Sky & Telescope - July 2021 - 24

Planets in Crowded Neighborhoods

Pr 0201
[1 hot Jupiter]
K2-100
[1 hot Neptune]

K2-102
[1 super-Earth]

K2-104
[1 super-Earth]

K2-264
[2 mini-Neptunes]

Pr 0211
[1 hot Jupiter]
[1 super-Jupiter]

K2-101
[1 super-Earth]

K2-95
[1 Neptune]

Between December 2003 and July 2006, Sato and his team
took radial-velocity measurements of Epsilon Tauri using the
1.88-m telescope at Okayama Astrophysical Observatory in
Japan, as part of an exoplanet search around G and K giants.
From the data they were able to tease out the unmistakable
sign of a 7.6-Jupiter-mass planet orbiting Epsilon Tauri every
595 days, at an average distance of 1.93 astronomical units
from the star - a trek slightly larger than the one Mars takes
around the Sun. The age of the Hyades is around 625 million
years, at the time providing the most secure upper limit for
how quickly giant planets form.
Besides a world around a star in NGC 2423 that might
or might not be a brown dwarf, it would be ﬁve years before
astronomers found more open cluster exoplanets: two gas
giants orbiting stars in M44, the Beehive Cluster. Further
discoveries trickled in afterwards, but it wasn't until three
Jupiter-mass planets were reported around three stars in M67
in 2014 that astronomers felt conﬁdent that the frequency of
giant planets in open clusters agreed with that for ﬁeld stars.
Today, we know of around 30 exoplanets in open clusters.

Planetary Migration
Back in 1995, when the ﬁrst exoplanet around a mainsequence star was announced, the most surprising aspect of
24

J U LY 2 0 2 1 * S K Y & T E L E S C O P E

its nature was that it was a Jupiter-mass planet orbiting close
to its parent star. The exoplanet makes one revolution around
the ﬁeld star 51 Pegasi every 4.2 days, at a distance just over
10% that of Mercury from the Sun. Planetary-formation
theories at the time didn't consider the possibility of such hot
Jupiters; we were expecting Jupiter-like planets in Jupiter-like
orbits, because we didn't think gas giants could form close to
stars. Attention quickly turned to the idea of planetary migration, planets moving to different orbits over time.
Two of the leading ideas for planetary migration involve
either interactions with the young circumstellar disk or gravitational interactions with other planetary or stellar companions. The time scales for these mechanisms are very different.
Circumstellar disks are only expected to survive for around
10 million years, and modeling suggests that the orbits of
any planets interacting with the disks would remain circular
while the drag brings them in closer to the star.
Gravitational interactions with other bodies can occur
over much longer timescales, and unlike disk drag, these lead
to the exoplanet's orbit becoming slightly elongated. Over
time, tidal interactions with the parent star will circularize the exoplanet's orbit again. There is thus a window in
a planet's evolution when the shape of its orbit may tell
astronomers how it wound up where it is around its star.

RON BRECHER

S M44 Astronomers have found about 10 exoplanet systems in the Beehive Cluster, many of which are marked here (some are out of the field of
view). The open cluster lies in Cancer, the Crab.

Sky & Telescope - July 2021

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