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The Milky Way Has Two Billion Earthlike Planets: How Many Will Prove to be Twin Earths?

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"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.

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 this month 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."

"If Kepler's really going to find the answer in a few years, I'm happy to just wait rather than speculate", Seager said. The other big concern is that, with the data Kepler can collect, it's impossible to say whether a given planet is truly Earthlike. Size alone, for example, doesn't say enough about habitability. "Earth and Venus are about the same mass and size," she said — and by some definitions both worlds fall in our sun's habitable zone.

"To me, for a planet to be called an Earth analog you have to have Earth's mass, size, orbit, and know whether there's liquid water on the surface. But you can't know that until we do atmosphere studies," she said. "Kepler can find only Earth-size planets — we'd never use the term 'Earth analog.'  Some people are expecting the number of Earths to be higher," Catanzarite said, adding that Seager "may be right" and the new estimate is too low, "but it's still unclear."

"We're assuming that for all 156,000 stars, it's possible to detect all small planets of Earthlike radii and orbital distance. If [Kepler] can't, that would make our number an underestimate."

What about the possibility of life not only in the Milky Way, but also in the 50 billion galaxies beyond our galactic home-base?  New findings from diverse fields are are being brought to bear regarding the central question of the 21st century: How common is life in the Universe?  Where can it survive, Will it leave a fossil record, How complex is it.

For decades, scientists have been debating the conditions that are needed to replicate an Earth-like probablility of complexity beyond the microbial level. There's not much doubt in the minds of most astrobiologists that based on extremophile life we've discovered recently on Earth (see prior posts below), that life on the microbial level will be discovered sometime in the next twenty years on Mars or on one of Jupiter or Saturn's moons.

The three recent key findings for astrobiology are extremophiles, extrasolar planets, and a sense that water may be more ubiquitous even in our own solar neighborhood (in meteors like the Mars' Lafayette, Europa, and the ice frost on polar Mars). This picture has evolved quite suddenly with 1000-plus extrasolar planets found in just the last decade (and none known before around 1995).

Even in the oldest globular cluster star systems in our Milky Way galaxy -- choked with stars that were born more than 10 billion years ago -- there are enough "metals" to make earth-like worlds. According to models of planet formation developed by Harvard's Dimitar Sasselov, a member of the Kepler team, such a planet should be about half again as large as the Earth and composed of rock and water, what the astronomers now call a “super Earth.”for evolved animal life to be present we need to find that sweet "Goldilocks" planet with an exceedingly complex host of conditions present that have given rise the "Rare Earth" hypothesis.

In their book of that title, "Rare Earth" [see below] authors Peter Ward and Donald Brownlee, both of the University of Washington, have outlined a short list of conditions needed:

The right distance from a star; habitat for complex life; liquid water near surface; far enough to avoid tidal lock; right mass of star with long enough lifetime and not too much ultraviolet; stable planetary orbits; right planet mass to maintain atmosphere and ocean with a solid molten core and enough heat for plate tectonics; a Jupiter-like neighbor to clear out comets and asteroids; plate tectonics to build up land mass, enhance bio-diversity, and enable a magnetic field; not too much, nor too little ocean; a large moon at the right distance to stabilize tilt; a small Mars-like neighbor as possible source to seed Earth-like planet; maintenance of adequate temperature, composition and pressure for plants and animals; a galaxy with enough heavy elements, not too small, elliptical or irregular; right position the galaxy; few giant impacts like 65 million years ago; enough carbon for life, but not enough for runaway greehouse effect; evolution of oxygen and photosythesis; and, of course, biological evolution.

In stark contrast to the habitats with highest probability of supporting complex life, the zones and regions of the known universe described below are the ones that astrobiologists have concluded have little or zero chance of supporting life as we know it.

The listing of "dead zones" was compliled for Rare Earth -- Why Complex Life is Uncommon in the Universe, by Uinversity of Washington scientists Peter D. Ward (Professor of Geological Sciences and Curator of Paleontology) and Donald Brownlee (Professor of Astronomy and member of the National Academy of Sciences).

