Are “Guardian” Jupiter-like Planets Needed for the Evolution of Intelligent Life?

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While the number of known exoplanets grows rapidly,  now exceeding 500, the January issue of The Astrophysical Journal reports that 12 years of ground-based extrasolar planet-hunting observations by the Anglo-Australian Planet Search (AAPS) shows that the number of Jupiter-like planets, in Jupiter-like orbits might not be as common as previously thought. The finding is reigniting the debate about whether the presence of a Jupiter-like planet is necessary for intelligent life.


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. They found that 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 and ardent planet hunter, Chris Tinney, a co-author of the study, from the University of New South Wales in Sydney.

Whether Jupiter-like planets are needed  has long been debated: some say that Jupiter protects Earth from civilisation-ending impacts that would frustrate evolution toward intelligent life and perhaps leave life at the bacterial level while others argue that Jupiter could just as easily send life-ending objects Earth's way.

"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," added Boss.

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 observes 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.
"The small occurrence of Jupiter-analogs bears directly on the occurrence of life in the universe," added Marcy.

Echoing Marcy and the "Jupiter-as-Guardian" advocates, Stephen Hawking believes that one of the major factors in the possible scarcity of intelligent life in our galaxy is the high probability of an asteroid or comet colliding with inhabited planets.

We have observed, Hawking points out in "Life in the Universe," the collision of a comet, Schumacher-Levi, with Jupiter (image above), which produced a series of enormous fireballs, plumes many thousands of kilometers high, hot "bubbles" of gas in the atmosphere, and large dark "scars" on the atmosphere which had lifetimes on the order of weeks.

It is thought the collision of a rather smaller body with the Earth, about 70 million years ago, was responsible for the extinction of the dinosaurs. A few small early mammals survived, but anything as large as a human, would have almost certainly been wiped out.

Through Earth's history such collisions occur, on the average every one million year. If this figure is correct, it would mean that intelligent life on Earth has developed only because of the lucky chance that there have been no major collisions in the last 70 million years. Other planets in the galaxy, Hawking believes, on which life has developed, may not have had a long enough collision free period to evolve intelligent beings.

“The threat of the Earth being hit by an asteroid is increasingly being accepted as the single greatest natural disaster hazard faced by humanity,” according to Nick Bailey of the University of Southampton's School of Engineering Sciences team, who has developed a threat identifying program.

What would happen to the human species and life on Earth in general if an asteroid the size of the one that created the famous K/T Event of 65 million years ago at the end of the Mesozoic Era that resulted in the extinction of the dinosaurs impacted our planet.

As Stephen Hawking says, the general consensus is that any comet or asteroid greater than 20 kilometers in diameter that strikes the Earth will result in the complete annihilation of complex life – animals and higher plants. (The asteroid Vesta, for example, one of the destinations of the Dawn Mission, is the size of Arizona).

How many times in our galaxy alone has life finally evolved to the equivalent of our planets and animals on some far distant planet, only to be utterly destroyed by an impact? Galactic history suggests it might be a common occurrence.

The first this to understand about the KT event is that is was absolutely enormous: an asteroid (or comet) six to 10 miles in diameter streaked through the Earth's atmosphere at 25,000 miles an hour and struck the Yucatan region of Mexico with the force of 100 megatons -the equivalent of one Hiroshima bomb for every person alive on Earth today. Not a pretty scenario!

Recent calculations show that our planet would go into another "Snowball Earth" event like the one that occurred 600 million years ago, when it is believed the oceans froze over (although some scientists dispute this hypothesis -see link below).

While microbial bacteria might readily survive such calamitous impacts, our new understanding from the record of the Earth's mass extinctions clearly shows that plants and animals are very susceptible to extinction in the wake of an impact.

Impact rates depend on how many comets and asteroids exist in a particular planetary system. In general there is one major impact every million years -a mere blink of the eye in geological time. It also depends on how often those objects are perturbed from safe orbits that parallel the Earth's orbit to new, Earth-crossing orbits that might, sooner or later, result in a catastrophic K/T or Permian-type mass extinction.

Vredefort The asteroid that hit Vredefort located in the Free State Province of South Africa is one of the largest to ever impact Earth, estimated at over 10 km (6 miles) wide, although it is believed by many that the original size of the impact structure could have been 250 km in diameter, or possibly larger(though the Wilkes Land crater in Antarctica, if confirmed to have been the result of an impact event, is even larger at 500 kilometers across). The town of Vredefort is situated in the crater (image).

Dating back 2,023 million years, it is the oldest astrobleme found on earth so far, with a radius of 190km, it is also the most deeply eroded. Vredefort Dome Vredefort bears witness to the world’s greatest known single energy release event, which caused devastating global change, including, according to many scientists, major evolutionary changes.

What has kept the Earth "safe" at least the past 65 million years, other than blind luck is the massive gravitational field of Jupiter, our cosmic guardian, with its stable circular orbit far from the sun, which assures a low number of impacts resulting in mass extinctions by sweeping up and scatters away most of the dangerous Earth-orbit-crossing comets and asteroids

Posted by Casey Kazan with Rebecca Sato

Note: Portions of this post was adapted from a news release issued by University of Southampton.

Sources:

http://www.rationalvedanta.net/node/131

Cosmosmagazine.com

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