Sky & Telescope - October 2022 - 30

Mass (Jupiter masses)
GREGG DINDERMAN / S&T, SOURCE: THE AUTHOR
Toasty Jupiters
orbit planets we see have completed tidal circularization or
were never on elongated orbits to begin with.
The mechanisms that put giant planets onto elliptical
orbits only rarely make hot Jupiters. We now have a good
handle on how common giant planets at different distances
from their host stars are. Although hot Jupiters
are less common than wider-separation giant
planets - about 10% of stars have a giant planet
and 1% have a hot Jupiter - they are not as rare as
we would expect from tidal migration. Therefore,
we think that some hot Jupiters must have originated
from in situ formation or disk migration.
Properties of hot Jupiters' host stars further
10%
1%
support the dual-scenario hypothesis. Tidal
circularization takes time, and we indeed see
that close-in Jupiters on elliptical orbits more often appear
around younger stars than those on circular orbits. However,
some worlds on circular orbits are too far away to have tidally
circularized, suggesting that their origins lie with one of the
other mechanisms. We also observe that the elliptical hot
Jupiters orbit stars that are enriched with heavier elements
like oxygen, carbon, and iron. This makes sense, because we
think that stars' compositions are similar to the material that
was in their planet-forming disks, and in disks with more solids,
more giant planets can form and either kick or gradually
torque each other into new, elongated orbits. In disks with
fewer solids, fewer giant planets tend to form; with fewer
planet interactions possible, these worlds tend to become hot
Jupiters through either disk migration or in situ formation.
Taking an inventory of which other planets, if any, accomHot
Jupiters
10
Warm Jupiters
Sun-like stars
with a gas giant
Sun-like stars
with a hot
Jupiter
pany close-in giants gives us further reason to think that dual
scenarios are commonly at work. Many hot Jupiters have no
other planets nearby in their system, consistent with disruptive
tidal migration. Farther out in their system, many have a
distant giant planet that might have caused the initial disturbance
(though we do not know for sure without
more detailed observations). However, there are
notable exceptions to hot Jupiters' loneliness,
such as WASP-47b, a hot Jupiter with two smaller
planets nearby. This planetary arrangement
couldn't have formed via tidal migration. Furthermore,
warm Jupiters orbiting farther from their
star - but still close-in compared to our own solar
system's cold Jupiter - commonly have nearby
planets, suggesting many of them originated from
disk migration or in situ formation.
Zooming in on Individual Systems
The past couple decades have brought us not only a huge
expansion in the collection of known close-in giant planets
but also more detailed views of individual systems. Astronomers
have discovered many close-in giant planets using the
transit technique, in which a planet passes in front of its star
and temporarily dims the star's light. High-precision measurements
and long, continuous observations from the Kepler
and TESS missions have also made it possible to use a valuable
new tool: transit-timing variations.
These variations arise because the gravitational influence
of a companion planet can cause a transiting exoplanet to
cross its star's face at slightly different times - sometimes
Cold Jupiters
1
Jupiter
Saturn
0.01
0.10
1
10
Distance from star (astronomical units)
A SEA OF JUPITERS Astronomers have discovered hundreds of gas giants around Sun-like stars, in much smaller orbits than Jupiter and Saturn
occupy in the solar system. Many of these fall in the " hot Jupiter " category, giants with an orbit less than one-tenth as wide as Earth's. (Note that
because each survey detects some kinds of planets better than others, the distribution here isn't the real distribution in the galaxy - it's just the
distribution of discoveries so far.)
30 OCTOBER 2022 * SKY & TELESCOPE
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Sky & Telescope - October 2022

Table of Contents for the Digital Edition of Sky & Telescope - October 2022