Early Universe: The most distant known galaxies are too young to have enough metals for formation of Earth-size inner planets. Hazards include energetic quasar-like activity and frequent super-nova explosions.

Elliptical Galaxies: Stars are too metal-poor. Solar mass stars have eveloved into giants that are too hot for life on inner planets.

Globular Clusters: Although they contain milllions of stars, the stars are too metal poor to have inner planets as large as earth. Solar mass stars have evolved to giants that are too hot for life on inner planets.

Small Galaxies: Most of the stars are too metal deficient.

Centers of Galaxies: Energetic star building and black-hole processes prevent development of complex life.

Edges of Galaxies: Most stars are too metal poor.

Planetary Systems with "Hot Jupiters": Inward spiral of the giant planets drives the inner planets into the central star.

Planetary Systems with Giant Planets in Eccentric Orbits: Unstable environments. Some planets lost to space.

Future Stars: Uranium, potassium, and thorium too rare to provide sufficent heat to drive plate tectonics.

Casey Kazan via NASA and nationalgeo.com

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Comments

a Jupiter-like neighbor to clear out comets and asteroids; plate tectonics to build up land mass, enhance bio-diversity, and enable a magnetic field; not too much, nor too little ocean; a large moon at the right distance to stabilize.

Could it possible for a planet to have an inhabitable zone between the poles and the equators if the poles are to cold and the equator to warm for life to exist (Poles change, inhabitable zone changes)?

In the conditions needed section. Couldn't there be other possibilities that are even better than ours? Two star solarsystems, a system with more protective shieldplanets like Jupiter + no similar keplers asteroid belt. Many planets have many moons could this affect living conditions in a positive way(if a planet has many moons could the moons protect from asteroid and comets hitting the planet)? I guess all this could be calculated, star distance, two star distance, moon(s) distance, nearby hot planets that could make a planet habitable even if the solarsystem sun is further away, an inhabitable planet with a warmer crust. I believe there are lot's of different options that doesn't have to be exactly like earths uniqe setup for life to exist.

Calculating all options would be quite a large job. If these calculations are made, computers could calculate the possibility of life when monitoring stars and planets.

A twin Earth ? another planet filled to overflowing with a voracious, merciless, Ignorant, murderous, primate hellbent on destroying anything It cannot enslave ?

The mind boggles.

I'm pretty sure life is common throughout the cosmos, but it's spread out over vast distances. The probability of intelligent life is even more rare, and the ability for these forms of life contacting one another is even greater given the fantastic distances between those systems or those galaxies.

It is not a matter of distances. It´s the trillions of interacting parameters involved and other considerations that make Earth a singularity

What if life can use other liquids instead of water, how big is the habitable zone then. Liquid helium is only a few degrees kelvin, for all we know there could be oceans and complex life on Pluto and Charon or rogue planets. How do we know for sure life even needs liquid. Just because it's hard to imagine doesn't mean it's impossible

We can't even agree on definition of what life is, so how can we make list of things life has to have.

I bet there are so many like Earth with an infinit amount of different outcomes of extinctions and evolutions. I wonder how computerized life will survive and evolve in the galaxies of high energy explosions. What is taking advantage of all the stuff that we don't call life and how.

There is 100 plus or minus that have the same life diversity as Earth. Refer to 'Your First Contact' book by Sheldan Nidle on his website.

2 billion Earth-like planets in our galaxy!?

Allright! That means we can spread out the current human population to only 3 people per planet.

For my planet, I hereby name it "Planet Paradise" and recommend that it just be for Halle Berry, Kim Kardashian, and myself.

No one else please -- especially not dudes. The rest of you dudes need to get your own planet.

What are the odds of finding humans on such an ideal planet? What are the odds of them being Humanists?

