Here ya go El-D

Nivekella

New member
It is a long one sorry, but so far one of my favs. I was floored I could still find it teehee.


Seeking other Earths
More than 120 extrasolar planets have been discovered, mostly gas giants. But what everyone really wants to find are "exo-Earths" orbiting other stars. Any day now, say the astronomers.
Robert Zimmerman

In 2001, a team of astronomers from Poland aimed the 1.3-meter Warsaw Telescope at the Las Campanas Observatory in Chile toward the center of the Milky Way. For 32 nights, they carefully recorded the faint light from 52,000 of more than 5 million stars in their field of view. Their goal: find planets.

With the light from so many stars, one stood out as unique. Despite the star's location 5,000 light-years away with an apparent magnitude of only 16.6, the scientists were able to discern a faint drop in the star's brightness. It lasted just 108 minutes and repeated every 1.2 days.

Although they weren't sure, the astronomers suspected they were seeing a planet as it traveled across its star.

Then another team of astronomers took a second look, using Doppler-shift techniques with the 10-meter Keck I telescope on Mauna Kea, Hawaii. In January 2003, they confirmed the detection, announcing that this ebb in brightness was caused by a Jupiter-size planet.

It was the first discovery of an extrasolar planet by the direct observation of a transit. Further analysis determined that the planet was a gas giant weighing only slightly less than Jupiter but having a diameter 30 percent greater.

More remarkably, the planet's 1.2-day orbit circled the star so tightly, at an average distance of about 2 million miles, that the temperature of its outer atmosphere was thought to hover around 3,000? Fahrenheit, making it almost as hot as the surface of many stars.

The new planet, inelegantly dubbed OGLE-TR-56b, joined the more than 120 other extrasolar planets discovered over the last nine years. In that time, the menagerie of planets has grown to include one planet 17 times more massive than Jupiter, and another only a little more than a tenth of Jupiter's mass. It includes planets in orbits nearly as short as OGLE-TR-56b's to as long as almost 15 years ? and planets in orbits ranging from virtual perfect circles to ones as eccentric as the paths of comets.

Planets have been found orbiting binary stars. Planets have been found orbiting pulsars. Some planets orbit so close to their stars that the heat and stellar wind is literally causing the planet to evaporate slowly.

Yet despite a now-lengthy list of discoveries, the Holy Grail of extrasolar planet research remains out of reach. Of the 107 known solar systems, not one resembles our own, even approximately. In fact, the very existence of earthlike terrestrial planets in most of these newly discovered solar systems is impossible. So it would seem that habitable planets like our own are rare and very unusual.

Nonetheless, after almost a decade of discovery, astronomers who specialize in this field have reached a far different conclusion. As they see it, a close look at the data suggests earthlike planets might not be unusual at all. Solar systems like our own can exist, they say, although they will not be scattered randomly throughout the galaxy. Moreover, only certain kinds of stars are likely to have planets, and of these, only a percentage can have planets like Earth, placed where life can prosper.

In other words, discoveries like OGLE-TR-56b not only show that today's extrasolar planet hunters have an increasing variety of amazing tools to look for planets, they also are giving scientists the necessary knowledge for determining exactly where to look. Based on these facts, exoplanet research should become very exciting in the next few years.

No Earths?
The first extrasolar planet discovered around an ordinary star, 51 Pegasi b, was a strange and unexpected revelation. A gas-giant planet more massive than Saturn, 51 Peg b orbits its star every 4.2 days at a distance of only 4.8 million miles. Considering that Mercury circles the Sun at 36 million miles every 88 days, the new planet was clearly in a toasty orbit.

Moreover, according to all theories then accepted about the formation and structure of solar systems, the planet should not exist. Gas giants were supposed to form in the chilly outback of a solar system and stay there. For a gas giant to coalesce this close to its star was supposedly impossible because the star's heat would destroy the lighter gases before they had time to assemble into a planet. What's worse, a gas giant in the inner part of a solar system would likely prevent the birth of any earthlike planets by its powerful gravity.

Nor was 51 Peg b alone. By 2000, scientists had discovered more than a dozen new planets, the majority having orbits closer to their star than Venus is to the Sun. Half had orbits of less than a month, and all were gas giants with masses ranging from 0.4 to 11 Jupiter masses.

Other early research was equally depressing. In 1999, a team of scientists used the Hubble Space Telescope to survey globular cluster 47 Tucanae for planets. Based on their sample of 34,000 stars, they figured on spotting approximately 15 to 20 planets in their search.

Instead, they found none.

"In a global sense," noted Ron Gilliland, lead scientist on the survey, "the result for 47 Tuc indicated that the existence of planets was less likely than we might otherwise have expected."

Better statistics
Despite the discouraging results, astron-omers were not worried. The Doppler detection techniques initially used in searches strongly favor the discovery of large planets (greater than 1 Jupiter mass) orbiting close to their stars (with orbits shorter than a year). Therefore, "hot Jupiters" like 51 Peg b were the most likely planets to be found first.

