[ExI] One star, five planets

[Damien Broderick]

http://www.cosmosmagazine.com/node/1687


Distant sun has system of five planets

Wednesday, 7 November 2007
Cosmos Online


SYDNEY: Astronomers have detected a fifth 
extrasolar planet circling 55 Cancri, a star 41 
light years away. This is the greatest number of 
confirmed planets yet found orbiting another star.

"This discovery of the first ever quintuple 
planetary system has me jumping out of my socks," 
said Geoff Marcy an astronomer at the University 
of California, Berkeley in the USA. "The 
significance is marvellous. We now know that our 
Sun and its family of planets is not unusual."

Liquid water?

55 Cancri is located 41 light-years away in the 
constellation Cancer and has nearly the same mass 
and age as our Sun (it is easily visible with 
binoculars from Earth). The researchers 
discovered the fifth planet using the Doppler 
technique, in which a planet's gravitational tug 
is detected by the wobble it produces in the parent star.

It weighs about 45 times the mass of Earth and 
may be similar to Saturn in its composition and 
appearance. It is the fourth planet out from 55 
Cancri and completes one orbit every 260 days.

The researchers have also discovered that the 
planet's location places it in the habitable 
zone: a band around the star where the 
temperature permits liquid water to exist – and 
therefore renders the discovery of life more 
likely. Though its orbit is slightly closer than 
Earth's is to our Sun, its star is slightly fainter.

"The gas-giant planets in our Solar System all 
have large moons," said Debra Fischer, an 
astronomer at San Francisco State University in 
California and lead author of a paper on the find 
upcoming in the Astrophysical Journal. "If there 
is a moon orbiting this new, massive planet, it 
might have pools of liquid water on a rocky surface," she said.

Fischer and Marcy are part of a team that 
discovered this planet after careful observation 
of 2,000 nearby stars with a telescope at 
California's Lick Observatory and the Keck 
Observatory on Mauna Kea, Hawaii. More than 320 
velocity measurements were required to 
disentangle signals from each of the planets.

Family of planets

The planets around 55 Cancri are somewhat 
different from those orbiting our Sun. The 
innermost planet is believed to be about the size 
of Neptune and whips around the star in less than 
three days at a distance from the star of 
approximately 3.5 million miles. The second 
planet is a little smaller than Jupiter and 
completes one orbit every 14.7 days, at a 
distance from the star of approximately 11.2 
million miles. The third planet, similar in mass 
to Saturn, completes one orbit every 44 days at a 
distance from the star of approximately 22.3 million miles.

The fifth and most distant known planet is four 
times the mass of Jupiter and completes one orbit 
every 14 years at a distance from the star of 
approximately 539.1 million miles. It is still 
the only known Jupiter-like gas giant to reside 
as far away from its star as our own Jupiter.

"Discovering these five planets took us 18 years 
of continuous observations at Lick Observatory, 
starting before any extrasolar planets were known 
anywhere in the universe," said Marcy. "But 
finding five extrasolar planets orbiting a star 
is only one small step. Earth-like planets are the next destination."

"This work marks an exciting next step in the 
search for worlds like our own," commented 
Michael Briley, an astronomer at the U.S. 
National Science Foundation. "To go from the 
first detections of planets around sun-like stars 
to finding a full-fledged solar system
 in just 
12 years, is an amazing accomplishment and a 
testament to the years of hard work put in by these investigators."