It should be very clear as to whether we are looking for 'another earth' or for life elsewhere in the universe. I think (from general observation and an open mind) the scientists are being too restricted in their view of what life is. Before the extremophiles were found water and sunlight was thought to be the primary requirements of life. Now that view is changing. Even on earth we are finding life in the most unexpected places. Imagine the extrapolated possibilities in the universe. Following from the conditions mentioned in the articel above, for example, protection from UV light might be in-built in the organisms skin. Maybe it produces something which when exposed to UV light produces another chemical which in turn can be consumed by the organism. I think the search for life is an endeavour taken up too early in the life of Homo Sapiens. We should probably focus on trying to understand the formation of the rocky and glowing objects first, before we go into such a complex entity as life.

I just wish that scientists would be more honest in what they report. Most of what is claimed is total speculation based on theories that cannot be proven, yet they speak about these theories as if they are true. I would have much more respect for them if they just said they do not know, but here is what we believe could be the possibility. Without absolute provable scientific evidence that is really all they should be saying about what is out there.

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Hey!

Yes there are Earth like World's out there waiting to be discovered,but a question rises,people say that Alien's have visited Earth,So how can they travel from that far distance(without breaking the speed of light ,almost 300,000 K/S)?so that would mean,if an extrateresstrial living thing want's to visit us from Kepler,it would take 36 year's to reach here on Earth,and they won't talk to Scientist , or do something imortant,but they would just appear to wierdo's?,that's totally funny,they would come so far just to appear to a guy who even doen't know what an ant is?,so far NASA radio signal's have not detected life in out Solar system,if there could be life we would hear their radio signal's.

My husband is from the planet of Zolon!

didnt you people watch my lastest movie ?
take a look at my name

Anyone who thinks there isn't life out there hasn't read the stats in this article and actually thought about it. We've already found DNA on meteorites that landed in Antartica millions of years ago.

Can't wait to meet the "aliens."

Becca
http://www.afformula.com/

The shear numbers alone are enough to allow me to look towards the heavens with absolute confidence in knowing within my line of vision life is looking back. I am thankful that I live in this age of discovery.

Maybe there's another planet that has dinosaurs like the ones from Earth. That would be incredible if they are still alive.

I find it extraordinarily arrogant that one would think that we are the only "living" creatures in exsistence. It is virtually impossible, statistically speaking, that we are the only earth. Even within our own galaxy, our solar system is merely a grain of sand on a mile long beach. What a waste of space it would be if we are the only intelligent life form in such a vast universe.

Personally, I suspect we will discover that humans are among the least evolved lifeforms in the multiverse.

"The rest of you dudes need to get your own planet."

If the Mormons are right, that can be arranged.
Such a universe wouldn't necessarily be any weirder than ours.

If I were to say that there is nobody else on the other side of the world that has the same hair color and eye color as you, you would say I was wrong and you couldn't believe I would suggest something so obviously incorrect.

The reason why you would have that reaction is because you clearly understand that there are billions of people on the other side of the world and at least one of them will have to match that description.
You don't know these people, or even what they look like, but you KNOW for a FACT that they are there BASED on the probabilities of the large numbers of people involved.

So, the same thing applies to life and planets, when people say there is no life in the universe, I have that same reaction of thinking they are wrong for suggesting something like that because I understand the size of the universe.

With that being said, if you truly understand and comprehend how truly large the universe is, or even just our galaxy alone, that there are TRILLIONS and TRILLIONS of stars, then it's pretty obvious that there is life out there.

To put the size of the universe into perspective, if the average person started counting when they were born and stopped when they died they would reach 2,144,448,000 seconds.
So, an average human can only count to 2.1 Billion seconds, with that being said, the Milky Way Galaxy alone contains 300 Billion stars, that means 1 human can't even count 1/30th of the amount of stars that there are, let alone catalog or see them all.


With Billions and Billions of years and Trillions of stars, the chances of something like life forming will happen many, many, many times.

first, the is no way the scientists could know the number of star. if yes, tell me how , the calculation.

and if the total number is 100 bilion, i guess 10 % is habitable,
10 bilions.

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