13 December 2010

Planetary ‘big brothers’ in short supply

Cosmos Online
While the number of known exoplanets continues to grow rapidly, the number of Jupiter-like planets, in Jupiter-like orbits might not be as common as thought.

Are Jupiter-like planets in long, circular orbits needed for the evolution of intelligent life? Credit: NASA

SYDNEY: While the number of known exoplanets continues to grow rapidly, the number of Jupiter-like planets, in Jupiter-like orbits might not be as common as thought.

A paper to be published in January in The Astrophysical Journal details 12 years of ground-based extrasolar planet-hunting observations by the Anglo-Australian Planet Search (AAPS).

The AAPS team analyzed data from 123 stars, all within 325 light years of our own Solar System and with at least eight years of observations, in search of ‘Jupiter analogs’ – gas giant planets on nearly circular orbits of over eight years.

Our Solar System is not common

Only 3.3% of the stars analysed harbour Jupiter analogs, and the team’s star-by-star computer simulations show that no more than 37% of these stars could possibly host such Jupiter analogs.

“Planetary systems like our own are not ubiquitous,” said astronomer Chris Tinney, a co-author of the study, from the University of New South Wales in Sydney.

The finding is reigniting the debate about whether the presence of a Jupiter-like planet is necessary for intelligent life.

Jupiter ejects comets

Astronomers have long argued about whether Jupiter-like planets are needed Jupiter protects Earth from more frequent civilisation-ending impacts that would frustrate evolution toward intelligent life and perhaps leave life at the bacterial level.

But Jupiter can both throw things our way and remove potential impactors from threatening Earth orbits, said Jonti Horner, an astronomer at the University of New South Wales in Sydney, who was not involved with the AAPS study.

Horner says that a large fraction of long-period comets pass through our inner solar system only once, before being ejected by the great gravitational influence of Jupiter. Jupiter analogs should also rid extrasolar planetary systems of rogue comets.

But, what about intelligent life?

“We do not know whether Jupiter-[analogs] are necessary for intelligent life,” said Alan Boss, a planetary scientist at the Carnegie Institution in Washington, DC said.

“The fact that groups like this can now put together the results of 12 years of monitoring nearby stars, with a good knowledge of [their data's] accuracy and limitations, means that for the first time, we can get good numbers about the census of extrasolar planets of all types,” said Boss.

Other recent observational results show that Solar System analogs are scarce, if not downright rare, said Jack Lissauer, a planetary scientist at NASA Ames Research Centre in Mountain View, California.

However, Lissauer remains sceptical that Jupiter analogs are a requirement of complex life.

Elliptical orbits can be damaging

Geoffrey Marcy, an astronomer at the University of California at Berkeley, who has been involved in the discovery of some 250 extrasolar planets, says that Jupiter analogs’ circular orbits may indeed be fortuitous for the habitability of fledgling inner solar systems.

Marcy notes that gas giant planets on long, elongated orbits could cause Earth-like planets to be gravitationally perturbed into highly eccentric orbits. That would likely result in untenable swings in such extrasolar Earths’ temperatures and climate.

“Thus,” said Marcy, “the small occurrence of Jupiter-[analogs] bears directly on the occurrence of life in the universe.”


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