Subsequent discoveries have given hot Jupiters (or "roasters," as one scientist calls them) less significance. Almost all the known nearby F-, G-, K-, and M-type single stars ? that is, those most like our Sun ? have been studied for periods ranging from 5 to 10 years. It's safe to say that practically all the hot Jupiters orbiting these stars have been found by now.

Smaller planets continue to be discovered. And as they join the list, the average planet size keeps falling and the average orbital distance keeps growing. At this point, hot Jupiters compose a decreasing percentage of all extrasolar planets. Also, the planet count seems to be increasing as astronomers' techniques inventory more thoroughly orbital distances of 1 astronomical unit (AU) or greater. Hot Jupiters are relative exceptions, not the rule.

Moreover, the growing catalog of extrasolar planets has revealed interesting patterns, some of which astronomers do not yet understand. For example, relatively few planets seem to have orbital periods of 10 to 100 days. Furthermore, in orbits greater than 100 days, very few planets have less than three-quarters of Jupiter's mass. Why these patterns exist is unclear.

The most intriguing discovery emerging from the data concerns the kinds of stars capable of producing planets. It appears, for example, that the more massive a star, the more likely it will have planets. Type M dwarf stars, with masses from a tenth to a third of the Sun's mass, almost never have planets. K stars, with masses of 30 to 70 percent of the Sun's, have planets about 3 to 4 percent of the time, while G stars, like our Sun, have planets some 7 percent of the time. And F stars, 30 to 50 percent more massive than the Sun, have planets about 10 percent of the time.

Furthermore, single stars seem to form planets more often than binary systems do. "Binaries, and in particular close binaries, probably don't permit any planetary systems to survive," notes Geoffrey Marcy of the University of California, Berkeley and one of the lead scientists in the California & Carnegie Planet Search. "Such stellar companions would gravitationally slingshot any of the planets right out of the system." Marcy believes that perhaps all single stars will prove to have planets.

Finally, a star's composition seems important. Right from the beginning, scientists noticed that extrasolar planets are more likely to orbit stars that are at least as rich in complex atoms as our Sun. These heavier atoms, which astronomers broadly term "metals," are the result of several episodes of star formation, with each new generation of stars adding its own heavier atoms to the star-making mix.

Recent exoplanet discoveries have confirmed that the greater a star's metallicity, the better its chances to have planets. In our neighborhood of the Milky Way, 20 to 30 percent of Sun-like stars with large metallicities are known to have planets. And that's after only a decade of study; given time and more discoveries, the percentage should rise significantly.

Maybe the lack of planets in globular cluster 47 Tucanae was not so surprising after all. The constant interaction of the cluster's closely packed stars would likely rip any planets from their stars, setting them adrift within the cluster. "Planets do not like very crowded environments," says Dimitar Sasselov of the Harvard-Smithsonian Center for Astrophysics.

Second, and possibly more important, globular clusters are old. They formed back when the Milky Way did and are full of first-generation stars deficient in metals. Compared to the Sun, 47 Tucanae has less than a third the metallicity. As Gilliland notes today, "Taken what we know now about metallicity and planet formation, it isn't really surprising that we found no planets in the cluster."

Globulars, however, should not be presumed to be entirely planet-free. An ancient gas-giant planet has been found orbiting a pulsar-and-white-dwarf binary in globular cluster M4 (see "Genesis planet," Astronomy, June 2004). But this planet's unusual history ? normal birth 13 billion years ago followed by ages of gravitational ping-pong ? make it rare, if not actually unique.

Better theories
No longer chained to a belief that solar systems have to resemble our own, theorists began to figure out why these newly discovered solar systems look as they do.

In 1996, astrophysicists Douglas Lin, Peter Bodenheimer, and Derek Richardson described how the interaction between a new gas-giant planet and the dust disk from which it formed would drive a planet like 51 Pegasi b inward. The mass of the disk would drag on the planet's orbital velocity, causing the orbit to shrink.

The orbit would stabilize only when the dust disk vanished, either blown away by the star's radiation or absorbed by new planets. The point when that happened would set the giant planet's final orbit. If it happened quickly, the gas giant would resemble Jupiter, orbiting far enough away that terrestrial planets could form closer to the star. "That's most likely what happened in our solar system," notes Sasselov.

If the disk took longer to vanish, the planet would migrate inward until it became a hot Jupiter. In some scenarios, the gas giant was absorbed by the star.

More rigorous computer simulations by others have since shown that hot Jupiters can form close to the star. Scientists at the Tokyo Institute of Technology found that because a planetary disk is denser within 1 AU (and orbits are shorter), the accretion time for a gas giant could be as brief as a thousand years. This is easily fast enough to build up a planet before the star ignites and its heat and stellar wind blow away the dust.

