National Geographic News
Image of the habitable-zone planets discovered to date alongside Earth.

Shown are the relative sizes of all of the habitable-zone planets discovered to date alongside Earth. Left to right: Kepler-22b, Kepler-69c, Kepler-62e, Kepler-62f and Earth (except for Earth, these are artists' renditions).

Photograph by NASA Ames/JPL-Caltech

Ker Than

For National Geographic

Published November 4, 2013

One in five sunlike stars harbors an Earth-size world that orbits in a "habitable zone" friendly to oceans and perhaps life, a new study suggests.

The findings, detailed in this week's issue of the Proceedings of the National Academy of Sciences, are based on a statistical analysis of observations made by NASA's now-crippled Kepler space telescope. (Related: "New Role for Disabled Kepler? Finding Exotic Alien Worlds.")

The astronomers estimate that 22 percent of sunlike stars may be orbited by small, rocky planets that reside within so-called habitable zones, where they receive Earth-like levels of sunlight.

"Our results show that small planets, with sizes similar to our Earth, are very common,” said study leader Geoffrey Marcy, an astronomer at the University of California, Berkeley.

What's New?

Launched in 2009, Kepler was tasked with finding alien planets, or "exoplanets," by looking for the telltale dimming of light that occurs when a world passes in front of, or "transits," its parent star.

For four years, until its malfunction this summer, Kepler stared at 150,000 stars situated in a patch of sky in the direction of the constellations Cygnus and Lyra.

Out of that group, Marcy and his colleagues Andrew Howard and Erik Petigura, focused on 42,000 stars that are like our sun or slightly cooler and smaller.

The team found 603 potential planets, or "planet candidates," orbiting these stars. Ten of these were Earth-size, that is, one to two times the diameter of Earth and orbiting their star at a distance where they are heated to lukewarm temperatures thought amenable for life.

The team next introduced 40,000 fake planets into the Kepler data as a test for their analysis software.

The fake planets ranged from Earth-size to ten times the size of Earth. They also had a range of orbital distances: Some were located within the habitable zone, while others were either too close or too far from their star.

Similarly, the team accounted for missed planets in their analysis, as well as the fact that only a small fraction of planets follow orbits that allow their crossings in front of their host star to be seen from Earth.

Overall, the team estimated that 22 percent of all sunlike stars in the galaxy have Earth-size planets in their habitable zones.

Why Is It Important?

The findings could also have implications for planet-formation theories that attempt to explain how different varieties of planets form in the ratios that they do, Marcy notes.

"People who construct theories about the formation of planets can now use our results to build computer models that simulate the formation processes of planetary systems," he said.

"They now have touchstones to test their theories. The theories must 'predict' the observed plethora of, and orbits of, Earth-size planets that we actually observe in the universe."

What Does This Mean?

The results also suggest that the nearest Earth-like alien world could be as near as 12 light-years away—close enough to be seen with the naked eye.

"Astronomers know the locations of all the thousands of stars near our solar system. Now we know that one in five sunlike stars has an Earth-size planet in the habitable zone," Marcy said in an email.

"You can imagine traveling outward from our solar system, stopping at the nearest sunlike star, then stopping at the second nearest, and so on. When you arrive at the fifth, you have passed five sunlike stars. On average, one of those five sunlike stars has an Earth-size planet in its habitable zone, with lukewarm temperatures, suitable for life."

That number could prove really important for NASA and other space agencies planning follow-up missions to Kepler, said study co-author Howard, a former UC Berkeley post-doctoral fellow who is now on the faculty of the Institute for Astronomy at the University of Hawaii.

"Successor missions to Kepler will try to take an actual picture of a planet, and the size of the telescope they have to build depends on how close the nearest Earth-size planets are," Howard said in a statement.

What's Next?

Now that they have an idea of what the abundance of habitable worlds in our galaxy is, Marcy and his team are planning an audacious next step.

"We will point the world's largest telescopes at the Earth-size planets, trying to receive the laser communications sent by any intelligent civilizations that live on those planets," he said.

"We will probably fail. But if we don't try, we will surely not detect them."

Follow Ker Than on Twitter.

65 comments
Julius Lim
Julius Lim

but there are still a problem how we get there rite?

we have no time travel spaceship or UFO air craft ....

Patrick Litherland
Patrick Litherland

Why can't the scientists and engineers build a telescope 10 times as good as the humble so they can see more clearly at what it is they are looking at.

