YSM Issue 95.2

FOCUS
Astronomy
HOW TO MAKE A HOT JUPITER
Investigating the orbits of large
exoplanets to uncover the mysteries
behind solar system formation
BY BRIANNA FERNANDEZ
If you’re reading this, you likely live in a
solar system. I’d venture to say that you
live on Earth, too, making you a resident
of its solar system. Given these assumptions,
you’re likely familiar with its structure: four
rocky planets on the inside followed by four
gaseous planets on the outside, all orbiting
the Sun in nearly identical equatorial planes.
This structure feels standard, safe. But every
tellurian astronomer was forced to confront
this biased assumption upon the discovery of
the first extrasolar planet.
In 1995, astronomers Didier Queloz and
Michel Mayor discovered planet 51 Pegasi b,
the first known planet found outside of the
solar system. This planet is large, hot, and
extremely close to the sun-like star that it
orbits—much closer than Mercury is to the
Sun. It is what is now called a “hot Jupiter,”
a gas giant planet with an orbit extremely
close to its star. This single planet scrambled
astronomers’ understanding of solar system
formation since they had previously assumed
that gas giants orbited far from their stars,
much like our beloved Jupiter. Now that over
5,000 exoplanets have been discovered, we can
create a “normal distribution” of solar systems,
and ours is far from the center of the bell curve.
Though they are familiar to us, our
system’s orbits, which transit the equator
of the Sun, are just as extreme as orbits
wherein a system’s planets orbit from pole
to pole. So while large, gaseous planets
completing orbits around their stars in
just a few days may seem foreign to us, it
is frequently observed in other planetary
systems. The only problem is that we don’t
understand how they do it.
Yale astronomy researchers Malena Rice
and Greg Laughlin are tackling this issue
head-on. Fifth-year Ph.D. student Malena
Rice was studying how a warm Jupiter, a
gas giant with an orbital period of over
ten days, fit into existing planet formation
theories when she noticed an interesting
correlation. “I made about a hundred
plots looking at how this planet fits in with
the bigger picture of all the [previous]
measurements… and I realized that, no
matter how you look at them, the eccentric
planets tend to be more misaligned,” Rice
said. This misalignment of eccentric hot
Jupiters, or those with more elliptical
rather than circular orbits, could be the key to
understanding how they form.
Heat Your Jupiter to 2000 °F
But how did hot Jupiters get their
characteristic elliptical orbits in the first
place? This issue has separated astronomers
into a few different camps. Perhaps hot
Jupiters formed in their original places in a
process called in situ formation. However,
this is difficult to do because there isn’t a lot
of planet-forming material near the stars
they orbit. Others argue that they might have
formed farther out and migrated inward,
shepherded by other planets in the solar
system through gravitational interactions.
In this process, the hot Jupiters would spiral
slowly and gradually toward the inner orbits
of the system. There is strong support for this
latter theory in astronomical communities.
The third camp argues in favor of higheccentricity
migration, a framework in which
hot Jupiters are born farther from their host
stars or the stars that they orbit. Through
scattering and nudging from other sources, the
hot Jupiters are knocked onto highly elongated
orbits and spiral inwards to reach much closer
orbits over time. “One way to distinguish
which of these is actually correct is by looking
at whether or not hot Jupiters are aligned with
the plane of their host star’s equator,” Rice said.
In general, we expect host stars to spin in the
same direction as their surrounding disks of
dust, gas, and other debris. These disks form
around newly-made stars to eventually form
planetary bodies through collisions of the
orbiting particles. In the beginning, everything
should form in the same plane. So, planets
that are misaligned or orbiting in directions
different from their host stars could indicate
that the system underwent a dynamically
dramatic process. These misalignments require
a kick hard enough to tilt entire systems, such
as one planet tossing another into a different
orbit, a star flying by and tilting the disk, or
one planet being thrown into the host star and
being engulfed. The fact that systems with hot
Jupiters are rarely observed with other planets
indicates that the Jupiters’ would-have-been
neighbors could have been thrown out early
on or engulfed by the chaos caused by the
dramatic inclination shift.
Observe Your Jupiter Transiting Its Star
Hot Jupiters are incredibly well-studied by
astronomers because they are the easiest to
observe. To detect these planets, astronomers
14 Yale Scientific Magazine May 2022 www.yalescientific.org