Sky & Telescope - July 2022 - 37

S-TYPE PLANET: GREGG DINDERMAN / S&T, SOURCE: MCDONALD OBSERVATORY;
P-TYPE SYSTEM: GREGG DINDERMAN / S&T, SOURCE: NASA / JPL CALTECH / T. PYLE
Of the 131 S-type exoplanet systems known, astronomers
have only found about 20 around a star that's less than
50 a.u. from its companion, confirming that S-type planets
exist around close binaries but are less common than they are
in wide binaries. It seems that when the stars in a binary get
too close, nature prefers a different solution: circumbinary
planets.
Two Suns in the Sky
When NASA's Kepler mission started operations in May
2009, astronomers knew it would find hundreds of eclipsing
binaries. The same sensitivity that enabled the space telescope
to detect small dips in starlight as planets crossed in front of
stars also revealed stars passing in front of each other. After
four years of operation, it had detected 2,878 of them.
Kepler scientists were interested in eclipsing binaries
because if the stars' orbital plane was aligned with our line of
sight, then it was likely that any planets the binary harbored
would be in the same plane. But astronomers were expecting
S-type planets in binaries with large separations.
" What happened with Kepler was that it was looking into
eclipsing binaries and then, entirely by accident, Kepler-16
was discovered, " Haghighipour says. Kepler-16 contains
two low-mass, main-sequence stars in a 41-day orbit, only
0.22 a.u. apart - less than Mercury's distance from the Sun.
A planet travels in a nearly circular orbit around the pair at
0.7 a.u., completing an orbit every 229 days.
" Once we found the first one, then we were able to find
other ones, " says Haghighipour. By the end of its mission,
Kepler had discovered a total of 12 circumbinary planets in
10 systems. One of them, Kepler-47, is the only multipleplanet
circumbinary system found to date. It has three,
roughly Neptune-size planets in 49.5-, 187-, and 303-day
154
Number of binary
star systems
with confirmed
planets
33
217
Number of
confirmed planets
in binary star
systems
49
SOURCE: RICHARD SCHWARZ / UNIVERSITY OF
VIENNA, HTTPS://IS.GD/UNIVIESCHWARZ
Number of planets
in multi-star
systems
Number of multistar
systems
with confirmed
planets
orbits around a tight binary. With a density similar to that
of Saturn, none of them seems to have a rocky surface. The
whole system could fit inside Earth's orbit around the Sun.
The fact that every circumbinary planet found so far is a
gas giant might confirm predictions made by Haghighipour
and others, even before these planets were found. They predicted
that in circumbinary systems, planets could only form
at distances greater than 5 a.u. from the stellar pair, where
they wouldn't feel its disrupting gravitational wobble. Out
there, in the vast expanse of chilly gas, the most likely worlds
to form would be giants.
Other scientists also predicted that, once formed, circumbinary
planets would migrate in towards the stars, where
Secondary star
Planet d
Primary star
Primary star
Planet b
Secondary
star
Planet c
Planet b
4 a.u.
0.3 a.u.
GAMMA CEPHEI Long disputed, the gas giant around Gamma
Cephei A is an S-type planet: It orbits only one of the stars in a binary
system, with the second, cooler star much farther away.
KEPLER-47 The three gas giants in the Kepler-47 system orbit a pair
of stars - one a near-twin to the Sun in size and temperature, the other
a much cooler red dwarf.
sk yand tele scope .o r g * JULY 2022 37
HTTPS://IS.GD/univieschwarz http://www.skyandtelescope.com

Sky & Telescope - July 2022

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

Contents
Sky & Telescope - July 2022 - Cover1
Sky & Telescope - July 2022 - Cover2
Sky & Telescope - July 2022 - 1
Sky & Telescope - July 2022 - Contents
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