Astronomers answer key question: How common are habitable planets?

Nov 04, 2013
NASA's Kepler spacecraft observed 150,000 stars within a field in the constellation Cygnus. Credit: Erik Petigura, UC Berkeley

UC Berkeley and University of Hawaii astronomers analyzed all four years of Kepler data in search of Earth-size planets in the habitable zones of sun-like stars, and then rigorously tested how many planets they may have missed. Based on this analysis, they estimate that 22 percent of stars like the sun have potentially habitable Earth-size planets, though not all may be rocky or have liquid water, a presumed prerequisite for life.

NASA's Kepler spacecraft, now crippled and its four-year mission at an end, nevertheless provided enough data to complete its mission objective: to determine how many of the 100 billion stars in our galaxy have potentially habitable planets.

Based on a statistical analysis of all the Kepler observations, University of California, Berkeley, and University of Hawaii, Manoa, astronomers now estimate that one in five stars like the sun have about the size of Earth and a surface temperature conducive to .

"What this means is, when you look up at the thousands of stars in the night sky, the nearest sun-like star with an Earth-size planet in its habitable zone is probably only 12 light years away and can be seen with the naked eye. That is amazing," said UC Berkeley graduate student Erik Petigura, who led the analysis of the Kepler data.

"It's been nearly 20 years since the discovery of the first extrasolar planet around a normal star. Since then we have learned that most stars have planets of some size and that Earth-size planets are relatively common in close-in orbits that are too hot for life," said Andrew Howard, a former UC Berkeley post-doctoral fellow who is now on the faculty of the Institute for Astronomy at the University of Hawaii. "With this result we've come home, in a sense, by showing that planets like our Earth are relatively common throughout the Milky Way galaxy."

Petigura, Howard and Geoffrey Marcy, UC Berkeley professor of astronomy, will publish their analysis and findings online the week of Nov. 4 in the journal Proceedings of the National Academy of Sciences.

Earth-size may not mean habitable

"For NASA, this number – that every fifth star has a planet somewhat like Earth – is really important, because 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. "An abundance of planets orbiting nearby stars simplifies such follow-up missions."

The team cautioned that Earth-size planets in Earth-size orbits are not necessarily hospitable to life, even if they orbit in the habitable zone of a star where the temperature is not too hot and not too cold.

This video is not supported by your browser at this time.
This is an animation explaining a new analysis of data from NASA's Kepler mission that concludes that about one in five sun-like stars have Earth-size planets in their habitable zone. Credit: Erik Petigura/UC Berkeley, Andrew Howard/UH, Geoffrey Marcy/UC Berkeley & Illumina Studios, Emeryville, Calif.

"Some may have thick atmospheres, making it so hot at the surface that DNA-like molecules would not survive. Others may have rocky surfaces that could harbor liquid water suitable for living organisms," Marcy said. "We don't know what range of planet types and their environments are suitable for life."

Last week, however, Howard, Marcy and their colleagues provided hope that many such planets actually are rocky. They reported that one Earth-size planet discovered by Kepler – albeit, a planet with a likely temperature of 2,000 Kelvin, which is far too hot for life as we know it – is the same density as Earth and most likely composed of rock and iron, like Earth.

"This gives us some confidence that when we look out into the habitable zone, the planets Erik is describing may be Earth-size, rocky planets," Howard said.

Transiting planets

NASA launched the Kepler space telescope in 2009 to look for planets that cross in front of, or transit, their stars, which causes a slight diminution – about one hundredth of one percent – in the star's brightness. From among the 150,000 stars photographed every 30 minutes for four years, NASA's Kepler team reported more than 3,000 planet candidates. Many of these are much larger than Earth – ranging from large planets with thick atmospheres, like Neptune, to gas giants like Jupiter – or in orbits so close to their stars that they are roasted.

To sort them out, Petigura and his colleagues are using the Keck Telescopes in Hawaii to obtain spectra of as many stars as possible. This will help them determine each star's true brightness and calculate the diameter of each transiting planet, with an emphasis on Earth-diameter planets.

Astronomers use the term "habitable zone" to indicate an orbit not too far from the star such that water freezes, and not too close such that water vaporizes. Habitable zones are orbital areas where the heat from the star creates lukewarm temperatures at which liquid water can exist, and water is the presumed prerequisite for life. Credit: Petigura/UC Berkeley, Howard/UH-Manoa, Marcy/UC Berkeley

Independently, Petigura, Howard and Marcy focused on the 42,000 stars that are like the sun or slightly cooler and smaller, and found 603 candidate planets orbiting them. Only 10 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 suitable for life. The team's definition of habitable is that a planet receives between four times and one-quarter the amount of light that Earth receives from the sun.

