Sky & Telescope - July 2022 - 35

O
n October 6, 1995, exoplanet science went from an
obscure research topic to making headlines. That
was the day astronomers announced the discovery of
51 Pegasi b, the first exoplanet orbiting a star like the Sun.
The discoverers, Michel Mayor and Didier Queloz (both
University of Geneva, Switzerland), measured how the star
51 Pegasi wobbled back and forth under the gravitational
pull of the planet. This method, known as the radial velocity
technique, enables observers to estimate an exoplanet's
mass. It took them only about two weeks of telescope time
to spot 51 Pegasi b, because it turned out to be a peculiar
planet of a type previously unknown: a gas giant lying
extremely close to its host star, so close that it completed
an orbit in four days. These planets are now called hot Jupiters,
and the discovery earned the two scientists the 2019
Nobel Prize in Physics.
Several years earlier, though, in 1987, Canadian astronomers
Bruce Campbell (then University of Victoria), Gordon
Walker, and Stephenson Yang (both then University of
British Columbia) had announced another discovery - a
Jupiter-sized planet around the primary star of a binary
system called Gamma Cephei. They had also used the radial
velocity technique, but their planet was more challenging
to detect because it needed 2.7 years to complete an orbit.
They had monitored the star for six years to pick up its
periodic signal.
That year, the three scientists announced their preliminary
results at a press conference at the summer meeting of
the American Astronomical Society in Vancouver. They hit
a wall of skepticism.
At that time, most scientists saw exoplanets as a matter
of science fiction rather than a legitimate field of research.
If any planets could be found around other stars, they were
expected to resemble those in our own solar system. Hence,
the discovery of a planet in a binary system, where the
stars are as close as the Sun is to Uranus, was a hard sell. It
seemed unlikely that a planet could form and survive in the
narrow space between the stars.
In 1992, doubtful of their own findings, Walker and
Yang joined with colleagues to retract their claim to the discovery
of the first exoplanet. They chalked the wobble they'd
picked up to the intrinsic variability of the primary star,
which was misclassified as a giant at the time and therefore
expected to suffer surface variations that could mimic the
presence of a planet. The third member of the team, Bruce
Campbell, had already quit astronomy in frustration when
he couldn't find a permanent position in the field.
But years later, in 2003, updated observations of
Gamma Cephei showed beyond doubt that Gamma Cephei
Ab is real. The star is actually a less active subgiant and
does not produce a planet-mimicking signal. Its exoplanet is
a gas giant at least 1.4 times more massive than Jupiter and
has an orbital period of 2.5 years, almost identical to the
orbit originally calculated by Campbell, Walker, and Yang.
Had they stuck to their guns, they might have been the ones
shaking hands with the King of Sweden.
Seeing Double: How Many Planets
Are There?
Around half of all Sun-like stars in our galaxy are in binary
or multiple systems - groups of gravitationally bound stars
that orbit a common center of mass. Binaries are the most
common, but there are also cases in which two binaries
are bound to each other to form a quadruple system. Even
three binaries can join to create a sextuple system, like the
recently discovered TYC 7037-89-1.
TWO SUNS Artist's concept
of a circumbinary system,
in which exoplanets orbit a
pair of stars
sk yand tele scope .o r g * JULY 2022 35
NASA AMES / JPL-CALTECH / T. PYLE
http://www.skyandtelescope.com
Sky & Telescope - July 2022

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