Many mysteries still
Despite their increasing knowledge, planet hunters are far from content about the state of their field. Although existing theories can explain things like the migration of hot Jupiters, for now, as Ron Gilliland notes, "We don't have an excellent predictive theory of planet formation."

The favored theory of planetary formation, dubbed core accretion, says that as a rotating planetary disk forms around a young star, collisions in the disk cause dust particles to stick together. These build small planetesimals several miles across; some of these will become cores on which material will accrete to form planets.

In the inner solar system, the process can make planets in less than 100,000 years, far less than the million-year life span of a planetary disk. The process can also make planets with enriched metallicities as seen in our own solar system, where the planets have proportions of heavy atoms several times greater than the Sun's.

Core accretion, however, has problems creating gas giants fast enough. In fact, it has trouble producing these planets in less than 100 billion years, far longer than the age of the universe.

Another theory, less popular, says that gravitational instabilities cause dense clumps to form as the planetary disk collapses, out of which planets consolidate. While this theory can make gas giants in distant orbits within the necessary time, it can't produce planets with the high metallicities seen in the solar system. Instead, the planets mimic the Sun, with compositional ratios about the same.

More important, neither theory can explain how almost all known exoplanets have orbits far more eccentric than the planets in our own solar system. "We actually don't understand yet the origin of so many highly eccentric orbits," says Sasselov. "It's clear that it has something to do with interactions between the planets and the disks, but it is not clear how."

Maybe the real issue is why the planets in our solar system have such perfectly circular orbits. "Therein lies the key question," says Marcy. "Is our own solar system a common type or some quirk?"

Then consider OGLE-TR-56b, the first extrasolar planet discovered as it transited across the face of its star. According to the Doppler surveys, a lot of planets have orbits ranging from 3 to 5 days, but none have shorter orbits. This led astronomers to assume that 3 days was the shortest natural orbit for a planet.

Then OGLE-TR-56b was discovered, circling its star every 1.2 days.

These inconsistencies and questions point out an essential fact. Surveys are still far from complete for a good reason: time. In our own solar system, the orbits of Jupiter and Saturn last 12 and 29.5 years, respectively. For astronomers to discover either planet using Doppler techniques would require a minimum of 24 and 60 years of observations, respectively.

Scientists have been searching with sufficient accuracy to spot a Jupiter for only about a decade, and even that accuracy is still inadequate to see a Saturn. In fact, with today's Doppler equipment, astronomers could look for a century and never detect Saturn. To discover planets this size and smaller, far from some other Sun, will require markedly better equipment and searches lasting literally for decades.

Where to look
Nonetheless, after almost ten years of discoveries, astronomers can now start drawing some conclusions.

First exoplanets lie in younger, more metal-rich regions of spiral galaxies. Notwithstanding the planet in M4, globular clusters make poor hunting grounds. Planets are more likely in the galactic arms, where star-making has proceeded for ages. Exoplanets also will be found in the quieter regions of those arms, where fewer massive stars can burn off planetary disks before planets form ? and where few other stars interfere closely enough to throw planets into interstellar space.

With this in mind, the Hubble Space Telescope was recently used to search for transits in the galactic bulge. Similar to the 47 Tucanae observations but looking in a better place, this new survey is expected to find between 50 to 100 planets.

Next, astronomers should focus on single stars, either G-type like the Sun or F stars that are more massive. Present surveys suggest at least 10 percent of these stars have planets ? and maybe all might prove to contain planetary systems once detection techniques improve.

In this regard, the California & Carnegie Planet Search is building a 2.4-meter telescope for Doppler observations three times more sensitive than anything today. If all goes as planned, this instrument will examine approximately 100 nearby stars per year and should be able to spot planets as small as 10 Earth masses within orbits of 1 AU.

Finally, astronomers need to look for a long time. NASA has plans to build several different planet-finders of much greater resolution. The Kepler mission, scheduled for launch in 2007, will hunt for transits of Earth-size planets by looking at more than 100,000 stars continuously. Following this, in 2009 the Space Interferometry Mission will be able to detect the presence of Earth-size planets in orbit around the nearest hundred or so stars. Last, in 2014, will come the two-part Terrestrial Planet Finder, possibly the most ambitious mission of all. It is designed to detect and study the feeble light of any earthlike planets within 45 light-years.

No other Earths are known at present, but the most exciting part of the hunt is about to begin. Stay tuned for big news.
 
<div class='quotetop'>QUOTE(Stavrose @ Dec 10 2005, 12:15 PM) [snapback]8758[/snapback][/center]
Mars is just around the corner. All it needs is some brains, technology, and money
[/b]

/agree with stavrose

Terraforming mars would be easier than starting a colony out side of our solar system. Travel time to mars is between 1month and 6 months depending on how much fuel you wana burn and when your attempting the journy. Whereas travel time to another solarsystem could be tens of thousands of years. In which time we could turn mars into a tolerable planet to live on.
 
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