A census of extrasolar planets

What distinguishes the team's analysis from previous analyses of Kepler data is that they subjected Petigura's planet-finding algorithms to a battery of tests in order to measure how many habitable zone, Earth-size planets they missed. Petigura actually introduced fake planets into the Kepler data in order to determine which ones his software could detect and which it couldn't.

"What we're doing is taking a census of , but we can't knock on every door. Only after injecting these fake planets and measuring how many we actually found, could we really pin down the number of real planets that we missed," Petigura said.

Analysis of four years of precision measurements from Kepler shows that 22±8% of Sun-like stars may have Earth-sized planets in the habitable zone. Credit: Erik A. Petigura.

Accounting for missed planets, as well as the fact that only a small fraction of planets are oriented so that they cross in front of their host star as seen from Earth, allowed them to estimate that 22 percent of all sun-like stars in the galaxy have Earth-size planets in their .

"The primary goal of the Kepler mission was to answer the question, When you look up in the night sky, what fraction of the stars that you see have Earth-size planets at lukewarm temperatures so that water would not be frozen into ice or vaporized into steam, but remain a liquid, because liquid water is now understood to be the prerequisite for life," Marcy said. "Until now, no one knew exactly how common potentially were around Sun-like stars in the galaxy."

All of the potentially habitable planets found in their survey are around K stars, which are cooler and slightly smaller than the sun, Petigura said. But the team's analysis shows that the result for K stars can be extrapolated to G stars like the sun. Had Kepler survived for an extended mission, it would have obtained enough data to directly detect a handful of Earth-size planets in the habitable zones of G-type stars.

"If the stars in the Kepler field are representative of stars in the solar neighborhood, … then the nearest (Earth-size) planet is expected to orbit a star that is less than 12 light-years from Earth and can be seen by the unaided eye," the researchers wrote in their paper. "Future instrumentation to image and take spectra of these Earths need only observe a few dozen nearby stars to detect a sample of Earth-size planets residing in the habitable zones of their host stars."

In January, the team reported a similar analysis of Kepler data for scorched planets that orbit close to their . The new, more complete analysis shows that "nature makes about as many planets in hospitable orbits as in close-in orbits," Howard said.

Explore further: How common are earths around small stars?

More information: "Prevalence of Earth-size planets orbiting Sun-like stars," by Erik A. Petigura, Andew W. Howard, and Geoffrey W. Marcy. www.pnas.org/cgi/doi/10.1073/pnas.1319909110

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discouragedinMI
1.4 / 5 (23) Nov 04, 2013
So they haven't answered it yet. They only have a "guess" at an upper bound.

NEXT!
Jeweller
1.5 / 5 (15) Nov 04, 2013
It's beginning to look like there will not be another planet exactly like Earth in our Galaxy.
It would have to have exactly the same
gravity
tilt
temperature
climate
orbit
moon
to have even similar life forms
I don't think it's likely at all.
Shootist
2.7 / 5 (21) Nov 04, 2013
It's beginning to look like there will not be another planet exactly like Earth in our Galaxy.
It would have to have exactly the same gravity tilt temperature climate orbit moon to have even similar life forms. I don't think it's likely at all.


exactly is too narrow a word.

Similar enough that you could walk in your shirtsleeves on its surface without suffering ill effect, 10s of thousands, anyway.
beleg
1.4 / 5 (9) Nov 04, 2013
Not if you make an opposite assumption:
No star exists that is completely void of any orbital held celestial object.
Why stop there? No planet is completely void of any orbital held celestial object.
No moon...etc., etc.,
GSwift7
3.3 / 5 (6) Nov 04, 2013
That's still rather staggering in terms of getting from one to another. You couldn't travel one way to anything 12 LY away with any propulsion concept on the drawing board in the time since the beginning of human civilation. We really need a propulsion breakthrough that makes everything we have ever thought of look like paper airplanes before we can even think about sending a probe.

And as suggested in previous comments, since only a fraction of that 22% will truely be hospitable to humans, that makes finding one of them something like looking for a particular grain of sand at the bottom of the ocean.
dogbert
2.1 / 5 (17) Nov 04, 2013
Only if a planet in the so called habitable zone actually has life and has had it for a long time can we expect the planet to be anything like the earth.

The earth has been shaped by life. Without life, we would not have an oxygen/nitrogen/carbon dioxide atmosphere.

