focus on research hen Barbara McArthur began looking “just for fun”
W
at 55 Cancri, a solar system 41 light-years away in
the constellation Cancer, she didn’t expect to find
a new planet hiding there. At the time, there were
only three known planets orbiting the distant sun, and
McArthur wanted to better measure the planets’ sizes.
In the process, and much to her delight, McArthur
discovered a new, smaller planet spinning closely
around 55 Cancri. The Neptune-sized planet, called
“55 Cancri e,” was the lowest mass extrasolar planet
known at the time. It takes a mere 2.8 days to whip
around its sun.
McArthur, a research scientist in astronomy, didn’t
Discovering
gaze for long hours at the night sky to find the planet.
What she did do is analyze data—a lot of data—col-
lected from very powerful telescopes.
Planets
One of these scopes, the Hobby-Eberly Telescope
(HET) located at the McDonald Observatory, took
snapshots of the 55 Cancri system for 100 days over
a 190 day period. “No one else could do this at that
time,” says McArthur. “We had the capability to tell a
big instrument to look at the system every day.”
The HET measured the star’s wobble, which is caused
by the gravitational pull of its planets. The wobble
causes light reaching Earth to shift in color. By mea-
suring the star’s color shifts, its planets’ masses and
orbits could be determined. McArthur then modeled
the solar system in a novel way, by combining the HET
data with data from NASA’s Hubble Space Telescope
and the Lick Observatory in Northern California.
“Barbara combined different data from different tele-
scopes,” says Fritz Benedict, senior research scientist at
the McDonald Observatory. “This had never been done
Barbara McArthur finds a in the past and that’s extremely powerful. That’s the way
she could dig out this tiny planet.”
Neptune-sized planet orbiting “We knew there were supposedly three planets,”
McArthur says. “When we looked at the data, we found
around a distant sun that there was this other pattern of 2.8 days in the data,
which was really cool.”
That 2.8-day pattern was the footprint of the new
planet, 55 Cancri e, which McArthur pulled out of the
weeds of data like an Easter egg.
McArthur’s discovery made 55 Cancri the first 4-
planet extrasolar system that we know. Not only does
the system boast a whopping 4 planets, but one of these
is a Jupiter-sized planet orbiting at a Jupiter distance
from the sun.
“This is the closest analog to our solar system, be-
cause it has this Jupiter-like planet and the three in-
ner planets,” says McArthur. This makes the 55 Cancri
system a premier laboratory for modeling planetary
Planet image by: Melody Brayton Lambert. system formation and evolution and for learning more
Far Right: Barbara McArthur. Photo by: Matt Lankes. about our own solar system. ✥
what we know
146 planetary systems
170 planets total
18 extrasolar systems with more
than one planet
1 extrasolar system with
4 planets (55 Cancri)
data as of: 12.09.05
f o c u s o n s c i e n c e 9
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