Swaminathan Vembu
Swaminathan Vembu

Larger habitable Planet size means more space and more water.

Atmospheric conditions published are facts and they always are unfavorable.

Therefore the search must carry on.

Joseph Avila
Joseph Avila

Just wondering, how long might it take a successor mission telescope to get near enough for a good closeup?

Tyrique Coleman
Tyrique Coleman

Ah, valid point, Les McNalley. What exactly are the standards for "earth-like"? As similar as the planets may be, it has to be very uncommon for those exoplanets to have conditions that would allow for us to live there. For all we know, on Kepler-69c , the closest thing to air in its atmosphere is chloramine, and on Kepler-22b, the air solution is like, I don't know, nitrous oxide mixed with diethyl ether or something. Would you mind explaining the requirements for a planet to be earth-like?

Ibok Jacob
Ibok Jacob

In my opinion, i believe our perspective on the search Intelligent lifeforms needs to be re-assessed.. what if we looking for the wrong things in the right places. what if out there they are indeed life-forms which are not carbon-based organisms... and from what i have been reading so far we are only searching for advanced life-forms send radio waves and hoping they reply, how r we so sure they have gotten to that stage.. we must understand that there was once a time when our most advanced technology was Fire.

Daryl Johanssen
Daryl Johanssen

This article and ensuing discussion must be sending members of  "The Voluntary Human Extinction Movement, ie." vhemt,"  to their pharmacies for major painkillers.

The Voluntary Human Extinction Movement (or vhemt) states:

"Phasing out the human race by voluntarily ceasing to breed will allow Earth's biosphere to return to good health.  Crowded conditions and resource shortages will improve as we become less dense."

Since they certainly have a valid point, maybe it would behoove us to  simply proceed with BOTH endeavors, as "Insurance"  against the extinction of man - if you will?

www.vhemt.org/

Seng Sze
Seng Sze

Property investment in other planet....Anyone?? Lol, well done Marcy and team!!!

Jillian Hull
Jillian Hull

C'est passionnant! Je ne peux pas attendre pour entendre plus!

Oscar Menvielle
Oscar Menvielle

In some way I agree with Simon, but also think that is a huge first step to know the future


simon vottero
simon vottero

i cant believe people have minds so closed ...  nasa search for "habitable" .. but they thinkin whit a human standard... who says aliens be the same like us? the can breath CO2 or anything... o not breathe  and this is only 1 thing... anyway.. open your minds  :p ( excuse my english.. im  a beginer .. im from argentina )

Colleen Przybylski
Colleen Przybylski

I was certainly not expecting that final paragraph. I feel like it would be a milestone for us to find life of ANY sort, let alone something intelligent who has created technology complex enough to communicate with us light-years away. (not that I doubt the existence of such beings SOMEWHERE in the universe, but..)

It would be more interesting if we were able to create some type of long-term mission that sent a rover to the nearest earth-like planet to examine it for life, instead of spending money on sending out lazer signals to who knows what.

Claudia Burns-Walters
Claudia Burns-Walters

Hopefully, by the time we earthlings find the means to visit other "habitable" planets, we will have evolved, learned or have wisdom enough to not exploit or destroy them.  (Hopefully, we reach that point before we destroy our own planet.)  I love planet earth!

Bill Doyle
Bill Doyle

This is good news for us, we just have to get there. We need to develop the technology needed to get to these planets and explore them because eventually we will either over populate this planet or pollute it to death or, failing that, disease or war will hasten our demise.

I know this may seem like a dark and a doom and gloom way to look at things but all you have to do is look at the news and see that we have discovered a new way to blow each other up.  The more planets we populate, the more of a chance we have to succeed as a species. However, by the time we develop the necessary technology we need for serious space exploration, it may too late for us.

Just my opinion.

Les McNalley
Les McNalley

Habitable to whom is the big question. Take Kepler-22b for example. What would happen to a human if we tried to walk on it? Crushed under our own weight?

chris Dolan
chris Dolan

A light year is a measure of distance.  The nearest star is only 4 light years away.  It almost certainly has an earth sized planet in its habitable zone.  Every star forms planets during the process of becoming a star.  Of course there are exceptions, but there is orbiting debris left over from solar formation in every case.  The problem is that the habitable zone is only one of the trillions of things that has to come together right to make an actual earth.  A true habitable zone.   That would bring the odds down to earth.  We are alone.  But we'll always have Pluto.  