We have not found life anywhere but here, so estimates of earth like worlds are simply guesses based on a lack of data.
goracle
1.3 / 5 (13) Nov 04, 2013
That's still rather staggering in terms of getting from one to another. You couldn't travel one way to anything 12 LY away with any propulsion concept on the drawing board in the time since the beginning of human civilation. We really need a propulsion breakthrough that makes everything we have ever thought of look like paper airplanes before we can even think about sending a probe.

And as suggested in previous comments, since only a fraction of that 22% will truely be hospitable to humans, that makes finding one of them something like looking for a particular grain of sand at the bottom of the ocean.

The latest Analog magazine has an article outlining ways to make the sublight voyage easier.
Sinister1811
2.3 / 5 (9) Nov 05, 2013
My guess is not very common. We haven't evolved anywhere else. Therefore, no other planet is going to be 100% habitable.
LarryD
3.7 / 5 (3) Nov 05, 2013
The basic assumption here is 'life AS WE KNOW IT'. Even on Earth some life exists in extreme conditions. What are the chances of life as we DON'T know it? No not a serious comment but it is just a 'guess' at the moment that life could exist on exoplanets that have water. But other conditions that we are not aware of yet (like the H2O containing toxic substances) might prevent life from evolving or us moving to one of the planets in hospitable orbits.
Fleetfoot
2.7 / 5 (3) Nov 05, 2013
That's still rather staggering in terms of getting from one to another. You couldn't travel one way to anything 12 LY away with any propulsion concept on the drawing board in the time since the beginning of human civilation. We really need a propulsion breakthrough that makes everything we have ever thought of look like paper airplanes before we can even think about sending a probe.


Nah, send the probes anyway, it's the only way to find out if they are worth visiting.

And as suggested in previous comments, since only a fraction of that 22% will truely be hospitable to humans, that makes finding one of them something like looking for a particular grain of sand at the bottom of the ocean.


Just send out a system that checks every star.
triplehelix
1.3 / 5 (14) Nov 05, 2013
The planet being in the habitable zone is just the beginning.

The main reason Earth has been able to bear fruit to sentient beings is because it is stable.

Jupiter mops up nearly all the asteroids near us. Since life began, we seem to have had only one major asteroid event.

If you have a habitable planet without a jupiter like planet relatively near it to mop up the asteroids, you'll be lucky to get intelligent life, and only small multicellular basic lifeforms, because the asteroids will continuously put evolution back to square 1.

Jupiter really has saved our bacon many times mopping up those asteroids with its huge gravitational pull.
beleg
1.4 / 5 (9) Nov 05, 2013
Relative asteroid free star systems are among plausible scenarios were considering.
Not enough is known about life to say there is a main reason for it.
Sinister1811
3.2 / 5 (9) Nov 05, 2013
1/5 from two people. I was talking about factors like gravity, weather, atmosphere, orbit, environment etc not being the same as Earth's. If either of them could tell me why that's wrong, that would be great.
Sinister1811
3.9 / 5 (7) Nov 05, 2013
Nah, send the probes anyway, it's the only way to find out if they are worth visiting.


But what about the enormous light year distances? That could literally take forever.
cantdrive85
1.3 / 5 (15) Nov 05, 2013
That's still rather staggering in terms of getting from one to another. You couldn't travel one way to anything 12 LY away with any propulsion concept on the drawing board in the time since the beginning of human civilation. We really need a propulsion breakthrough that makes everything we have ever thought of look like paper airplanes before we can even think about sending a probe... that makes finding one of them something like looking for a particular grain of sand at the bottom of the ocean.

The latest Analog magazine has an article outlining ways to make the sublight voyage easier.

And from that article...
" I (John G. Cramer, complete tool!) gave the next talk, which was based on my May 2012 AV Column, "Shooting Wormholes to the Stars."

This from a symposium that included the noted GR/BB sci-fi blathering of the likes of Stephen Hawking, Paul Davies and this fool.

Wormholes!?!?! COMPLETE MORONS!
GSwift7
5 / 5 (2) Nov 05, 2013
Nah, send the probes anyway, it's the only way to find out if they are worth visiting


and

Just send out a system that checks every star


Name a machine that can function for 10 thousand years, which is only a fraction of the time required to reach just one star 12 LY away. There are thousands of engineering problems we have no answer for, which must be solved just to get a robot probe to work that long.

