Astronomers spy fifth planet orbiting nearby star

Largest extrasolar system to date

By Lester Haines, 7th November 2007

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Astronomers have spotted a fifth planet orbiting 55 Cancri, making the (relatively) nearby star the centre of the most populous extrasolar system to date, New Scientist reports.

55 Cancri lies 41 light years from Earth, and is "slightly cooler and dimmer than our own Sun". It was already known to boast four planets, three giants orbiting closer than Mercury is to the Sun, and a fourth at roughly Jupiter's distance from the Sun, but four times as massive as the former.

The first four planets were identified using the radial-velocity method, or "the way their gravity tugs on the parent star". Researchers led by Debra Fischer of San Francisco State University and Geoff Marcy of the University of California in Berkeley, used the same technique to pinpoint the fifth, which lies between the hot inner trio and the distant world.

The new planet has been dubbed 55 Cancri f, and "appears to be a gas giant like Saturn". Significantly, it orbits 55 Cancri at a distance of 117 million kilometres, "about eight per cent farther than Venus is from our Sun", which means it's in the possible liquid water zone.

Sadly, the planet itself "probably does not boast the right conditions for life" since its mass is between that of Neptune and Saturn, "suggesting it has a gas-rich composition that is unfavourable for life as we know it".

Fischer explained: "We can only imagine that it might look something like a beefy Neptune-like planet or perhaps a Saturn-like planet with rings and moons around it."

She then postulated: "The gas-giant planets in our solar system all have large moons. If there is a moon orbiting this new, massive planet, it might have pools of liquid water on a rocky surface."

Which means, of course, the possibility of life. However, Marcy cautioned that "such a moon would need to be at least as massive as Mars in order to have enough gravity to hang onto its water over the long term... and none of the moons in our solar system are that heavy."

He concluded: "We have no evidence of any moon at all nor rings around this planet."

Still, hopes springs eternal, and Jonathan Lunine of the University of Arizona in Tucson, told New Scientist there's a sizeable and apparently planet-free "gap" between 55 Cancri's latest family member and the most distant brother.

He asked provocatively: "Is there perhaps a much smaller, terrestrial-sized planet lurking in that gap – one that might be habitable?" ®

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