Monday, May 28, 2012

Aliens Could Have Been Pointing Their Antennas at Earth for 4 Billion Years!


Aliens could have been pointing their antennas at Earth for 4.6 billion years, without picking up a signal. Maybe the inhabitants [of a Twin Earth] are at the level of the classical Romans ... or maybe trilobites. We need to check out hundreds of thousands of Earthlike worlds."
Dimitar Sasselov -- Harvard Smithsonian Center for Astrophysics

Our Milky Way galaxy may be home to at least two billion Earthlike planets, a new study based on initial data from from NASA's Kepler space telescope says -- a number that is actually far lower than many scientists anticipated, which could make it hard to find twin "Earths" in our galaxy.
Based on what Kepler's found so far, the study authors think that up to 2.7 percent of all sunlike stars in the Milky Way host so-called Earth analogs. As of this February, Kepler has confirmed 15 new planets and found an additional 1,235 planet candidates, including the smallest planet yet spied outside our solar system. Kepler will collect transit data for a minimum of three and a half years, allowing for a more complete planetary census at a later date.
"There are about a hundred billion sunlike stars within the Milky Way," said study co-author Joe Catanzarite, a scientist with NASA's Jet Propulsion Laboratory (JPL). "Two percent of those might have Earth analogs, so you have two billion Earth analog planets in the galaxy," he added. "Then you start thinking about other galaxies. There are something like 50 billion, and if each one has two billion Earthlike planets, it's mind boggling."

Although the figure seems large, Catanzarite and co-author Michael Shao, also of JPL, say their results actually show that Earths are "relatively scarce," which means a substantial effort will be needed to identify suitable target stars for followup missions designed to study the chemical signatures of Earth-size worlds. The chemical signals may hint whether the planets have oxygen atmospheres, liquid water -- or even signs of life.

The Kepler telescope has been scanning a patch of sky near the constellation Cygnus which can be considered a representative sample of what exists throughout the Milky Way. The Kepler space telescope analyze the light from the 156,000 stars in its field of view searching for stars that dim periodically—signs that significant objects are orbiting these stars.
To extrapolate the number of possible Earths in the Milky Way, Catanzarite and Shao started by defining an Earth analog based on a transiting planet's size and the distance at which the planet orbits its star.
"A famous 1993 paper calculated the inner and outer distance of the so-called Goldilocks zone"—not too hot, not too cold — "where liquid water could exist on a planet's surface," Catanzarite observed. 

"But more recently people have been saying those boundaries are conservative. Maybe you could go closer or farther. For instance, because of greenhouse gases a planet could be farther away and still be warm, or because of clouds, which the previous models didn't take into account, you could be closer but keep the surface cool."

In general, though, an orbit similar to Earth's seems ideal: "Closer than Earth and you'll fry; water would turn to steam. If you're too far, water would freeze into ice."

"People generally agreed that the smallest habitable planet would be 0.8 Earth radii ... or roughly half of Earth's mass. The reason is that a lower mass planet wouldn't be able to hold on to an oxygen atmosphere," Catanzarite said. "Out to two Earth radii is the largest planet we'd call Earthlike. More massive planets start to accumulate thick hydrogen atmospheres, like Neptune or Uranus," with unbearable atmospheric pressures.
Neptune
Other planets may exist that we can't see because of their orbital inclination, so the team used previous exoplanet data to make mathematical estimates for the probabilities of these unseen worlds.

The results published online on arXiv.org showed that, according to the traditional boundaries of the Goldilocks zone, 1.4 percent of sunlike stars should have Earth analogs. If you accept the wider, more modern version of the habitable zone, 2.7 percent of sunlike stars likely host Earths. The study authors predict that Kepler will eventually find 12 Earth analog planets in its field of view — and may have already found 4 such worlds among its initial candidates.
"This study completely underestimates the frequency of Earths," according to MIT planetary scientist Sara Seager, a member of the Kepler science team who noted that  Kepler's just getting started, so its data is far from complete. "Say you're doing a census of the United States," she said. "If you go to California and knock on every door, you can then extrapolate out to the rest of the country. That's what Kepler's doing."
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