.
Three Scenarios
Scientists have proposed three general scenarios to explain
the existence of close-in giant planets. Each expands our origins
theory into a richer and more dynamic sequence, with
more crosstalk between the inner solar system and regions
farther out than once predicted.
In the first scenario, the giant forms right where we see it
today: near the star. Dust, pebbles, asteroids, or even a whole
core travel through the gas disk from the outer region to the
inner region, where the lack of abundant material would
have stymied a giant's formation. The resulting large core
accretes surrounding gas and becomes a gas giant, forming
very close to the star instead of in the outer region of the
disk. This scenario is inspired by another type of extra-solar
system: large, rocky super-Earths close to their stars. If superEarths
can form or arrive close to the star during the gas disk
stage, some may grow into hot Jupiters.
In the second scenario, close-in giant planets originally
form much farther from the star, like our own giant planets.
Then, through interactions with the gas disk, the full-fledged
gas giant moves inward.
Both the in situ and disk-migration scenarios keep the gas
giant on a circular, coplanar orbit - at least while the gas
is still around - and can plant it at a wide range of separations
from its host star. These scenarios may also allow other
planets to form or arrive nearby. Even if the giant planets are
gravitationally disturbed after migrating or coalescing close
HD 80606b
migrate after the gas disk disappears,
this time through a series of dramatic
events. First, another planet or star in
the system kicks - or perhaps gradually
pulls - the gas giant onto a highly
elliptical orbit. The elliptical orbit brings
the gas giant periodically very close to its
star. Tidal forces stretch the planet during its close approach;
as it moves far away from the star, the planet returns to its
spherical shape. The process repeats at the next passage. This
periodic stretching generates frictional heat and saps energy
from the planet's orbit, shrinking and circularizing the orbit
over many passages. This tidal migration likely destroys any
intervening planets along the way, leading to a lonely giant
planet that's close to its star.
To make a hot Jupiter through tidal migration, the change
to the orbit needs to be just right: too strong, and the gas
giant will come so close to the star that it will be either
ripped apart by tides or hit the star; too weak, and the giant
won't approach its sun close enough for tides to shrink its
orbit over the star's lifetime. When the close pass is just
right, the orbit circularizes over time, but we may catch
younger or more recently disturbed planets that are still on
elongated orbits.
The Power of Populations
More than 25 years after the discovery of the first hot Jupiter,
we now know of hundreds of close-in giant planets. We can
make use of other properties of these planets, their stars, and
their systems to test the three formation scenarios: in situ
formation, disk migration, and tidal migration. Studies so
far tell us that no one scenario can explain all the observed
properties, and that at least two scenarios are likely at work.
Orbital properties provide powerful tests of the scenarios.
If the tidal migration mechanism is at work, for example,
then we expect to find young, close-in giant planets on highly
elongated orbits, still in the midst of tidal circularization. In
contrast, giants that formed in situ or by disk migration will
always inhabit circular orbits. We now know of about two
dozen hot Jupiters on elliptical orbits that point to ongoing
tidal migration. What we don't know is whether the circular3
Earth days
Typical orbital period
(or " year " ) of a hot Jupiter
sk yand tele scope .o r g * OCTOBER 2022 29
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Sky & Telescope - October 2022

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