It is much more reasonable to continue to improve our telescopes until we can't build them any better, and that's a LONG way off. The photons are whizzing past us, begging for us to look at them, carrying enough information about these planets to tell us if they are habitable. Then we can think about what it would take to get a probe to one of them.
Fleetfoot
4 / 5 (4) Nov 05, 2013
Nah, send the probes anyway, it's the only way to find out if they are worth visiting


and

Just send out a system that checks every star


Name a machine that can function for 10 thousand years, which is only a fraction of the time required to reach just one star 12 LY away.


It's less than 400 years with current technology. You are right, we need self-repairing, self -replicating probe technology of course.

There are thousands of engineering problems we have no answer for, which must be solved just to get a robot probe to work that long.


It's fewer than you think, AFM manipulators (see IBM) on nano vehicles such as those being researched by RICE could build photon sails using aluminium extracted from asteroids. There's decades of work needed, but not centuries.

It is much more reasonable to continue to improve our telescopes ..


Other than cost, I wouldn't expect that to stop even if we did launch the probes.
Torbjorn_Larsson_OM
4 / 5 (2) Nov 05, 2013
Considering that the frequency of Earth sized planets was estimated to 20 - 40 % of stars, and that the established lower limit of habitable Earth sized so far is ~ 5 % [HEC], this is hearthening!
Torbjorn_Larsson_OM
5 / 5 (4) Nov 05, 2013
@discouraged: They have "answered it", given an estimate, as the news release shows. We will get better observations, but this constrained observation is unlikely to change much.

Indeed, it was close to earlier estimates, see my previous comment.

@Jeweller, Sinister, LarryD, triplehelix:

You are mixing together 2-3 separate questions:

1. It is well known that no planet will produce "similar life forms" as Earth has today, and that not even Earth would do so if put in the same initial conditions. Evolution is contingent.

2. Planetary systems have been found to be very diverse. Categories of planets less so, as the news release shows.

Then some of you try a "Rare Earth" idea, which is ludicrous. An open ended bayesian model can predict any probability you want, just add more factors, so are not relevant for the question of life.

3. We now know that life clade within geochemistry, hydrothermal vents to be precise, and even Mars has such.

[tbctd]
Torbjorn_Larsson_OM
5 / 5 (2) Nov 05, 2013
[ctd] Maybe life got started and is still extant in refugia there. That is the next planet over.

#3 is the astrobiology question that the research illuminates.

@triplehelix:

We now know that Jupiter is more harmful than protecting, from modeling what would happen instead of guessing from observation. It throws large objects into the inner system, while it is not very good at sweeping them up later. The only good thing is that a normal size giant of ~ 0.2 Jupiter masses would have been the worst possible planet - Jupiter is a bit better.

Anyway, Mojzics [sp?] et al showed that life survivse up to 10x the rate of the late bombardment, since prokaryotes proliferate and spread faster than any reasonable sterilizing rate.

We have had many asteroid event since life arose, they have found a tail of at least 7 large impactors after the first fossils appeared. You are thinking of land life, and not even that is true.

[tbctd]
Torbjorn_Larsson_OM
5 / 5 (2) Nov 05, 2013
[ctd]

Despite many impacts, and even larger IIRC, during land life existence, only one was promoting a mass extinction. It happened because it hit unlucky in calciferous and sulfurous sediments that modern life has laid down.
Andrew Palfreyman
1 / 5 (13) Nov 05, 2013
We can get to Alpha Centauri in less than 10 years using current tech. The sun pumps out 10^26 Watts continuously. That makes it possible, but you will need to do the due diligence to understand it all. I'd start with the Icarus Interstellar site. I am assuming that you understand high school physics. If not, trust me.
RealScience
5 / 5 (4) Nov 05, 2013
@Andrew - If by "we can get to Alpha Centuri" you mean sending people, then you appear to be confusing 'current tech' with 'perhaps-possible technology that we are starting to think about'.

I took a quick look at the Icarus web site, and calling any of the ideas there 'current' tech is a wee bit of a stretch. We don't yet have controlled fusion, or warp drives, or practical magnetic drives, and we can't yet even collect a significant fraction of the 10^26 Watts from the sun, let alone direct it into a beam...

(But it is pretty cool that people are actually getting together to start thinking that way outside of a Sci-Fi convention.)
goracle
1 / 5 (10) Nov 06, 2013
That's still rather staggering in terms of getting from one to another. You couldn't travel one way to anything 12 LY away with any propulsion concept on the drawing board in the time since the beginning of human civilation. We really need a propulsion breakthrough that makes everything we have ever thought of look like paper airplanes before we can even think about sending a probe... that makes finding one of them something like looking for a particular grain of sand at the bottom of the ocean.