Liz Ricker
Liz Ricker

Astounding work. Thank you to the teams and individuals with the ability, the opportunity, and the drive to push forward these studies.

Ali Bukhari
Ali Bukhari

More Land to live!!! Great Marcy and Team, Appreciated Work!

Kaushik Saha
Kaushik Saha

We feel happy when dreams come true......... But a dream that is 12 light years away from all of us shouldn't be seen for the next 12 light tears time. Now its up to the scientists who will make this dream come true for us and of-course themselves too. 

Albert Wang
Albert Wang

If someday we can really establish a new home in that planets, do the people in Earth want to go?

Dallin Jensen
Dallin Jensen

@Joseph Avila The James Webb telescope coming out in 2018 is supposed to be able to analyze the atmospheres of earth sized worlds.

bob jakl
bob jakl

@Tyrique Coleman

Earth like planets are planets that are candidates for life. This is, planets in what astronomers explain a zone that is not too hot and not too cold and just the right size for life as we know it to evolve.  These Earth size planets orbit in the habitable zone, where conditions permit surface liquid water

Levi Goodman
Levi Goodman

@Ibok Jacob what would humans do if we found an intelligent race that was behind us by only a couple centuries?

Joseph Avila
Joseph Avila

@Ibok Jacob ...or conversely, how can we be so sure that these life-forms will not be advanced so far beyond us that we lack the intelligence, simply, to understand them?

bob jakl
bob jakl

@simon vottero I agree simon.  Every species evolved in their own environment.  That is why we find living organisms on Earth In the hottest and coldest conditions.  Not to mention caves with no sunlight.  Out thought of life is different then what we thought just a decade ago.  Life exists everywhere.  Life as adaptable, in the smallest and largest form. 

Keith Lyle
Keith Lyle

And what if inhabitants of that planet have discovered earth ?

bob jakl
bob jakl

@Claudia Burns-Walters It seems as though, unfortunately, humans are a cancer to the Earth.  Dinosaurs existed for 100's of millions of years.  Humans a fraction of that.  We are our own demise. 

Zoie Wittenberg
Zoie Wittenberg

We don't need to run to another planet for refuge from our own. In fact, it is very unlikely that we will have the technology to traverse light-years  to reach other Earth-like planets. It is more likely that we will move to the Moon, or have a Wall-e situation (For those of you whom are not Disney Pixar fans, an orbiting space craft  that is large enough to emulate Earth's conditions and hold the entire Earth's population.).

bob jakl
bob jakl

@Bill Doyle Bill, I agree completely.  We are our own destruction.  With no regard for future life.    

Levi Goodman
Levi Goodman

@Les McNalley well with some of these not too large ones you might be able to just get some cloves and trot around on all fours.

Charles Becker
Charles Becker

@chris Dolan

"Fifteen hundred years ago everybody knew the Earth was the center of the universe. Five hundred years ago, everybody knew the Earth was flat, and fifteen minutes ago, you knew that humans were alone on this planet. Imagine what you'll know tomorrow. " Kay

Daryl Johanssen
Daryl Johanssen

@Kaushik Saha 

Ah yes, what a marvelous dream to stop all unnecessary self-induced suffering.

 All mankind  would now listen to SCIENTISTS - and NOT philosophers........

Daryl Johanssen
Daryl Johanssen

@Albert Wang 

"We  cannot solve problems with the same thinking that we used when we created them."

Albert Einstein

Levi Goodman
Levi Goodman

@Keith Lyle Why are we not sending out laser signals? why should we expect others to say hi if we are not willing to?

bob jakl
bob jakl

@Charles Becker @chris Dolan exactly chris.  our galaxy has billions of stars.  the universe has billions of galaxies.   each with billions of stars.  each star having an average of 5-10 planets.  to think earth is the only planet teaming with life is ridiculous.  people cant comprehend true time and evolution. 

Daryl Johanssen
Daryl Johanssen

@Albert Wang 

As we are watching for the first time in history, even the VATICAN is listening to Einstein today. WHY the POPE is asking Catholics to tell Bishops their CONCERNS - already knowing what they are.  In that way, they can make the needed "changes in dogma" to STAY IN BUSINESS and not become "Extinct" also.

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