The latest Analog magazine has an article outlining ways to make the sublight voyage easier.

And from that article...
" I (John G. Cramer, complete tool!) gave the next talk, which was based on my May 2012 AV Column, "Shooting Wormholes to the Stars."

This from a symposium that included the noted GR/BB sci-fi blathering of the likes of Stephen Hawking, Paul Davies and this fool.

Wormholes!?!?! COMPLETE MORONS!

I don't think you even read the title.
GSwift7
2 / 5 (4) Nov 06, 2013
It's less than 400 years with current technology. You are right, we need self-repairing, self -replicating probe technology of course


What kind of propulsion are you suggesting?

If you're talking about an ion drive that would need to run continuously for the majority of the trip, then that's out of the question. We can't get ion drives to work that long. I would even question whether your fuel system could work that long (heaters to keep the fuel from freezing, pumps, valves, stirrers, seals, etc). We don't have anything that could realistically get to another star.

BTW, I think several of you guys started talking about the trip from here to alpha centauri, rather than the 12 LY average trip the article is talking about.

Solar sails aren't a proven tech yet either. There have only been limited attempts to prove the concept so far. Building one on the scale you would need for interstellar travel is a whole different ballgame (if the concept works at all).

Jonseer
1 / 5 (10) Nov 06, 2013
So they haven't answered it yet. They only have a "guess" at an upper bound.

NEXT!

So true, and luckily for them Astronomers never get hit with the harsh backlash other scientists who specialize in fields directly related to life on Earth experience whenever they get a few facts wrong while stating a theory that is right overall (Iike climate change).

Way too much time is spent sitting around fantasizing answers to questions then presenting there conclusions as "facts" that get disproved the moment tech becomes advanced enough to see the fallacies.

Of all the fields of science astrobiologists live in a bubble where they are paid to sit around and play 24/7. They get away with it, because they are in charge of developing the standards all the "facts" that justify their work.

I'm sure quite a few laugh themselves to sleep when they think how fortunate they are to be smart enough to create a job where all the playing they do is considered genius work by the sheeple.
JES
3 / 5 (2) Nov 07, 2013
My guess is not very common. We haven't evolved anywhere else. Therefore, no other planet is going to be 100% habitable.


Ehhh...what a very logic conclusion. Wonder why no one have thought about that!
Fleetfoot
not rated yet Nov 07, 2013
What kind of propulsion are you suggesting?


High temperature solar sail.

BTW, I think several of you guys started talking about the trip from here to alpha centauri, rather than the 12 LY average trip the article is talking about.


You need to communicate back to Earth and the way to do that with best SNR is to use a series of minimum length hops, i.e. a relay at every star.

Solar sails aren't a proven tech yet either. ... Building one on the scale you would need for interstellar travel is a whole different ballgame (if the concept works at all).


Ikaros has been very successful. Their aluminium layer is the right thickness but the substrate is much too thick, it needs to be replaced by an aerogel based on carbon nanotubes which is a good black-body radiator.

The self-repair aspect is the hardest part but allows self-replication automatically and a small version can be launched which "repairs" itself back to full size in the asteroid belt.
Judgeking
not rated yet Nov 07, 2013
One thing that's not mentioned; Sun-like stars only make up about one third of stars, so it would be one if five of these. I don't know how visible the other 2/3rds are from Earth though.
discouragedinMI
1 / 5 (8) Nov 09, 2013
Well, this discussion has gone off course. Read their assumptions. It's mostly guess work. I don't call that science. I call it an idea.
Sinister1811
3 / 5 (2) Nov 11, 2013
My guess is not very common. We haven't evolved anywhere else. Therefore, no other planet is going to be 100% habitable.


Ehhh...what a very logic conclusion. Wonder why no one have thought about that!


I couldn't agree more. I think it's important not to be too optimistic. Early astronomers thought Venus had forests and rivers under its thick atmosphere, and Mars had canals and civilization. We know better today. ;)
yaridanjo
1 / 5 (6) Nov 18, 2013
I can remember the days when most astronomers thought that only our Sun had planets.

A more reasonable thought is that nearly every star has planets, and they are deployed in a Titus Bode law type formation meaning some type of planet in the Goldilocks zone is likely.

What is really being sought is sentient life like humans or Cetaceans.

Sentient life forms in groups of seven, and they must be close enough to interact.

The military is way ahead of astronomers in this field, because we are a Warrior sentient species.

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